Free Universal Unit Converter: Length, Weight, Volume, Temperature & More
Convert any value across seven categories — length, weight, volume, temperature, area, speed, time — with NIST-exact conversion factors and an all-units snapshot for every input. 100% client-side.
Seven Categories
Length, weight, volume, temperature, area, speed, and time. Switch with one tab — no dropdown digging, no separate calculator per category.
NIST-Exact Factors
We use the exact internationally-agreed multipliers (1 mile = 1609.344 m, NOT 1609 or 1600). Temperature uses true offset transforms, not linear approximations.
All-Units Snapshot
Every input shows the result in EVERY other unit of that category in one grid — perfect for cross-referencing without re-typing for each target unit.
100% Client-Side
Conversion math runs in your browser. Measurements stay on your device — useful for cooking recipes, lab data, and any value you do not want logged.
Free unit converter for length, weight, temperature & more
This unit converter changes a value from one unit to another across seven categories: length, weight/mass, volume, temperature, area, speed, and time. Type a number, pick a from and to unit, and see the result plus an all-units snapshot for the whole category. It uses exact NIST factors (1 mile = 1609.344 m, not 1609) and proper temperature offset formulas. It runs 100% in your browser, free, with no upload.
Pair it with the Percentage Calculator for tax and discount math, the Age Calculator for calendar-aware date math, the Number Base Converter for binary, octal, and hex, or browse the full Toolk tools hub.
How to convert units
- Pick a category tab — Length, Weight, Volume, Temperature, Area, Speed, or Time.
- Type the value you want to convert into the input field. The result updates instantly as you type.
- Choose the from unit and the to unit from the two dropdowns.
- Use the swap button to flip the from/to pair (e.g. turn cm → in into in → cm) without re-typing.
- Scroll the all-units snapshot to read your value in every other unit of that category at once.
- Tap a quick-preset chip (cm → in, kg → lb, °C → °F…) to jump straight to the most-searched conversions.
How unit conversion actually works
Most units convert with a single multiplier. Each unit declares a factor to the category's base unit (the SI unit where one exists), and conversion is simply (value × fromFactor) ÷ toFactor. For example, the metre is the length base, an inch is 0.0254 m and a foot is 0.3048 m, so 12 inches × 0.0254 ÷ 0.3048 = 1 foot exactly. These factors are not guesses: the 1959 international yard-and-pound agreement fixed 1 yard = 0.9144 m exactly, which makes 1 inch = 2.54 cm and 1 mile = 1609.344 m by definition, as documented by NIST's unit-conversion reference.
Temperature is the exception. Celsius, Fahrenheit, and Kelvin scales have different zero points, so a single multiplier cannot convert them — you need an offset plus a scale. Per NIST's SI temperature definitions, °F = °C × 9/5 + 32 and K = °C + 273.15. This converter runs every temperature through Celsius as a pivot, then applies the correct offset formula — never a length-style factor.
"An interval of one degree Celsius corresponds to an interval of 1.8 degrees Fahrenheit… zero degrees Celsius is 273.15 K."— NIST, SI Units: Temperature
Worked examples: input → output
Length · 10 cm → inches
10 × 0.01 ÷ 0.0254 = 3.93701 in
Weight · 1 kg → pounds
1 ÷ 0.45359237 = 2.20462 lb
Temperature · 0 °C → Fahrenheit
0 × 9/5 + 32 = 32 °F
Edge case · temperature is not a simple factor
A factor-only tool treats 0 °C as 0 × (9/5) = 0 °F — wrong by 32 degrees. The offset matters: the correct answer is 32 °F. The same trap hits −40, the one point where the scales meet (−40 °C = −40 °F). For a temperature difference, drop the offset and use only the scale — a 10 °C change equals an 18 °F change, not 50 °F.
The Seven Categories & Their Base Units
| Category | SI Base Unit | Units We Support |
|---|---|---|
| Length | metre (m) | mm, cm, m, km, in, ft, yd, mi, nautical mile |
| Weight / Mass | kilogram (kg) | mg, g, kg, t, oz, lb, stone, US ton |
| Volume | litre (L) | mL, L, m³, tsp, tbsp, fl oz, cup, pt, qt, gal (US/UK) |
| Temperature | Celsius (°C) | °C, °F, K (offset transforms) |
| Area | square metre (m²) | mm², cm², m², ha, km², in², ft², yd², acre, mi² |
| Speed | m/s | m/s, km/h, mph, ft/s, knot, Mach |
| Time | second (s) | ms, s, min, h, day, week, month (avg), year (avg) |
Where Unit Conversion Has Famously Gone Wrong
Mars Climate Orbiter (1999)
$327M spacecraft crashed into Mars because one team used pound-force-seconds and another used newton-seconds. Different units, same number, no conversion done. NASA report: "the root cause was the failed translation of English units to metric."
Gimli Glider (1983)
Air Canada Boeing 767 ran out of fuel at 41,000 ft because the fueller used pounds where the crew expected kilograms. Pilot glided to an unused runway. No fatalities, but a perfect "always specify your units" case study.
Recipe Disasters
A US recipe calling for "1 cup" in the UK is 16.5% smaller than expected (US cup = 237 mL, UK cup = 284 mL). Enough to ruin batter ratios on the first attempt.
Temperature Linear-Approximation Bugs
Some calculators forget the +32 offset for °C-to-°F conversion, getting answers ~32° too low. We always apply the offset transform.
Conversions Worth Memorising
| If you know | Multiply by | To get |
|---|---|---|
| Inches | 2.54 | Centimetres |
| Feet | 0.3048 | Metres |
| Miles | 1.609 | Kilometres |
| Pounds | 0.4536 | Kilograms |
| Kilograms | 2.2046 | Pounds |
| US gallons | 3.785 | Litres |
| Square feet | 0.0929 | Square metres |
| Acres | 4047 | Square metres |
| mph | 1.609 | km/h |
| Knots | 1.852 | km/h |
Notice that mph → km/h is the same multiplier as miles → km (1.609) — because dividing both sides by "per hour" cancels the unit. That symmetry is a useful sanity check when you need to convert quickly without a calculator.
The precision and rounding rule most converters hide
The displayed result is rounded to 6 significant digits with trailing zeros trimmed (toPrecision(6)), so 1 kg shows as 2.20462 lb, not 2.2046226218.... For values smaller than 0.0001 or larger than 1,000,000,000, the output switches to scientific notation like 1.2345e-7 so tiny and huge results stay readable instead of collapsing to 0 or a wall of digits. The underlying math keeps full IEEE-754 double precision — only the display is rounded.
Two factors are easy to get wrong elsewhere. US and UK gallons differ: 1 US gallon = 3.785411784 L, but 1 UK (imperial) gallon = 4.54609 L — about 20% larger — so this tool exposes both as separate units. And "month" and "year" are averages: a month is the Gregorian mean of 30.4375 days (2,629,746 s) and a year is 365.2425 days (31,556,952 s). For real calendar boundaries (leap years, a Feb 29 birthday, adding a month to Jan 31), use the Age Calculator or Date Difference Calculator instead.
Runs 100% in your browser
Your data never leaves your device. Every conversion is computed locally in JavaScript — no uploads, nothing leaves your device. That matters for cooking recipes, lab measurements, or salary figures you would rather not have logged. I tested all seven categories against known reference values: 1 mile = 1609.344 m, 1 kg = 2.20462 lb, 0 °C = 32 °F, −40 °C = −40 °F, 1 Mach = 343 m/s at sea level, and confirmed the scientific-notation cutoffs at the 0.0001 and 1e9 boundaries. The output stays instant on every keystroke.
Frequently asked questions
Is this unit converter free?
Yes — 100% free with no signup and no usage limit. Every category and conversion is fully available.
Is my input sent to a server?
No. The conversion math runs entirely in your browser. No values you enter are uploaded, so the tool works offline once the page loads.
Why use 1609.344 instead of 1609 for miles?
Because 1609.344 m is the exact internationally-agreed value (1 mile = 1760 yards × 0.9144 m). The rounded 1609 introduces about 0.02% error per mile, which compounds in scientific and engineering work.
Why is temperature handled differently?
Temperature scales have offsets, not just multipliers. Celsius to Fahrenheit is °F = °C × 9/5 + 32, and Kelvin is K = °C + 273.15. A factor-only approach gives nonsense like 0 °C = 0 °F instead of the correct 32 °F.
Can I convert between two categories, like length and volume?
No — they measure different physical quantities, so a metre is not equal to any number of litres. Units are interchangeable only within a category (1 m = 100 cm = 3.28 ft). Crossing dimensions needs a shape formula, e.g. cylinder volume = π × r² × h.
What is the difference between US and UK gallons?
Same name, two units. 1 US gallon = 3.785411784 L; 1 UK (imperial) gallon = 4.54609 L, about 20% larger. This converter lists both separately so a UK recipe's "1 gallon" is not silently under-measured.
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Last updated: June 2, 2026 · Runs 100% in your browser — no uploads, nothing leaves your device.
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