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Unit Converter

Convert length, weight, temperature and more

About Unit Converter

Unit Converter handles the six measurement categories you encounter most often in daily life, study, and engineering: length, weight, volume, temperature, speed, and area. Type a value in any field and every related unit updates instantly without needing to click a button. The tool uses NIST and SI-standard conversion factors, so a mile is exactly 1.609344 kilometres and a kilogram is exactly 2.20462262185 pounds — not rounded approximations. Temperature deserves special mention because Fahrenheit and Celsius are offset scales, not simple multiplications: the converter applies the full two-step formula so you always get the correct answer. Length covers everything from nanometres to light-years; weight from milligrams to metric tons; volume from teaspoons to cubic metres; speed from centimetres per second to Mach; area from square millimetres to square miles. Six decimal places of.

Why use Unit Converter

Six Categories, One Tool

Length, weight, volume, temperature, speed, and area are all handled under one URL. You never need to hunt for a separate converter when a different unit type comes up mid-task.

Live Conversion as You Type

Results update with every keystroke — no submit button required. When you adjust a value, every related unit in the category refreshes simultaneously, giving you a complete picture of equivalences instantly.

SI-correct Precision

Conversion factors are taken from NIST and SI standards rather than rounded textbook approximations. Up to six decimal places are preserved, which matters for scientific, medical, and high-accuracy engineering work.

Offset-aware Temperature

Fahrenheit-to-Celsius conversion uses the full (F − 32) × 5/9 formula, and Kelvin offsets are handled correctly. The tool never applies a naive multiplication that produces wrong results for temperature.

One-click Swap

A single swap action reverses source and target units, so converting in the opposite direction takes one click rather than reselecting both dropdowns. Useful when you know the answer and need to verify the original.

Offline Capable

All conversion logic is pure JavaScript with no external API calls. Once the page is loaded, it keeps working without an internet connection — on a plane, in a basement lab, or anywhere with a weak signal.

How to use Unit Converter

  1. Choose a category tab — Length, Weight, Volume, Temperature, Speed, or Area
  2. Select the source unit from the first dropdown or click the unit label next to the input field
  3. Type your value in the source field
  4. Read the automatically updated result in the target field alongside all related units
  5. Click Swap to reverse source and target units in one action
  6. Adjust decimal precision using the precision control if you need more or fewer decimal places

When to use Unit Converter

  • When a recipe gives measurements in cups and you need millilitres, or vice versa, especially for US versus metric cookbooks
  • When a product specification lists weight in pounds or ounces and you need kilograms for a shipping label or import form
  • When comparing road distances across countries that use miles versus kilometres on their maps and road signs
  • When a fitness tracker reports speed in mph but your treadmill displays km/h and you need to match them
  • When an engineering drawing gives area in square feet and your material supplier charges by the square metre
  • When a weather app reports Fahrenheit and you need a Celsius equivalent for travel planning or a lab protocol

Examples

Length — miles to km

Input: 1 mile

Output: 1.609344 km (exact SI-defined value)

Weight — kg to pounds

Input: 1 kilogram

Output: 2.204623 lb

Temperature — Fahrenheit to Celsius

Input: 100 °F

Output: 37.778 °C

Tips

  • Temperature is an offset scale, not a ratio scale — never multiply Fahrenheit directly by a constant to get Celsius; use the full (F − 32) × 5/9 formula
  • For cooking, US fluid ounces and UK fluid ounces differ by about 4 percent — check which the recipe was written for before scaling
  • Verify any conversion used in medication dosing or lab protocols against a second source, since a rounding error at microgram or milligram scale can be significant
  • The swap button reverses source and target in one click — faster than resetting both dropdowns when you want the inverse conversion
  • Nautical miles (used in aviation and sailing) and statute miles (used on roads) differ by about 15 percent — make sure you are using the right one for navigation

Frequently Asked Questions

What is the exact factor for miles to kilometres?▾
One international mile equals exactly 1.609344 kilometres by definition. This is not an approximation — the US, UK, and international standards organisations all agree on this exact value. Using 1.6 or 1.61 introduces a small but cumulative error in multi-step calculations.
Why is Fahrenheit-to-Celsius not just a multiplication?▾
Celsius and Fahrenheit have different zero points as well as different scale sizes. The full conversion is °C = (°F − 32) × 5/9. Skipping the subtraction step gives the wrong answer because the scales are offset, not just stretched relative to each other.
What is the difference between US and UK gallons?▾
A US liquid gallon equals 3.78541 litres, while a UK (imperial) gallon equals 4.54609 litres — about 20 percent larger. This distinction matters for fuel economy, brewing, and shipping. The converter defaults to US gallons but labels them clearly.
How accurate are the conversions?▾
Conversion factors follow NIST and SI definitions and are stored at full IEEE 754 double-precision floating point. Results are displayed to six decimal places by default. For most practical purposes this exceeds the accuracy of the measurement being converted.
Why doesn't 1 lb equal exactly 0.45 kg?▾
One international avoirdupois pound equals exactly 0.45359237 kilograms by definition. The rounded figure of 0.45 is an approximation that accumulates error over multiple conversions. This tool uses the full defined value.
Can I convert between volume and weight — for example ml to grams?▾
Volume and weight measure different physical quantities, so a direct conversion requires the density of the specific substance. Water at 4°C has a density of 1 g/ml, so 100 ml of water weighs 100 grams, but 100 ml of olive oil weighs about 92 grams. This tool converts within physical quantity families, not across them.
What unit does 'fl oz' mean — US or UK?▾
The tool uses US fluid ounces (1 fl oz = 29.5735 ml) by default, which is most common in American recipes and product labels. The UK fluid ounce is slightly larger at 28.4131 ml. Labels in the tool make the distinction clear when both variants are available.
Are nautical miles and statute miles the same?▾
No — a nautical mile equals exactly 1.852 kilometres, while a statute (land) mile equals 1.609344 kilometres. Nautical miles are used in aviation and maritime navigation and are defined as one minute of arc along a meridian. The converter includes both mile variants in the Length category.

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Glossary

SI Unit
A unit belonging to the International System of Units — the modern form of the metric system. SI base units include the metre for length, kilogram for mass, and second for time, from which all other SI units are derived.
Imperial Unit
A unit from the traditional British system, including miles, yards, feet, inches, pounds, ounces, and gallons. The US customary system uses similar but not identical units; US and UK gallons, for example, differ in volume.
Conversion Factor
A numerical multiplier used to change a measurement from one unit to another. For example, 1 inch = 2.54 centimetres, so the conversion factor from inches to centimetres is exactly 2.54.
Metric System
A decimal-based system of measurement in which units differ by powers of ten. Prefixes such as kilo- (×1000), centi- (÷100), and milli- (÷1000) indicate scale. Most countries use the metric system for everyday measurement.
Significant Figures
The meaningful digits in a number, starting from the first non-zero digit. A measurement of 1.609 km has four significant figures. Retaining sufficient significant figures is important when conversion results feed into further calculations.
Base Unit
The fundamental unit within a category from which all other units in that category are derived. The metre is the SI base unit for length; all other length units (kilometre, centimetre, inch, mile) are defined relative to it.