📏 Unit Converter
A unit converter across 12 categories and 90+ units that shows the factor used — and whether it is exact by definition or merely conventional.
Common conversions
1 cm = 0.3937007874 in — exact by definition.
- Defined as exactly 2.54 cm since 1959
What Unit Converter Does
Most unit conversions are just multiplication, and the interesting question is which number to multiply by. A surprising share of the common ones are exact by definition rather than measured: an inch is precisely 2.54 centimeters, a pound is precisely 0.45359237 kilograms, a US gallon is precisely 231 cubic inches. Those are not approximations that got very good — they are definitions, agreed in 1959, carrying no uncertainty whatsoever.
Others are not exact and never can be. Degrees to radians runs through π, which is irrational. Inches of mercury depend on a conventional fluid density and a conventional value for gravity. The distinction matters when you are chaining conversions or reporting significant figures, and no other converter tells you which kind you are looking at. This one says so on every result.
The other thing worth knowing is where the same word means two different quantities. A US gallon and an imperial gallon differ by about 20%. A "gigabyte" from a drive manufacturer and a "GB" from your operating system differ by about 7%, which is why a 1 TB disk shows up as 931 GB and nothing is actually missing. Mechanical and metric horsepower differ by 1.4%.
And temperature is not multiplication at all. Celsius and Fahrenheit disagree about where zero sits, so converting between them needs an offset as well as a scale factor — which is why they cross at exactly −40, and why this converter routes every temperature through kelvin rather than pattern-matching pairs.
How to Use Unit Converter
- Pick a category — or tap one of the common conversions to jump straight there
- Type the value; the result updates as you type, with no button to press
- Use the swap arrows to reverse the direction without re-picking both units
- Read the conversion factor under the result, including whether it is exact by definition
Formula Used by Unit Converter
The general conversion
result = value × (factor of source unit) ÷ (factor of target unit)
- factor
- How many base units one of this unit is worth — meters, kilograms, liters and so on
- Why a base
- Routing through one base unit per category means n units need n factors, not n² pairs
Worked example
Converting 5 miles to kilometers
- Mile factor: 1609.344 m exactly
- Kilometer factor: 1000 m
- 5 × 1609.344 ÷ 1000
Result: 8.04672 km — exact, because both factors are definitions
Temperature needs an offset too
°F = °C × 9/5 + 32 · K = °C + 273.15 · °R = K × 9/5
- Offset
- The scales disagree about zero, so a scale factor alone is not enough
- Absolute scales
- Kelvin and Rankine start at absolute zero, so between those two it IS pure multiplication
Worked example
The point where Celsius and Fahrenheit agree
- Set x = x × 9/5 + 32
- x − 1.8x = 32
- −0.8x = 32
Result: x = −40. The two scales read the same only at −40°
The exact definitions everything else follows from
1 in = 0.0254 m · 1 lb = 0.45359237 kg · 1 US gal = 231 in³
- 1959
- The international yard and pound agreement, adopted by the US, UK and Commonwealth
- Derived
- Foot, yard, mile, ounce, ton, quart, cup and fluid ounce all follow exactly from these
Worked example
Deriving the mile from the inch
- 1 ft = 12 in = 0.3048 m
- 1 yd = 3 ft = 0.9144 m
- 1 mi = 5280 ft
Result: 1,609.344 m exactly — no rounding anywhere in the chain
Decimal against binary prefixes
1 GB = 10⁹ bytes but 1 GiB = 2³⁰ = 1,073,741,824 bytes
- GB
- Decimal, as used by drive manufacturers and networking
- GiB
- Binary, and what most operating systems mean when they print "GB"
Worked example
A 1 TB drive as your computer reports it
- 1 TB = 10¹² bytes
- 1 GiB = 1,073,741,824 bytes
- 10¹² ÷ 1,073,741,824
Result: 931.32 — the drive is not short, the two are counting differently
Exact by definition — no uncertainty at all
These are agreed definitions, not measurements. Any converter disagreeing with these in the eighth decimal place is using a rounded factor.
| Unit | Exactly equals | Since |
|---|---|---|
| 1 inch | 0.0254 m | 1959 international agreement |
| 1 foot | 0.3048 m | Derived: 12 inches |
| 1 yard | 0.9144 m | Derived: 3 feet |
| 1 mile | 1,609.344 m | Derived: 5,280 feet |
| 1 nautical mile | 1,852 m | Defined directly |
| 1 pound | 0.45359237 kg | 1959 international agreement |
| 1 ounce | 0.028349523125 kg | Derived: 1/16 pound |
| 1 US gallon | 3.785411784 L | Defined as 231 cubic inches |
| 1 acre | 4,046.8564224 m² | Derived: 4,840 square yards |
| 1 atmosphere | 101,325 Pa | Defined directly |
| 1 calorie (thermochemical) | 4.184 J | Defined directly |
| speed of light | 299,792,458 m/s | It defines the meter |
Source: NIST Special Publication 811, Appendix B — Conversion Factors
Same name, different quantity
Each of these pairs shares a word and does not share a value. Getting the wrong one is the most common real-world conversion error.
| Pair | Difference | Where it bites |
|---|---|---|
| US gallon vs imperial gallon | 3.785 L vs 4.546 L — 20.1% apart | Recipes, fuel economy, UK vs US measures |
| GB (10⁹) vs GiB (2³⁰) | 7.4% apart | A 1 TB drive reports as 931 GB |
| Mechanical hp vs metric PS | 745.70 W vs 735.50 W — 1.4% apart | Car specs quoted in different markets |
| Short ton vs long ton vs tonne | 907.18 vs 1,016.05 vs 1,000 kg | Freight and commodity weights |
| Julian year vs calendar year | 365.25 vs 365.2425 days | Astronomy against civil timekeeping |
| International vs US survey acre | differ in the 6th significant figure | Land surveying and legal descriptions |
The conversions people actually search for
Each value here is exact, because both units in every pair are defined rather than measured. These are one tap on the converter above.
| Conversion | Factor | Reverse |
|---|---|---|
| cm → inches | 0.393700787402 | 1 in = 2.54 cm |
| kg → pounds | 2.204622621849 | 1 lb = 0.45359237 kg |
| km → miles | 0.621371192237 | 1 mi = 1.609344 km |
| meters → feet | 3.280839895013 | 1 ft = 0.3048 m |
| liters → US gallons | 0.264172052358 | 1 gal = 3.785411784 L |
| mph → km/h | 1.609344 | 1 km/h = 0.621371192237 mph |
| knots → km/h | 1.852 | 1 km/h = 0.539956803456 kn |
| °C → °F | not a factor — × 9/5 then + 32 | °C = (°F − 32) × 5/9 |
How to Read Your Result
Exact and approximate are genuinely different
When both units are defined, the conversion carries no uncertainty and you can keep as many digits as you like — 1 mile is 1,609.344 meters, full stop. When one is conventional or irrational, extra digits are false precision. This matters most when chaining: converting inches to centimeters to meters loses nothing, but running a measurement through degrees and radians and back does. A converter that hands you eight decimal places without saying which kind you have is inviting you to over-report.
Your hard drive is not lying to you
Manufacturers sell in decimal units, where a terabyte is 10¹² bytes. Windows and most operating systems display in binary units but label them with decimal prefixes, so what they call "931 GB" is really 931 gibibytes. Both numbers describe the same disk. The IEC introduced KiB, MiB and GiB in 1998 precisely to end this ambiguity, and the industry has adopted them unevenly ever since. macOS switched to decimal reporting in 2009; Windows did not.
Why temperature is the odd one out
Every other category in this converter is a ratio scale: zero means none of the quantity, so conversion is pure multiplication. Celsius and Fahrenheit are interval scales — their zeros are arbitrary points, one the freezing point of water and the other a brine mixture — so they need an offset as well. This is also why it is meaningful to say 20 kg is twice 10 kg but wrong to say 20 °C is twice as hot as 10 °C. In kelvin, where zero is absolute, that ratio statement becomes valid again.
The 1959 agreement quietly fixed a real problem
Before it, the US and UK inches differed slightly, and precision engineering across the Atlantic had to account for it. The agreement defined the inch as exactly 2.54 cm — pinning the imperial system to the metric one rather than the other way round. Every imperial length unit in use today is therefore formally a metric derivative. The one exception is the US survey foot, retained for legacy land records and differing by about two parts per million, which is why the international and US survey acre are not quite the same.
Where the top results put their effort
The most popular converters are not winning on interface — two of them still use unstyled multi-select list boxes, and the best-looking of them is not the most popular. They win on coverage and structure: dozens of categories, hundreds of units, and a separate indexed page for each common conversion. Coverage is the real bar to clear, which is why this tool went from 4 categories to 12 and from 23 units to 95.
Significant figures, not decimal places
Fixing the number of decimal places makes small results useless and large results unreadable — a meter in kilometers becomes "0.001000" while a light year in meters overflows into noise. Formatting to significant figures instead keeps the same amount of real information at every magnitude, and falls back to scientific notation only when a plain decimal genuinely cannot be read. Your input, not the arithmetic, is almost always what limits how many of those digits mean anything.
Limitations & Accuracy Notes
- Currency is not included. Exchange rates change continuously and belong to a live data source, not a static factor table.
- US customary and imperial units share names but not always values. Volume units here are US unless labeled imperial; dry measures differ again from liquid measures.
- The international acre and foot are used, not the US survey versions retained for some land records. The two differ by roughly two parts per million.
- Results are computed in double precision, so a very long chain of conversions can accumulate error in the final digits even where every individual factor is exact.
- Fuel economy is not a straight conversion: miles per gallon and liters per 100 km are reciprocals of one another, so they cannot be handled by the same multiply-and-divide as everything else.
- Displayed digits are not a claim about your measurement. A conversion cannot add precision that the input never had.
Frequently Asked Questions
Is 1 inch exactly 2.54 cm?
Why does my 1 TB drive show as 931 GB?
Why can't temperature be converted by multiplying?
At what temperature do Celsius and Fahrenheit read the same?
Is a US gallon the same as an imperial gallon?
Which conversions are not exact?
References & Further Reading
- NIST Special Publication 811 — Guide for the Use of the International System of Units — The US National Institute of Standards and Technology reference for SI conversion factors; Appendix B is the source for the exact values used throughout this tool
- BIPM — The International System of Units (SI Brochure) — The defining document for the SI, including the 2019 redefinition that fixed the electronvolt and the speed of light exactly
- NIST — International and US survey foot — Why two slightly different feet existed in the United States, and the basis for the international/survey acre distinction noted above
- unitconverters.net — The #1 result for this term, and the widest competing coverage at roughly 80 category converters — the benchmark this tool was expanded to approach