Unit Converter
Convert between units of length, weight, temperature, volume, area, and speed.
Last reviewed: April 2026| Unit | Value |
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Quick Answer
A unit converter changes a measurement from one unit to another using fixed conversion factors — for example, 1 mile = 1.609 kilometers, 1 kilogram = 2.205 pounds, and 1 liter = 0.264 gallons. Temperature uses a formula rather than a factor (°F = °C × 9/5 + 32). Choose a category and units above and enter a value to convert instantly.
This unit converter handles the ten measurement categories you actually encounter in daily work: length, mass, volume, area, temperature, energy, pressure, speed, time, and digital storage. It covers SI (metric), US customary, and imperial units, and applies the exact internationally agreed conversion factors set by NIST and BIPM. The sections below explain how each conversion is derived, where the customary systems diverge, and why the 2019 SI redefinition matters even if you never touch a laboratory.
What This Calculator Does
The unit converter performs single-step conversions across ten categories: length (meter, kilometer, centimeter, millimeter, micrometer, mile, yard, foot, inch, nautical mile), mass (kilogram, gram, milligram, metric ton, pound, ounce, stone, short ton), volume (liter, milliliter, US gallon, quart, pint, cup, fluid ounce, tablespoon, teaspoon, cubic meter, cubic foot), area (square meter, square kilometer, hectare, acre, square foot, square yard, square inch, square mile), temperature (Celsius, Fahrenheit, Kelvin), speed (m/s, km/h, mph, knots, ft/s), digital storage (byte, kilobyte, megabyte, gigabyte, terabyte, petabyte, bit, kilobit, megabit), and time (milliseconds through years).
For every non-temperature category, the converter uses a base-unit factor table. Each unit is recorded as its size in a canonical base — meters for length, kilograms for mass, liters for volume, square meters for area — and any conversion becomes a single multiplication: output = input × (factor_from / factor_to). Temperature is the one exception, because Celsius and Fahrenheit are offset scales rather than purely multiplicative ones, so the calculator routes those conversions through a dedicated function that handles the +32 / −32 offsets and the 9/5 ratio explicitly.
This separation matches how NIST and BIPM publish their conversion standards: a master table of multiplicative factors for additive units, plus explicit affine formulas for the three temperature scales. Because all internal arithmetic is done in IEEE 754 double-precision floating point, the calculator is accurate to roughly fifteen significant digits — far more precision than any real-world physical measurement actually requires.
The output also includes a full reference table showing the input value expressed in every unit of the selected category at once. This is useful when you need to cross-check a result or share a single conversion as a complete picture — for example, converting 65 mph to every other speed unit simultaneously so a reader can pick whichever they understand best.
How to Use It
Start by choosing a Category from the dropdown — length, mass, temperature, volume, area, speed, digital storage, or time. The From and To unit selectors automatically repopulate with units valid for that category, so you cannot accidentally try to convert kilograms into kilometers. Enter the numeric value you want to convert in the Value field; the result updates live as you type, with no need for a "Calculate" button.
The ⇄ swap button between the From and To selectors flips the direction of the conversion instantly — handy when you realize you entered the units backwards, or when you want to convert in both directions during the same session. Below the result, the quick reference table shows your input value translated into every unit in the current category at once, with your chosen target unit highlighted. The Export CSV button below the table saves that full reference as a spreadsheet-ready file, which is useful for engineering documentation or recipe scaling.
The calculator state — category, from-unit, to-unit, and value — is encoded into the page URL each time you change an input. That means the Copy button in the Share section produces a link that reproduces the exact conversion when opened on another device. You can bookmark common conversions ("65 mph to km/h" or "175 lb to kg") and revisit them with a single click. The Reset and Print buttons restore defaults and produce a clean print layout respectively.
For accessibility, every input is keyboard-navigable: Tab cycles through the controls in logical reading order, and the result region carries aria-live="polite" so screen readers announce updates without interrupting the user. Dark mode toggles via the moon icon in the header and persists across visits via localStorage.
Worked Example: 65 mph in Every Common Speed Unit, and 175 lb to Kilograms
Step 1 — Establish the Canonical Conversion Factors
The international yard and pound agreement of July 1, 1959, fixes 1 mile = 1,609.344 meters exactly, and 1 pound = 0.45359237 kilograms exactly. These are not approximations — they are legal definitions adopted by the US, UK, Canada, Australia, New Zealand, and South Africa. One hour is exactly 3600 seconds by SI definition, and one nautical mile is exactly 1852 meters under the 1929 International Hydrographic Conference standard.
Step 2 — Convert 65 mph to Meters per Second
Convert miles to meters and hours to seconds in a single step:
65 mph × (1609.344 m / 1 mi) × (1 hr / 3600 s)
= 65 × 1609.344 ÷ 3600
= 104,607.36 ÷ 3600
= 29.058 m/s ≈ 29.06 m/sStep 3 — Convert 65 mph to Kilometers per Hour
Since 1 mile = 1.609344 km exactly:
65 mph × 1.609344 = 104.607 km/h ≈ 104.6 km/hStep 4 — Convert 65 mph to Knots
One knot is exactly one nautical mile per hour, so:
65 mph × (1609.344 m / 1852 m) = 65 × 0.868976 = 56.484 kn ≈ 56.5 knotsStep 5 — Convert 175 lb to Kilograms
Using the exact 1959 international pound definition:
175 lb × 0.45359237 kg/lb = 79.3787 kg ≈ 79.4 kgThe calculator carries the full 15-digit precision internally and rounds for display only — so the same 175 lb input converts equally cleanly to 79,378.7 grams, 12.5 stone, or 0.0794 metric tons.
Why the Same Number Has Three Different Speed Readings
104.6 km/h, 29.06 m/s, and 56.5 kn all describe identical physical motion — the same car moving at the same speed. The numbers differ because each unit measures distance over time using different "yardsticks." Aviation and maritime navigation prefer knots because charts are scaled in nautical miles (which correspond to 1 arc-minute of latitude). Scientists prefer m/s because the meter and second are SI base units and most physics equations are dimensionally cleanest in SI. Highway signs use mph in the US and km/h almost everywhere else because that is what drivers were taught.
Figure: The seven SI base units (blue circles) combine to produce the named derived units (yellow boxes). For example, force in newtons = kg·m/s² combines mass, length, and time; pressure in pascals = N/m² adds an area term. All physical quantities in science reduce to combinations of these seven.
Common Use Cases
International Scientific Collaboration
Every peer-reviewed scientific journal in the world publishes in SI. When an American research team submits a paper alongside European or Asian collaborators, every measurement — sample mass, reaction temperature, beam wavelength, voltage, pressure — must be reported in SI. A unit converter is essential for taking lab-bench notes recorded in customary units and translating them cleanly for publication.
US Travel to Metric Countries
An American driver in Germany sees road signs in km/h, fuel pumps in liters, and weather forecasts in Celsius. Mental conversion takes practice; a converter on a phone makes the trip far less stressful. Speed limits of 130 km/h on the autobahn convert to 80.8 mph; a 50-liter fuel tank holds 13.2 US gallons; a 25 °C summer day is 77 °F.
Recipe Conversion
European baking recipes report flour and butter by mass (grams), while American recipes use volume (cups, tablespoons). Converting 250 g of flour to cups requires knowing flour density (roughly 125 g per US cup of all-purpose), so the conversion is approximate. Liquid ingredients are simpler: 250 mL = 1.057 US cups. Always weigh dry ingredients when precision matters — King Arthur Baking's reference table publishes per-ingredient gram-to-cup conversions for exactly this reason.
Engineering Specifications
US engineering drawings often specify dimensions in inches and tolerances in thousandths of an inch ("mils"), while parts manufactured in Asia or Europe arrive with metric drawings. Converting a 0.250-inch shaft diameter to 6.35 mm and matching it to ISO standard bearing sizes is daily work for mechanical engineers. Pressure-vessel ratings reported in psi must be converted to pascals or bar for international compliance documentation.
Fuel Economy: MPG vs L/100km
US fuel economy is reported in miles per gallon (higher is better); the rest of the world uses liters per 100 km (lower is better). The conversion is not linear: 1 / MPG × 235.215 = L/100km (using US gallons). A 30 MPG car consumes 7.84 L/100km; a 40 MPG car uses 5.88 L/100km. Note that UK MPG figures use imperial gallons, so a "40 MPG" Range Rover in a UK magazine equals only about 33 MPG to an American.
Kitchen Cooking
Oven temperatures shift between Celsius (most cookbooks worldwide), Fahrenheit (US), and Gas Mark (UK). 180 °C = 356 °F = Gas Mark 4 — a standard moderate-oven baking temperature. Cups, tablespoons, and teaspoons differ subtly across countries: a US tablespoon is 14.79 mL, a UK tablespoon is 17.76 mL, and an Australian tablespoon is exactly 20 mL by national standard. For dessert and pastry work where ratios matter, these small differences compound.
Edge Cases
Temperature Offset (Fahrenheit to Celsius Is Not Multiplicative)
Most unit conversions are simple multiplication: 100 cm = 100 × 0.01 m = 1 m. Temperature is different because Celsius and Fahrenheit have different zero points. Water freezes at 0 °C but 32 °F. To convert F to C, you must subtract 32 first, then multiply by 5/9: (98.6 − 32) × 5/9 = 37.0 °C. To convert C to F, multiply by 9/5 first, then add 32: (20 × 9/5) + 32 = 68 °F. Kelvin shares Celsius's degree size but starts at absolute zero (−273.15 °C), so K = C + 273.15. A "temperature difference" of 5 °C, however, equals a difference of 9 °F — for differences only (not absolute temperatures), the offset cancels out.
US Customary vs Imperial Inconsistencies
"Imperial" and "US customary" are not the same system, even though both descend from English units. A US gallon is 3.785 liters; a UK imperial gallon is 4.546 liters — about 20% larger. A US fluid ounce is 29.5735 mL; a UK fluid ounce is 28.4131 mL — counterintuitively smaller, even though the UK gallon is larger, because a US gallon contains 128 fl oz while a UK gallon contains 160 fl oz. UK pints and quarts likewise differ from US pints and quarts. This calculator uses US definitions for volume by default.
Area Is Squared (1 km² ≠ 1000 m²)
Linear conversions cannot be applied directly to area or volume. Since 1 km = 1000 m, then 1 km² = 1000² = 1,000,000 m², and 1 km³ = 1000³ = 1,000,000,000 m³. The same rule applies to feet and inches: 1 ft² = 12² = 144 in², not 12. A common construction-estimating error is to convert a linear measurement to metric, then square it after — you must either square first or multiply by the squared factor, never both partially.
Troy Ounce vs Avoirdupois Ounce
Precious metals (gold, silver, platinum) are weighed in troy ounces (31.1035 g), while groceries, postal mail, and almost everything else use the avoirdupois ounce (28.3495 g). A "1 ounce" gold coin is about 10% heavier than a "1 ounce" of cheese at the deli counter. The troy system also has its own pound (12 troy ounces = 373.24 g), which is smaller than the standard avoirdupois pound of 16 ounces = 453.59 g. Always check which "ounce" is meant when calculating bullion value, pharmaceutical doses (which historically used apothecary units), or jewelry weight.
Pound-Force vs Pound-Mass
In US engineering, "pound" can mean either a unit of force (lbf, the weight of one avoirdupois pound at standard gravity = 4.4482 newtons) or a unit of mass (lbm = exactly 0.45359237 kg). Casual usage conflates them, but Newton's second law F = ma requires consistent units. The US engineering convention uses the slug as the mass unit that pairs with lbf in F = ma: 1 slug ≈ 14.59 kg. This calculator's "pound" treats the value as mass (lbm). When working with forces, torques, or pressures, double-check which is meant — engineering drawings should specify lbf, lbm, or psi to remove ambiguity.
Behind the Science
The 2019 SI Redefinition
On May 20, 2019 — World Metrology Day — the General Conference on Weights and Measures redefined all seven SI base units in terms of fixed numerical values of seven fundamental physical constants. For over a century the kilogram had been defined by the mass of a platinum-iridium cylinder stored in Sèvres, France ("Le Grand K"), but periodic intercomparisons with national copies showed drift of roughly 50 micrograms over a century — a small amount in absolute terms, but unacceptable for precision metrology.
The 2019 redefinition fixed Planck's constant h at exactly 6.62607015 × 10⁻³⁴ J·s. The kilogram is now whatever mass makes that number exact, realizable in any laboratory through a Kibble balance (which equates mechanical and electromagnetic force). The second is fixed by the cesium-133 hyperfine transition at exactly 9,192,631,770 Hz. The meter is fixed by the speed of light at exactly 299,792,458 m/s — meaning the meter is now defined by the second, not measured against any physical bar.
Metric System History: French Revolution to BIPM
The metric system was officially adopted by France on December 10, 1799. The French Academy of Sciences had spent the previous decade rationalizing the chaotic feudal weights and measures into a single decimal system based on natural references — the meter was defined as one ten-millionth of the distance from the equator to the North Pole through Paris (measured by Delambre and Méchain on a seven-year survey), and the kilogram as the mass of one liter of water at 4 °C.
The Convention of the Metre, signed in 1875 by seventeen nations including the United States, established the Bureau International des Poids et Mesures (BIPM) and committed signatories to maintain a coherent system of weights and measures. Physical prototypes — the International Prototype Metre and Kilogram, both made of platinum-iridium alloy — were manufactured in 1889 and stored at Sèvres. Each signatory nation received a national copy. The 2019 redefinition retired those artifacts entirely, replacing them with constant-based definitions that need no physical reference at all.
Why the US Holdouts: Mendenhall Order 1893
The United States never formally adopted the metric system for public commerce — gas pumps still display gallons, road signs still display miles, grocery scales still display pounds. But on April 5, 1893, Thomas Mendenhall, Superintendent of the US Coast and Geodetic Survey, issued the Mendenhall Order, which secretly defined US customary units in terms of metric prototypes. The yard became exactly 3600/3937 meter (later refined in 1959 to exactly 0.9144 m). The pound became exactly 0.45359237 kg.
From that date forward, every "inch" in an American engineering drawing has been, by federal definition, a metric quantity. The Metric Conversion Act of 1975 declared the metric system the "preferred system of weights and measures for United States trade and commerce," but adoption remained voluntary. NASA, NIST, DARPA, the FDA, and every published scientific paper use SI exclusively. The 1999 loss of NASA's Mars Climate Orbiter — caused by a unit mismatch between pound-force-seconds and newton-seconds in two pieces of contractor software — remains the canonical cautionary tale about why mixed-unit work is dangerous, and why every working engineer should be fluent in both systems.
History
The imperial system was codified by the British Weights and Measures Act of 1824, replacing centuries of regional English and Scottish variants. The imperial gallon was originally defined as the volume of 10 pounds of distilled water at 62 °F, yielding approximately 4.546 liters. That definition was refined by the UK Weights and Measures Act of 1985, which fixed the imperial gallon at exactly 4.54609 liters — a metric definition for an imperial unit, mirroring what the Mendenhall Order had done in the US in 1893.
US customary units still trace their formal definitions to the Mendenhall Order of 1893 and its 1959 refinement under the international yard and pound agreement. The US fluid ounce, US gallon, and US pint diverged from their British counterparts in the early 1800s when the US chose to keep the old wine-gallon and Queen Anne gallon definitions rather than adopt the new 1824 imperial standard. As a result, the US is the only industrialized country in the world using gallons of exactly 3.785 liters — making US-to-UK fuel-economy and recipe-volume conversions a perennial source of errors for travelers and writers crossing the Atlantic.
The metric system itself has been refined seven times since 1889: the 1889 prototype era, the 1960 establishment of SI proper, the 1967 cesium-based redefinition of the second, the 1983 redefinition of the meter via the speed of light, and finally the 2019 constant-based redefinition of all seven base units. Each refinement increased precision without changing any practical conversion factor — a quart of milk still pours the same way regardless of which definition of the liter applied when the carton was filled.
Limitations
Floating-point arithmetic introduces rounding error at the 15th significant digit. For nearly every practical purpose this is invisible — converting 65 mph to km/h to mph and back yields exactly 65 again. But when chaining many conversions ("inches to mm to meters to feet to inches"), errors of one part in 10¹⁵ can compound to one part in 10¹³ over a dozen operations. Always carry conversions through to the final unit in a single step where possible, rather than performing intermediate conversions.
Significant figures matter. Converting "175 lb" to "79.3787 kg" reports more precision than the input justified — if the original measurement was a bathroom-scale reading good to ±1 lb, then the converted value is realistically 79.4 ± 0.5 kg, and reporting it as 79.3787 is misleading. The calculator displays results to 8 significant figures by default; trim that back to match the precision of your input when documenting results for publication, legal records, or regulatory submission. NIST Special Publication 811 has explicit guidance on appropriate rounding for converted values.
Currency conversion is not supported by this tool. Currency rates change continuously based on foreign exchange markets and require a live data feed; static factors would produce stale, misleading results. For currency conversion, consult xe.com, your bank, or the official central-bank reference rate for the day of the transaction.
Some categories use conventions that are nearly — but not exactly — universal. The "month" used in time conversion is 30.44 days (one-twelfth of a 365.25-day Julian year); a calendar month varies between 28 and 31 days. The "year" is 365.25 days; the tropical year used by astronomers is 365.2422 days. For most everyday purposes these differences are negligible; for orbital mechanics, financial accrual, or legal deadlines, use the exact calendar.