Conversion Calculator

Length · Mass · Temperature · Volume · Area · Speed

Value

From

Quick examples

📌

Length uses meters as the SI base. Inch = 0.0254 m exactly.

🔄 Enter a value and choose units to convert.

Unit Conversion Formulas

Most unit conversions multiply by a factor that links a unit to an SI base. Temperature is different because Fahrenheit and Celsius scales have different zeros.

General method

value_to = value_from × (base_from / base_to) base_* = size of one unit in SI base units Example: 5 ft → m: 5 × 0.3048 = 1.524 m

Temperature

°C = (°F − 32) / 1.8 °F = °C × 1.8 + 32 K = °C + 273.15

Exact anchors

1 in = 0.0254 m · 1 lb = 0.45359237 kg US gallon = 231 in³ = 3.785411784 L

💡

Convert through SI first. That keeps every pair consistent and avoids chaining rounded factors.

Categories & FAQ

Category	SI base	Examples
Length	meter	in, ft, mi, km
Mass	kilogram	oz, lb, ton
Temperature	kelvin path	°C, °F, K
Volume	cubic meter	L, gal, cup
Area	square meter	ft², acre, ha
Speed	m/s	mph, km/h, knot

How do you convert units?

Pick a category, enter a value, choose From and To. The calculator converts through the SI base for that category and lists every other unit too.

How do you convert Celsius to Fahrenheit?

°F = °C × 1.8 + 32. Example: 20 °C → 68 °F. Open the Temp tab to convert live.

How to Use This Conversion Calculator

Choose Length, Mass, Temp, Volume, Area, or Speed. Enter the value, select From and To units, and read the hero result. Swap flips the units. The list below the metrics shows the same value in every unit of the category so you can compare scales at a glance without leaving the tool.

Factors follow NIST Guide to the SI style conversions, including exact inch and pound definitions where those international agreements apply.

What unit conversion means

A conversion expresses the same physical quantity with a different unit label. One meter and 100 centimeters describe the same length. Good converters never change the underlying quantity, only the unit scale.

Length conversions

The meter is the SI length unit. One inch is exactly 0.0254 meter. Therefore one foot is exactly 0.3048 meter and one yard is exactly 0.9144 meter. The international mile is 5280 feet, so 1 mi = 1609.344 m exactly.

Worked example: convert 10 miles to kilometers. 10 × 1609.344 = 16093.44 m = 16.09344 km.

Mass conversions

The kilogram is the SI mass unit. One avoirdupois pound is exactly 0.45359237 kg. One ounce is 1/16 pound. A short ton is 2000 pounds. Everyday “weight” converters almost always mean these mass units.

Worked example: 150 lb → kg: 150 × 0.45359237 = 68.0388555 kg.

Temperature conversions

Celsius and Fahrenheit are affine scales. Water freezes at 0 °C / 32 °F and boils near 100 °C / 212 °F at standard pressure. Kelvin is absolute: 0 K is absolute zero and K = °C + 273.15.

Worked example: 98.6 °F → °C: (98.6 − 32) / 1.8 = 37 °C.

Volume conversions

The SI volume derived unit is the cubic meter. One liter is exactly 0.001 m³. The US liquid gallon is defined as 231 cubic inches, which equals 3.785411784 liters because the inch is exact.

Worked example: 2 US gal → L: 2 × 3.785411784 = 7.570823568 L.

Area and speed

Area units are length squared. One acre is 43560 square feet. Speed units are length per time. One mile per hour equals 0.44704 m/s exactly because of the exact mile and the 3600 second hour. One international knot is 1852 m/h = 0.514444… m/s.

How to convert manually

1. Identify the quantity type. 2. Look up the factor to SI. 3. Multiply. 4. Divide by the target unit’s SI size. 5. For temperature, use the °C / °F / K formulas instead of a single multiply.

Common mistakes

Using UK imperial gallon factors for US recipes
Treating temperature like a simple scale factor without the 32 offset
Mixing mass pounds with force pounds without saying so
Rounding intermediate factors too early
Confusing statute miles with nautical miles

Practical scenarios

Travelers convert km/h speed limits. Cooks convert cups and milliliters. Engineers convert drawings between inches and millimeters. Runners convert race distances. Weather readers convert °F forecasts to °C.

Pair this tool with the Time Calculator for pure time units, or the Date Calculator for calendar spans. Length and speed here already cover mph and km/h. For percent change between two measured values after conversion, use the Percent Calculator on the converted numbers.

US customary versus imperial

US customary length and mass units share the international inch and pound with modern imperial usage. Volume is where recipes break: the US liquid gallon is smaller than the UK imperial gallon. This calculator uses US liquid volume units only and states that clearly in the Volume hint.

Derived exact chains

Because the inch is exact, many area and volume factors become exact too. One square inch is exactly 0.00064516 m². One cubic inch is exactly 1.6387064×10⁻⁵ m³. Building conversions from those anchors avoids inconsistent rounded tables.

Scientific notation

Very large or very small results also appear in scientific notation in the metrics row. That helps when converting micrometers to miles or square millimeters to square kilometers without losing place value.

Swap workflow

Use Swap when you typed the units backward. The value stays the same and From/To flip, then the result recalculates immediately. Preset buttons load common homework and travel pairs for each category.

When this calculator is useful

Use it for homework unit changes, travel planning, lab prep, recipe scaling across metric and US customary units, and quick checks against datasheets.

Significant figures

Exact definitions (inch, pound, liter relation to m³) can keep many digits. Measured inputs should keep only as many digits as the measurement supports. The calculator shows several display precisions so you can round to your need.

Knots and nautical miles

One international nautical mile is exactly 1852 meters. A knot is one nautical mile per hour. Aviation and marine charts use these units. Do not mix them with statute miles when planning distances over water or airways.

When this calculator may not be enough

It does not cover every specialized unit (survey feet variants, Troy ounces, UK gallons). Fuel economy as L/100 km is not a simple reciprocal of km/L without care. Currency conversion needs live exchange rates, not SI factors.

How to interpret your result

The hero is the To unit value. Metrics show SI base size, scientific notation, and the inverse (how many From units equal one To unit for linear categories). Temperature inverse is not a single multiply factor.

Limitations

Factors are NIST aligned for common SI and US customary units listed here. Regional grocery ounces by volume and fluid ounces differ; this volume set uses fluid measures. Results are for general conversion, not legal metrology certificates.

⚠️

For trade measurements, use calibrated instruments and the legal units required in your jurisdiction.

ℹ️

Inch and pound exact values and temperature formulas follow NIST Guide to the SI conversion appendices and NIST temperature unit pages.

How many meters are in an inch?

Exactly 0.0254 meter. That international inch definition is the length anchor for feet, yards, and miles in this calculator.

How many kilograms are in a pound?

Exactly 0.45359237 kilogram for the avoirdupois pound used in US customary mass conversion.

What is the difference between mass and weight?

Mass is matter in kilograms. Weight is force. Consumer converters usually treat pounds as mass. This Mass tab does the same.

How do I convert mph to km/h?

Open Speed, choose mph From and km/h To. Because 1 mi = 1.609344 km, 1 mph = 1.609344 km/h exactly.

Sources & References

NIST Guide to the SI, Appendix B.8 conversion factors: nist.gov
NIST Guide to the SI, Appendix B.9 factors by quantity: nist.gov
NIST SI Units — Temperature: nist.gov/pml/owm/si-units-temperature