Interest-Only Mortgage Payment Calculator
An **interest-only (IO) mortgage** charges only *interest* during an initial window (typically 3, 5, 7 or 10 years) — the principal does not fall at all. When the IO window ends, the loan converts to a normal principal-and-interest payment over the remaining term, and the monthly payment jumps from "interest only" to "principal + interest". **The tool returns**: (1) the IO monthly payment = P × r; (2) the post-IO amortising payment = P × r / (1 − (1+r)⁻ⁿ) where r = annual rate / 12 and n = amortising months; (3) an equivalent fully-amortising payment over the *same total* term as a benchmark; (4) interest paid in the IO phase, the amortising phase, and the full IO scheme vs the fully-amortising benchmark. **Typical uses**: (a) **property investors** — maximise cash flow while rental yields catch up; (b) **bridge financing** — while waiting to sell the old property or refinance; (c) **high-but-lumpy income earners** — make extra principal payments in good months; (d) **commercial-real-estate (CRE) loans** — relieve cash-flow stress before the project stabilises. **Watch out**: the payment jump at the end of the IO period (this tool shows the exact figure) can cause "payment shock" if you cannot refinance or sell the property by then.
Enter a valid principal (> 0), rate (≥ 0) and term (amortising period > 0).
Monthly payment during IO period
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Total cost comparison
During the interest-only period the principal does not fall — when the period ends, the monthly payment jumps to the fully-amortising level. Unless you intend to sell, refinance or repay the principal before then, the total cost is usually higher than a normal mortgage.
Formula
Notation: P = loan principal R = annual rate (in percent) r = R / 100 / 12 (monthly rate) Y₁ = IO years; n_io = Y₁ × 12 Y₂ = amortising years; n_am = Y₂ × 12 N = (Y₁ + Y₂) × 12 (full term, months) IO-period monthly payment: payIO = P × r After the IO period, the amortising monthly payment (annuity formula): payAm = P × r / [1 − (1 + r)⁻^n_am] Benchmark — fully amortising over the same total term: payFA = P × r / [1 − (1 + r)⁻^N] Interest: interestIO = payIO × n_io interestAm = payAm × n_am − P interestTotal = interestIO + interestAm interestFA = payFA × N − P (benchmark) Total cost: totalIO = interestIO + payAm × n_am = P + interestTotal totalFA = payFA × N = P + interestFA Degenerate r = 0 case: payIO = 0; payAm = P / n_am; payFA = P / N.
- · **Why is an IO mortgage usually *more* expensive overall?** — During the IO period the principal does not fall, so the amortising phase has to repay the *entire* principal over a *shorter* window, pushing both the monthly payment and the total interest above a normal fully-amortising loan. Example with US$500 000 at 6 %: (a) **30-year fully-amortising**: $2 997/mo, total interest ≈ $579 000; (b) **5-year IO + 25-year amortising (same 30-year total)**: IO payment $2 500/mo, amortising payment $3 221/mo (7 % higher than the FA benchmark), total interest ≈ $616 000 — **about $37 000 more** than the standard 30-year loan. **Bottom line**: an IO mortgage is not a money-saver; it shifts cash flow forward, trading lower short-term payments for higher long-term cost.
- · **"Payment shock" is the biggest IO risk** — the month the IO period ends, the payment jumps from the IO level to the amortising level. This tool prints the exact jump. **Example** (500 k, 6 %, 5 + 25): the payment leaps from $2 500 to $3 221, a **29 % rise**. **Mitigation**: (a) verify your income at IO-end will cover the new payment (don't assume future pay rises); (b) make voluntary principal pre-payments during the IO window to lower the amortising base; (c) plan to refinance at IO-end, but model the rate-rise risk (US benchmark rates rose from 0.25 % to 5.5 % over 2022–2024, making a refi much more expensive); (d) build in a sale-of-property exit, with enough appreciation to repay the principal.
- · **Post-2008 regulatory tightening** — the US Dodd-Frank Act (2010) and CFPB's **Qualified Mortgage (QM)** rule (effective 2014) excluded IO mortgages from "QM" safe-harbour, meaning lenders take more *Ability-to-Repay (ATR)* legal risk. Owner-occupied IO mortgages have collapsed from ≈ 20 % of US originations at the 2006 peak to under 2 % today. **But IO is still common in**: (a) commercial real estate (CRE) — outside QM scope; (b) Non-QM private-bank lending (jumbo, high-net-worth); (c) other jurisdictions — UK buy-to-let investment mortgages (≈ 4 % of UK market, FCA-regulated), Australia (IO ≈ 30 % of investor mortgages, APRA tightened in 2017).
- · **When is an IO mortgage actually attractive?** — not on "total cost" but on "cash-flow timing": (a) **investment property + high rental yield** — if the cash freed up each month can be deployed into another investment earning more than the mortgage rate, the net return is positive; (b) **transition scenarios** — waiting to sell the old place, expected bonus, simulated income; (c) **US tax optimisation** — mortgage interest on investment property is fully deductible, and IO maximises the deductible base (though TCJA 2017 capped owner-occupied deductibility at $750 k); (d) **planned early repayment** — if you expect to sell or refinance within 3–5 years, IO monthly interest can be lower than the P&I interest *component*: P&I customers do pay principal each month (reducing interest later), but IO interest is always on the full principal. **Counter-example**: owner-occupied + long-term hold + stable income + no other investment opportunity — a normal fully-amortising loan is simply better.
- · **References**: (1) Brealey, Myers & Allen, *Principles of Corporate Finance*, 13th ed. (2019), §3.2; (2) Consumer Financial Protection Bureau (CFPB), "What is an interest-only loan?", consumerfinance.gov; (3) 12 CFR § 1026.43 (Regulation Z, Ability-to-Repay rule); (4) Bank of England, *Financial Stability Report*, June 2017 (IO mortgage section); (5) APRA (Australia), *Prudential Practice Guide APG 223*, "Residential Mortgage Lending" (2017); (6) Frederic S. Mishkin, *The Economics of Money, Banking, and Financial Markets*, 13th ed., Pearson (2022), ch. 13.
Frequently asked
Why is the interest-only monthly payment so low — is there a catch?
**Because you are not paying any principal — just the bank's "rental fee" on the money**. **Example**: US$500 000 at 6 %, IO monthly = 500 000 × 0.005 = **$2 500/mo**. The 30-year fully-amortising payment for the same loan is **$2 997/mo** — about $500 less per month. **But that $500 has not disappeared**: (a) **after 5 years of IO** you still owe the original $500 000; a fully-amortising borrower has already repaid about **$42 000** of principal and owes only $458 000; (b) **the next 25 years of amortising payments** are **$3 221/mo** (7 % higher than the FA benchmark); (c) **total interest**: IO scheme ≈ $616 000 vs FA benchmark ≈ $579 000 — the IO mortgage costs roughly **$37 000 more** in interest. **When is IO sensible?** (a) you are certain you will sell within 5 years (relocation, promotion) and the sale repays the principal; (b) it is an investment property and rental yield exceeds the mortgage rate (the spread is positive cash flow); (c) your income is highly variable (self-employed, bonus-heavy) — pay the low IO base and pre-pay principal in good months; (d) it is a bridge loan while you arrange refinancing. **Otherwise** — owner-occupied + long-term hold + steady income + a goal of being debt-free sooner — a normal fully-amortising mortgage is the better choice.
How big is the payment jump at the end of a 5-year IO period — how do I estimate it?
**A typical IO-end payment jump is 25–40 %**, depending on the rate, IO length and amortising length. **Closed-form**: jump = (payAm − payIO) / payIO = [1 / (1 − (1+r)⁻ⁿ)] − 1, where r = monthly rate and n = amortising months. **Examples** (30-year total term): (a) **6 % rate, 5y IO + 25y amort**: +29 % ($2 500 → $3 221); (b) **6 % rate, 10y IO + 20y amort**: +43 % ($2 500 → $3 582); (c) **3 % rate, 5y IO + 25y amort**: +90 % ($1 250 → $2 371) — the lower the rate, the bigger the relative jump (because the IO base is small); (d) **8 % rate, 5y IO + 25y amort**: +16 % ($3 333 → $3 859) — high rates flatten the jump. **Mitigation**: (a) use this tool to see the actual jump; (b) ensure that your income at IO-end can absorb new annual payment / 0.3 (housing cost ≤ 30 % of income); (c) make voluntary principal pre-payments throughout the IO period (e.g. 5 %/year) — this lowers the IO-end principal and the jump; (d) lock in a fixed-rate IO mortgage at origination to avoid the double whammy of rising rates *plus* the IO-end jump.
How does IO mortgage availability differ across Hong Kong, the UK and the US?
**United States — almost extinct for owner-occupied since 2008**: the CFPB Qualified Mortgage (QM) rule (effective 2014) excluded IO mortgages from QM safe-harbour, and owner-occupied origination fell from ~20 % at the 2006 peak to under 2 % today; IO survives in jumbo, commercial-real-estate and investment-property loans. **United Kingdom — owner-occupied IO available but constrained**: the FCA's 2014 Mortgage Market Review (MMR) requires borrowers to demonstrate a "credible repayment plan" (e.g. insurance, ISA, sale of the property). Owner-occupied IO is now ≈ 4 % of UK originations; **buy-to-let investment** is still dominated by IO (over 60 %). **Hong Kong — HKMA caps**: the HKMA Residential Mortgage Supervisory Guidelines (since 2009, multiple revisions) enforce a strict 50 % DSR with a +3 pp interest-rate stress test plus LTV caps (e.g. 70 % below HK$10 m), so pure-IO residential mortgages essentially do not exist; however, **developer "first-mortgage" / "Stage Payment" schemes** in the primary market function similarly (2–3 years of interest-only or minimal principal), at the regulatory fringe. **Australia — common on investment loans**: APRA's 2017 intervention pulled IO investment-mortgage share back to ≈ 30 %; owner-occupied IO is under 5 %. **Bottom line**: this tool reports the *cost structure of an IO scheme* — it does not deal with regional regulation or product availability.
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Enter total ad spend, impressions, clicks and conversions; the tool returns CPM (cost per mille), CPC (cost per click), CPA (cost per acquisition) plus CTR and CVR. Foundational metrics for digital advertising — Google Ads, Meta Ads, TikTok Ads — and any media-buying budget exercise.
Customer Churn & Retention Rate Calculator
Enter the start-of-period customer count, end-of-period count and new customers acquired; the tool returns the churn rate, retention rate and expected average customer lifetime (1 ⁄ churn). The two most-watched health metrics for SaaS, subscription and membership businesses.
Straight-Line Depreciation Calculator
Enter the asset cost, salvage value and useful life; the tool returns annual depreciation (cost − salvage) ÷ life, plus the accumulated depreciation and end-of-year book value for every year of the schedule. The most-used depreciation method in corporate accounting, fixed-asset registers and tax filings.
Betting Odds Converter (Decimal / American / Fractional / Implied Probability)
Enter odds in any one format and the tool returns the other three: European decimal (2.50), American (+150 / −200), British fractional (3 / 2) and the implied probability (40%). Essential conversion for sports betting, poker, value-betting and arbitrage analysis.
Prorated Salary Calculator
Enter the annual salary, days actually worked and the days-in-year basis (260 working days, 365 calendar days, etc.); the tool returns prorated_salary = annual_salary × days_worked ⁄ days_in_year. The standard formula HR uses for mid-year hires / leavers, unpaid leave and part-year contracts.
Funding Round Equity Dilution Calculator
Enter the pre-money valuation, round size, new option-pool top-up % and your pre-round founder stake; the tool applies the standard VC pre-money pool convention to return post-money valuation, new-investor ownership, existing-shareholder share, the founder's post-round stake and the dual dilution (investor + pool). The cap-table sanity check for founders negotiating a term sheet.
Equivalent Annual Cost (EAC) Calculator
Enter the project NPV (or initial cost), discount rate and life in years; the tool returns the Equivalent Annual Cost via EAC = NPV × r ⁄ (1 − (1 + r)⁻ⁿ), the annuity factor, the nominal total payments and the discount-free flat average for comparison. The standard capital-budgeting tool for comparing projects of different lifespans (e.g. a 5-year vs 8-year machine).
Dividend Payout Ratio Calculator
Enter dividends per share (DPS) and earnings per share (EPS) — or total dividends paid and net income — and the tool returns the payout ratio plus its complement, the retention ratio. The standard metric for gauging dividend sustainability and benchmarking blue-chip vs growth stocks.
Price-to-Sales (P/S) Ratio Calculator
Enter share price and revenue per share — or market cap and annual revenue — and the tool returns the P/S ratio with an industry-benchmarked band. The go-to valuation metric for high-growth or pre-profit companies (SaaS, biotech) where P/E breaks down.
Days Sales Outstanding (DSO) Calculator
Enter accounts receivable, credit sales for the period and the number of days; the tool returns DSO = (AR ⁄ Credit Sales) × Days — the canonical metric for how quickly a company converts customer credit into cash. Core gauge of working-capital efficiency and liquidity.
Loan-to-Income (LTI) Ratio Calculator
Enter the loan principal and gross annual income; the tool returns LTI = Loan ÷ Gross Annual Income and benchmarks it against common regulatory caps (UK PRA 4.5×, Irish Central Bank 4×, equivalent HKMA stress-test DSR bands). The standard screen for mortgage affordability and leverage risk.
Free Cash Flow (FCF) Calculator
Enter operating cash flow, capital expenditure and (optionally) the change in working capital; the tool returns Free Cash Flow = OCF − CapEx ± ΔWC and the FCF margin — the core input for DCF valuation, cash-on-cash return and margin-of-safety analysis.
Basis Points (bps) ↔ Percentage Converter
A basis point (bp) is the standard unit for small changes in financial rates — 1 bp = 0.01 %, 100 bp = 1 %. Enter bps or % and the tool converts both ways, with optional application to a notional principal, rate or bond spread — eliminates the ambiguity behind "Fed hikes 25 bps" headlines.
Double-Declining Balance Depreciation Calculator
Enter the asset cost, salvage value and useful life; the tool builds a year-by-year double-declining balance (DDB) schedule with depreciation expense, accumulated depreciation and book value — an accelerated method permitted by US GAAP and IFRS, widely used for vehicles, computers and machinery whose value drops sharply in the early years.
Forex Pip Value Calculator
Enter a currency pair, lot size, quote price and account currency; the tool returns the cash value of one pip (and one pipette) for that trade — essential for MT4 / MT5 traders, forex risk management and stop-loss / take-profit placement.
Continuous Compound Interest Calculator (A = P·e^(rt))
Continuous compounding is the limit of more-frequent periodic compounding: A = P·e^(rt). Enter principal, rate and time; the tool returns the final amount and total interest, and contrasts it with finite-period compounding (monthly / quarterly / semi-annual / annual) — used in options pricing, Black-Scholes and academic finance.
Bond Current Yield Calculator
Current yield = annual coupon ÷ current market price — the simplest measure of a bond's instant cash return. Enter par value, coupon rate (or coupon $) and market price; the tool returns current yield and contrasts it with the coupon rate — a quick first look at bonds trading at premiums or discounts.
Black-Scholes Option Price Calculator
Enter spot price, strike, time to expiry, risk-free rate, volatility and optional dividend yield; the tool applies the Black-Scholes-Merton 1973 model to return the European call and put prices, d₁ / d₂ and the full set of Greeks (Delta, Gamma, Vega, Theta, Rho) — the standard reference for option pricing, sanity-checking quotes and exploring implied volatility.
Tiered Sales Commission Calculator
Enter cumulative sales and 1–5 tiered commission rates (e.g. 5 % on the first 100 k, 6 % on the next, 8 % above) and the tool returns total commission, marginal and effective rates, plus the contribution of each tier — used by sales reps, sales managers and year-end payroll.
Customer Lifetime Value Calculator (CLV / LTV)
Enter average order value, purchase frequency, gross margin, churn (or expected lifetime) and discount rate; the tool applies the steady-state formula CLV = (AOV × Freq × Margin) / (Churn + Discount) to give the net present value of one customer over their entire relationship — the standard SaaS, subscription, retail and e-commerce metric.
Average True Range Calculator (ATR)
Paste the high, low and close for the last N bars; the tool applies Wilder's True Range max(H − L, |H − C_prev|, |L − C_prev|) and his RMA smoothing to give ATR — the standard volatility measure for sizing futures, FX and crypto stops and positions.
Dividend Reinvestment Plan (DRIP) Calculator
Enter starting shares, share price, annual dividend yield (DPS / price), expected annual price growth and horizon; the tool runs the year-by-year compounding loop — each dividend is fully reinvested into new shares, which earn the next round of dividends — and reports the final share count, market value, cumulative dividends and annualised CAGR. The standard analysis tool for long-term dividend reinvestment in stocks such as SPY, AAPL and KO.
Holding Period Return (HPR) Calculator
Enter buy price, sell price, total cash received during the holding period (dividends / coupons / distributions) and the length of the period; the tool computes the holding-period return (HPR) and the annualised HPR — works for stocks, bonds, ETFs and funds and shows the complete picture of capital gain plus cash distributions.
Personal Net Worth Calculator
Enter assets (cash, investments, property, vehicles, other) and liabilities (mortgage, credit cards, student loans, car loans, other); the tool returns total assets, total liabilities and net worth, plus the debt-to-asset ratio (DTA) and liquid-to-debt ratio commonly used to gauge financial health.
Cash-on-Cash Return Calculator
Enter the annual pre-tax cash flow from a property or investment and the actual cash you invested (down payment plus closing costs); the tool returns CoC = annual_cash_flow ⁄ total_cash_invested — real-estate investors' standard benchmark for cash-on-cash efficiency.
Car Loan Monthly Payment Calculator
Enter the car price, down payment, annual interest rate and loan term; the tool applies the standard amortization formula to give the monthly payment, total interest, total repayment, and a 12-month principal/interest split.
Safety Stock & Reorder Point Calculator
Enter the average daily demand, demand standard deviation, lead time and target service level; the tool applies Safety Stock = Z × σ_d × √L and ROP = d̄ × L + SS to give the reorder point that meets your stock-out tolerance.
Car Lease Monthly Payment Calculator (Depreciation + Money Factor)
Enter the cap cost, residual value, term, money factor and down payment; the tool applies the industry-standard lease formula to split the monthly bill into depreciation, finance charge and pre-tax total.
Loan Extra Payment Savings Calculator (Time & Interest Saved)
Enter the loan principal, annual rate, scheduled monthly payment and the extra you can pay each month; the tool runs month-by-month amortisation to show the time and interest saved, and compares the with- vs without-extra payoff timelines.
Two-Asset Portfolio Variance / Std-Dev Calculator (Markowitz)
Enter two assets' weights, standard deviations and correlation ρ; the tool applies σ²p = w₁²σ₁² + w₂²σ₂² + 2w₁w₂σ₁σ₂ρ to give portfolio variance / std-dev, covariance, diversification benefit and the minimum-variance weights — the heart of Markowitz modern portfolio theory.
Forward Exchange Rate Calculator (Covered Interest Rate Parity)
Enter the spot rate, the quote-currency (domestic) and base-currency (foreign) annual interest rates, and the contract days; the tool applies covered interest rate parity F = S × (1 + r_d × t/basis) / (1 + r_f × t/basis) to give the theoretical forward rate, forward points and annualised premium / discount — essential for FX-forward pricing, carry-trade analysis and cross-border settlement.
Mortgage Balloon Payment Calculator
Enter principal, rate, amortising term and actual loan term; the tool calculates the monthly payment using the longer amortisation schedule and the balloon (lump-sum) due at loan maturity, plus total interest and a comparison vs a fully-amortising loan of the same term.
Enterprise Value to EBITDA (EV / EBITDA) Multiple Calculator
Enter equity market cap, total debt, cash, minority interest and preferred stock, plus trailing-12-month EBITDA; the tool returns Enterprise Value, the EV / EBITDA multiple and an industry benchmark band — the cornerstone multiple in M&A, private-equity and value-investing valuation.
Modified Duration Calculator (Bond Price Sensitivity to Yield)
Enter a bond's coupon rate, yield to maturity, remaining years and coupon frequency; the tool returns Macaulay duration, modified duration D* = D_M / (1 + y/k), convexity, DV01 and the approximate price change for ±1% yield moves — the standard interest-rate-risk metric for fixed-income portfolios.
Earned Value Management (EVM) Calculator (CPI, SPI, EAC, ETC, VAC)
Enter Planned Value (PV), Actual Cost (AC), Earned Value (EV) and Budget at Completion (BAC); the tool instantly returns the PMI / PMBOK-standard Cost Performance Index (CPI), Schedule Performance Index (SPI), Cost / Schedule Variance (CV / SV), Estimate at Completion (EAC), Estimate to Complete (ETC), Variance at Completion (VAC) and To-Complete Performance Index (TCPI) — the core indicators a PMP / PRINCE2 project manager uses to track project health.
Rent vs Buy Total-Cost Comparison
Enter property price, down payment, mortgage rate, expected property appreciation, current rent, rent growth, alternative investment return and intended holding period; the tool computes the NPV of renting vs buying and identifies the break-even year — a universal framework without country-specific tax handling.
Graham Number Calculator (Value-Investing Fair Price √(22.5 × EPS × BVPS))
Enter earnings per share (EPS) and book value per share (BVPS); the tool applies Benjamin Graham's classic formula √(22.5 × EPS × BVPS) to estimate the theoretical fair-value ceiling for a defensive stock pick — the entry-level screen used by value investors and equivalent to the combined P/E ≤ 15 and P/B ≤ 1.5 Graham criteria.