Bicycle Helmet Size Converter (Head Circumference cm ↔ XS/S/M/L/XL)

Cm, m, km, in, ft, yd, mi — common length units, converted instantly.

Convert between Celsius (°C), Fahrenheit (°F) and Kelvin (K) instantly.

Enter your foot length or a size in any system to see the equivalent US / UK / EU and centimetre sizes.

Pick a paper size (A0–A10, B / C series, Letter, Legal, card sizes) to see its mm / cm / inch / pixel dimensions.

Convert a number simultaneously between binary, octal, decimal and hexadecimal — useful for programmers, students and electronics work.

From an aspect ratio and one side, find the other; or from a diagonal and ratio, get real width and height (4:3, 16:9, 21:9 …).

Convert between bytes, KB, MB, GB, TB and PB — distinguishing decimal (1000-base, drive labels) from binary (1024-base, OS-reported).

Convert between HEX (#RRGGBB), RGB(0–255) and HSL(0–360, 0–100, 0–100) instantly — with a live colour preview swatch.

Convert a P-metric tire spec (e.g. 225/65R17) into sidewall height, overall diameter, circumference and revolutions per km — and compare against a reference size to see speedometer error.

Convert between joules, kilojoules, calories, kilocalories, kilowatt-hours, BTU and watt-hours in a single view — for nutrition labels, electricity bills, heating and fuel comparisons.

Convert between Pa, kPa, bar, mbar, atm, psi, mmHg, Torr and inHg in a single view — all the everyday pressure units.

Convert between N·m, kgf·m, kgf·cm, lbf·ft, lbf·in and ozf·in — all the torque units used on torque wrenches, bicycle bolts and car service specs.

Convert between degrees, radians, gradians, turns, arcminutes, arcseconds, milliradians and NATO mil — with live sin/cos/tan values for reference.

Convert between L/100km, km/L, US MPG and UK (imperial) MPG — compare car efficiency figures across regions without arithmetic.

Convert between W, kW, MW, mechanical horsepower (hp), metric horsepower (PS), BTU/hr, ft·lbf/s, cal/s, kcal/h and refrigeration tons in one view.

Pick a root font-size (defaults to 16 px) and convert any CSS pixel, rem, em, percentage or point value into the others, with a reference table for common sizes.

Convert alcohol by volume (ABV %) to US proof and UK proof, plus compute grams of pure alcohol and "standard drinks" per pour for US / UK / AU / HK.

Convert network bandwidth (kbps, Mbps, Gbps) to file-transfer rate (KB/s, MB/s, MiB/s) and estimate how long a given file size takes.

Kilograms, grams, milligrams, pounds, ounces, stones, tonnes, jin and tael — common mass units, converted instantly.

Enter battery capacity (mAh) and voltage (V) to get watt-hours (Wh) instantly — with a live IATA airline check (100 Wh / 160 Wh cut-offs).

Enter underbust and bust (or pick a local size) to convert bra sizes across US, UK, EU, AU and JP standards.

Enter inner circumference (mm) or diameter to cross-reference US, UK, EU, JP and HK ring sizes.

Square metres, hectares, acres, square feet, square miles and more — common area units, converted instantly.

Litres, millilitres, cubic metres, US and imperial gallons, quarts, pints, fluid ounces, cups, tablespoons and teaspoons — converted instantly.

Convert freely between km/h, mph, m/s, ft/s, knots and Mach number.

Enter foreground and background hex colors to compute the WCAG 2.x contrast ratio (1:1 to 21:1) and see whether the pair passes AA / AAA accessibility levels for normal and large text.

Look up the diameter (mm), cross-sectional area (mm²), resistance per kilometre and safe ampacity for any American Wire Gauge (AWG) size — handy for low-voltage wiring design.

Convert between Hz, kHz, MHz, GHz, THz, RPM and BPM — one input fills every other unit instantly.

Convert between newtons (N), kilonewtons (kN), pound-force (lbf), kilogram-force (kgf), dyne and ounce-force (ozf) — bridging SI, imperial and CGS force units used in physics and engineering.

Convert magnetic flux density between tesla (T), millitesla (mT), microtesla (µT), gauss (G) and milligauss (mG) — handy for comparing MRI fields, Earth field, speakers and lab magnets across very different scales.

Enter parcel length × width × height and DIM divisor (IATA air 6000 cm³/kg, express 5000, UPS/FedEx US 139 in³/lb) to compute volumetric weight and compare it with actual weight to find the chargeable weight.

Convert Cartesian (x, y) ↔ polar (r, θ) coordinates in either direction with a degree/radian toggle — useful for math, physics, engineering and game-development work.

Enter any distance and the tool converts it across metres, kilometres, miles, Earth–Moon distance, astronomical units (AU), light-years, parsecs and light-travel time — useful for astronomy, science fiction and classroom prep.

Enter a scale ratio (e.g. 1:25 000) and a measured map distance; the tool returns the real-world distance, or works in reverse from real distance to map distance — useful for hiking, map reading and town planning.

Two-way conversion between degrees–minutes–seconds (D° M′ S″) and decimal degrees — the standard need when juggling GPS coordinates, astronomy, surveying and marine navigation. Supports signed values for south latitude and west longitude.

Enter your head circumference (cm or inches); the tool maps it to US (fitted and fractional), UK, EU (cm) and Japanese hat sizes plus the S/M/L/XL range — useful for buying New Era caps, fedoras, baseball caps and helmet liners online.

Convert between kilowatt (kW), mechanical horsepower (HP), metric horsepower (PS / cheval-vapeur), brake horsepower (BHP), and foot-pound-force per second — the standard unit set for cars, engines, generators and power tools across UK / US / EU / JP markings.

Enter the luminous flux (lumens, lm) of a fixture and the lit area; the tool returns the illuminance (lux = lm/m²) and can solve for the lumens required to hit a target lux — used for room lighting design, photography, grow lights and workplace illuminance planning.

Enter wheel diameter (or circumference) and RPM (or speed); the tool applies C = π·d to return rolling distance per revolution, distance per minute and the matching road speed (km/h, mph) — useful for bikes, cars, conveyors, gears and physics class.

Enter image pixels (e.g. 24 MP or 6000 × 4000) and target print resolution (300 DPI photo, 240 DPI relaxed, 150 DPI poster); the tool returns the maximum print size in inches and cm, and works in reverse to find the megapixels needed for a desired print — a workhorse for photo printing, poster design and album layout.

Enter a track's BPM and the tool returns the millisecond delay time for every common note value — whole, half, quarter, eighth, sixteenth, thirty-second — plus dotted and triplet variants. Essential for producers setting echo, reverb pre-delay and sidechain timing in any DAW.

Enter video duration (minutes), video bitrate (Mbps) and audio bitrate (kbps); the tool computes the resulting file size in GB ⁄ MB and runs the inverse: given a target file size, what bitrate fits. Handy for YouTube uploads, streaming budgets and estimating SD-card / cloud-storage requirements.

Enter text or Morse code; the tool converts in either direction using the ITU-R M.1677-1 international Morse alphabet (letters, digits and standard punctuation), with conventional 3-space letter gaps and 7-space word gaps. Standard reference for amateur radio (HAM), scouting and emergency-comms training.

Enter wind speed in any unit (m/s, km/h, mph or knots); the tool maps it to Beaufort force 0–12 per the WMO standard, with the official sea-state and land-condition description. The international shorthand for sailors, hikers, aviators and outdoor sports.

Enter R-value (thermal resistance) or U-value (thermal transmittance); the tool flips them via U = 1 ⁄ R and converts between SI (m²·K/W) and US (ft²·°F·h/BTU) units — the foundational conversion for roof, wall and window insulation specifications.

Enter a paper weight in either GSM (metric grammage) or US-system pounds (bond, cover, text, index or tag) and the tool converts in both directions via the basis-weight ratio, plus shows typical use cases (copy paper, business cards, posters, magazine covers) — handy for print buying, design specs and comparing imported paper stock.

Slope can be expressed three ways: percent grade (rise⁄run × 100 %), degrees (arctan(rise⁄run)) or ratio (1:N). Enter any one and the tool instantly converts to the other two — useful for hiking, cycling, road signs, accessibility ramps and roof pitch.

Enter a blood glucose value; the tool converts between mg/dL and mmol/L — the two globally-used units (mg/dL in the US, mmol/L in Europe, China, Hong Kong and elsewhere). Essential when reading lab reports across borders.

Enter your HbA1c value; the tool converts between NGSP units (%) and IFCC units (mmol/mol) — both are used in clinical practice worldwide, and diabetes monitoring requires reading reports in the right unit.

Enter total cholesterol, LDL, HDL or triglycerides; the tool converts between mg/dL and mmol/L using the standard conversion factors (38.67 for cholesterol classes, 88.57 for triglycerides) — useful when reading US-style and European-style lipid panels.

Enter a gas concentration (ppm or mg/m³), molecular weight, temperature and pressure; the tool converts in both directions via the ideal-gas law — used in indoor air quality, occupational safety (OSHA / NIOSH PELs) and environmental monitoring.

Enter diopters (D) or focal length (mm / m / inch); the tool converts both ways via P = 1 / f(m) — used for eyeglass prescriptions, myopia / presbyopia correction, close-up filters and magnifier ratings.

Enter a duration in any unit (seconds, minutes, hours, days, weeks, months, years, decades, centuries, millennia) and the tool converts to every other unit simultaneously — using year = 365.25 days (IAU Julian) and month = 30.4375 days.

Enter the lamp wattage W and luminous efficacy (lm / W) — or known lumens and efficacy — and the tool uses lm = W × η to convert between input power and light output, with built-in reference efficacies for incandescent, halogen, CFL and LED bulbs — essential when picking the right LED brightness to replace an old wattage bulb.

Pick gender (men / women) and garment type (tops / bottoms), enter any region size; the tool instantly returns the matching US, UK, EU and JP sizes alongside the standard bust / chest / waist range — essential for online shopping, travel and overseas purchases.

Enter the karat (24K = pure), percent purity, or millesimal fineness (999 = pure); the tool simultaneously shows all three values and the international hallmark grade it corresponds to (e.g. 750, 585, 375) — useful for buying jewellery, scrap-gold valuation and customs checks.

Enter a frame rate (24 / 25 / 29.97 / 30 / 60 / 120 fps or custom) and clip duration; the tool reports frame duration (ms), total frames, and cross-references industry standards — film, PAL, NTSC, web, high-speed — useful for post-production, animation timing and game-engine work.

Enter image width, height, colour mode (grayscale / RGB / RGBA / CMYK) and bit depth (8 / 10 / 12 / 16 bit); the tool returns the uncompressed raw size and estimates for high-quality JPEG, standard JPEG, low-quality JPEG and lossless PNG.

Enter water hardness in any unit (gpg, ppm/mg-L CaCO₃, mmol/L, German degrees °dH, French °fH, English °eH); the tool converts to all other units at once and classifies the water as soft, moderately hard, hard or very hard.

Enter density in any unit; the tool converts to kg/m³, g/cm³ (= g/mL), lb/ft³, lb/US gal, lb/UK gal and kg/L at once — useful in engineering design, chemistry labs and shipping payload calculations.

Enter a volumetric flow rate in any unit (L/s, L/min, m³/h, US gpm, UK gpm, ft³/s (cfs), ft³/min (cfm), etc.); the tool converts to all the other common units at once — handy for pumps, HVAC, fire-protection and irrigation work.

Convert between effective-dose units (Sievert, Rem, mSv, µSv, mrem) and absorbed-dose units (Gray, Rad, mGy) using the standard 1 Sv = 100 Rem and 1 Gy = 100 Rad ratios — essential for radiation safety, medical imaging and nuclear-engineering work.

Convert between dynamic viscosity (Pa·s, Poise, cP, lbf·s/ft²) and kinematic viscosity (m²/s, Stokes, cSt) — essential for fluid mechanics, lubricants, chemical engineering and food processing.

Convert instantly between metres-per-second² (SI), feet-per-second² (imperial), g (standard gravity 9.80665 m/s²), gal (CGS, used in geophysics), km/h-per-second and mph-per-second — essential for physics class, vehicle testing, seismology and aerospace.

Convert refrigeration capacity between US tons of refrigeration (TR / RT), kilowatts (kW), BTU/h, kcal/h and horsepower — essential for HVAC sizing, chiller specs and data-centre heat-load estimation.

Enter a hardness value in Vickers (HV), Brinell (HB) or Rockwell C (HRC); the tool converts to the other two scales and the approximate ultimate tensile strength (UTS, MPa) using the ASTM E140 / ISO 18265 conversion table — essential for heat-treatment work, mechanical engineering and QA on steels.

Convert between farads (F), millifarads (mF), microfarads (μF), nanofarads (nF) and picofarads (pF) — instantly. The grid below shows every unit at once for quick cross-reference. Essential for circuit design, datasheet reading and decoding legacy schematics that still use "mfd" or "MFD".

Convert between common volumetric flow-rate units (L/s, L/min, m³/h, m³/s, US GPM, UK GPM, CFM, CFS, barrels/day) — essential for water treatment, HVAC, chemical engineering, irrigation and oil & gas.

Convert between color temperature (K) and mireds (M = 10⁶ / K), with reference values for common sources (candle, tungsten, daylight, overcast) — essential for photography filters, cinema lighting and smart-LED tuning.

Enter a value in any CSS length unit and the tool simultaneously shows the equivalents in px, rem, em, pt, pc, cm, mm and in — using the W3C-standard 16 px = 1 rem and 96 px = 1 in conversions, with a custom root-font-size knob for design-system and typography work.

Enter feet + inches (e.g. 5'10") or cm / m and the tool returns all three forms simultaneously — the one-to-one conversion between the US / UK foot-and-inch height format and metric cm / m, suited for medical records, visa forms, athlete profiles and apparel sizing.

Air-conditioners, heat pumps, heaters and saunas are usually rated in BTU/h, but engineering and electrical nameplates use W or kW. The tool converts both ways using the international 1 W = 3.412141633 BTU/h, and also shows the equivalent kW, mechanical horsepower (HP) and refrigeration tons (TR) — a standard HVAC-sizing, appliance-comparison and energy-audit tool.

Enter file size (GB / MB) and connection bandwidth (Mbps / Gbps); the tool returns the theoretical minimum download time (seconds / minutes / hours) and warns that TCP/IP overhead adds 5–15 % to real-world figures — the standard estimate before streaming uploads, cloud backups or Steam game pulls.

Enter your hand circumference (measure around the palm excluding the thumb); the tool returns equivalents in US numeric (6–11), EU numeric (same as palm inches), UK / Japan cm and the S/M/L/XL letter band — handy when ordering work, ski, motorcycle or fashion gloves from overseas.

Enter any value with its unit — wan (10⁴), yi (10⁸), lakh (10⁵), crore (10⁷), million, billion or trillion; the tool simultaneously outputs the other eight common magnitudes — built for reading Indian, Chinese, Japanese, Korean and Anglo-American financial news, cross-border prospectuses and annual reports.

Type a string or name; the tool instantly spells it out using the ICAO / NATO 1956 phonetic alphabet (Alfa, Bravo, Charlie…) — the global radio-spelling standard for phone calls, aviation, military and emergency services to avoid confusing letters that sound alike (B vs P, M vs N).

Enter plain text or a Base64 string; the tool encodes / decodes both directions per RFC 4648 (with UTF-8 encoding) — used for data: URLs, HTTP Basic Auth, JWTs, email attachments, SSH keys and other developer use cases. Shows input and output byte length.

Enter plain text or an encoded URL; the tool applies RFC 3986 percent-encoding both directions (space → %20, non-ASCII → UTF-8 byte sequence) — used for URL query strings, form submissions and API parameters. Choose between encodeURI and encodeURIComponent modes.

Enter a single character or codepoint (decimal, hex, or U+NNNN); the tool converts both directions and lists the Unicode codepoint, UTF-8 byte sequence and HTML numeric entities (&#dec; / &#xhex;) — useful for debugging character encoding, emoji, CJK text and control characters.

Paste a JSON Web Token (JWT); the tool splits header.payload.signature per RFC 7519, Base64URL-decodes the first two segments into readable JSON, surfaces common claims (alg, typ, iss, sub, aud, exp, iat, nbf) and converts Unix timestamps to local dates — signature verification is intentionally out of scope; this is a debugging tool.

Convert concentration units: 1 % = 10 000 ppm = 10⁷ ppb = 10¹⁰ ppt. Enter any one and the tool fills in the other three; for dilute water solutions (density ≈ 1 g/mL) it also shows the equivalent mg/L (≈ ppm), µg/L (≈ ppb) and ng/L (≈ ppt) — useful for lab chemistry, water testing, food-safety standards and air-quality reference.

Enter two colour temperatures in Kelvin; the tool converts them to Mireds via M = 10⁶ ÷ K and uses the difference to recommend the nearest standard CTO (orange, warming) / CTB (blue, cooling) gel strength (Full, 1/2, 1/4, 1/8) — the working unit for photo, video and mixed-source lighting.

Recipe calls for 3 Large eggs but your fridge has Medium or Jumbo? Enter the recipe size and count, get the equivalent count for every other size (Small / Medium / Large / Extra Large / Jumbo) based on USDA standard weights, with whole eggs + part-egg whites / yolks to top up.

Paste your SMS message — the tool auto-detects GSM-7 vs UCS-2 encoding and returns character count and number of SMS segments (160/153 for GSM-7, 70/67 for UCS-2) so you can estimate sending cost.

Enter an incandescent bulb wattage (e.g. 60 W); using Energy Star's lumens-per-watt standards, the tool returns the equivalent LED watts, CFL watts and total lumens — useful when swapping out old bulbs.

Enter fresh herb quantity (tsp / Tbsp / cup); using the industry 3 : 1 fresh-to-dried substitution rule, returns the dried equivalent. Reverses, includes 12 common herb defaults, and accepts a custom ratio.

Enter the recipe salt amount (tsp / tbsp / g / oz) and source type; the tool returns the equivalent volume in Diamond Crystal, Morton, table, sea and Maldon salt using Serious Eats / Cook's Illustrated bench-tested densities.

Enter a route or boulder grade in any one system; using the IFSC / UIAA / REI conversion table, simultaneously shows the equivalent in the other four common climbing grade systems.

Enter audio bitrate (kbps / Mbps) and duration (h / m / s); returns the resulting MP3 / AAC / WAV / FLAC file size in MB / GB, with common podcast and music encoding presets for quick reference.

Convert between seconds, minutes, hours and HH:MM:SS form — useful for video editing, sports timing and meeting durations.

Convert between m/s, ft/min, km/h, knots and ft/s — the five vertical-speed units used in aviation instruments, mountaineering and dive ascent-rate guidance.

Baking butter conversions: enter any one quantity (sticks, grams, tablespoons or cups) and the tool returns the equivalent in the other three units.

Enter syrup temperature (°C or °F) to instantly see the candy stage (thread / soft ball / firm ball / hard ball / soft crack / hard crack / light caramel / dark caramel), its full range and typical dessert use.

Enter the LED panel pixel pitch (P-value, e.g. P2.5 / P4) plus screen width × height — get the effective resolution, pixel density and the minimum / comfortable / visual-fusion viewing distances so you size the right panel for any AV install.

Enter the horizontal / vertical pixel count and the corresponding field of view (FOV) — get the pixels-per-degree (PPD), the industry-standard metric for comparing VR headsets, monitors and TVs against the retinal-resolution benchmark (~60 PPD).

Enter an aspect ratio (e.g. 16:9 or 16/9) — instantly get the padding-bottom percentage and matching CSS snippet (plus the modern aspect-ratio property), for responsive video / iframe containers.

Enter RGB (0–255) and instantly get the CMYK percentage values (K, C, M, Y), with matching hex code and printing-friendly tips on the screen-sRGB → CMYK gamut gap.