Air Changes per Hour (ACH) Calculator

Hong Kong service charge, extra tipping and bill-splitting in one tool.

Estimate monthly bill using the latest CLP / HKE tariff.

Estimate the fuel cost of a trip from distance, vehicle consumption and fuel price (one-way or round-trip).

Enter the bill, service charge, tip and party size to get each person’s share — then round each share up to the nearest $1 / $5 / $10 / $20 / $50 to make cash collection painless.

Enter the original price and a percent off (or fixed amount off) to see the final price, money saved and effective discount.

Convert between cups, tablespoons, teaspoons, millilitres, ounces and grams — for following overseas recipes or halving / doubling portions.

Compare two products by price-per-unit (per g, kg, ml, L or each). Enter price + amount + unit for each — see which is cheaper and by how much.

Convert hourly wage to monthly / annual salary or back — with customisable hours per week and weeks worked per year. Works for part-time, freelance and salaried jobs.

Estimate a dog's human-equivalent age from its actual age and size (small / medium / large / giant), with both the classic "×7" rule and modern research curves side by side.

How many people, how hungry, and how many slices per pizza? Get the exact number of pizzas to order — no leftovers, no shortages.

Enter room dimensions and tile size to estimate the number of tiles needed (including a configurable waste allowance) — for floor or wall jobs.

From room length, width and height, door and window area, paint coverage per litre and the number of coats, estimate how many litres of paint you need.

Pick a brew method (pour-over, French press, cold brew or espresso) and a desired output, and get the exact coffee dose and water weight to match the recommended ratio.

Enter the room size, roll dimensions and pattern repeat to get the number of wallpaper rolls to order (with pattern-match waste included).

Enter cigarettes per day, price per pack and years smoked — see your total spend and how much you could save by quitting.

Convert any roof rise / run into the X-in-12 pitch notation used by builders, plus the slope angle in degrees, slope percentage and the rafter-length multiplier.

Enter slab length, width and thickness with a waste factor and get the concrete volume (m³, yd³, ft³) plus the approximate number of 25 kg, 40 kg or 80 lb pre-mix bags required.

Enter total rise and a target riser height to get the number of risers, actual riser height, tread depth, stringer length, pitch and a side-by-side compliance check against the IRC, IBC and UK Approved Document K.

Enter room area, ceiling height, sun exposure, occupants and climate to find the recommended air-conditioner BTU rating / horsepower.

Enter your wedding date to see the anniversary number, days until the next one, and the traditional / modern gift theme (paper, silver, gold…).

Convert oven temperatures between Celsius, Fahrenheit and British gas marks, with descriptive labels (slow / moderate / hot / very hot).

Enter original servings and desired servings to scale every ingredient proportionally, with friendly rounding to common cookware-sized amounts.

Enter your cat's real age to estimate the human-equivalent age using the standard veterinary formula and see the matching life stage (kitten, adult, senior, geriatric).

Enter price, expected number of uses and upkeep cost to compute true cost per use — see whether that item is really worth it.

Estimate hiking time with the classic Naismith's Rule (1 h per 5 km plus 1 h per 600 m of ascent), with optional descent and rest-time corrections.

Enter power (W / kW / hp) and weight (kg / lb) to instantly compute W/kg, hp/kg and hp/lb ratios with cyclist and car performance bands.

Enter battery capacity (kWh), charger power (kW) and the start / target state of charge to estimate how long an EV charging session takes, with adjustments for charging efficiency and the constant-current → constant-voltage taper above 80 %.

Enter the original gravity (OG) and final gravity (FG) of a beer or wine ferment and the calculator returns the standard homebrewing ABV estimate ABV ≈ (OG − FG) × 131.25, plus apparent attenuation and a calorie estimate.

Enter the area and target depth to compute the volume of mulch, soil or gravel needed (in litres, m³ and cubic yards), and estimate bag count and total cost.

Enter the dimensions of your tank — rectangular, cylindrical or bow-front — to get the water volume in litres, US gallons, UK gallons and water weight, with a fill-percentage adjustment for substrate and décor.

Enter flour weight and target hydration to get water, salt and yeast quantities via baker's percentage — with Neapolitan, NY and Sicilian style presets.

Enter an original price and two discounts applied in sequence (e.g. "30% off then an extra 10%") — the tool returns the final price, the equivalent single discount and the savings, sidestepping the common mistake of simply adding the percentages.

Enter a TV or monitor diagonal and an aspect ratio (e.g. 16:9, 21:9) and the tool returns the actual width, height and area in both inches and centimetres — handy for furniture planning, wall-mount fit and seeing why a "55-inch" is so much bigger than a "50-inch" in area.

Enter great-circle distance (km / mi / nm), cabin class and segment type (domestic / short / long-haul); the tool applies DEFRA 2024 emission factors to estimate per-passenger CO₂-equivalent kilograms, total trip emissions and tree-month offsets — the standard checklist for business-travel reporting, personal carbon budgets and offset-purchase decisions.

Enter an amount and a VAT / GST rate; the tool computes both directions — adding tax to a net price, and extracting the tax already embedded in a gross price. Works for any country that uses VAT / GST / IVA / TVA (UK 20%, EU 17–27%, AU & SG GST, India GST, etc.).

Enter daily energy use (kWh), peak sun hours, panel wattage and overall system efficiency; the tool returns the required array size in kW and the number of panels needed — a starter sizing tool for rooftop PV, off-grid cabins and RV / van conversions.