Unit Converter — Length, Weight, Temperature, Volume & Speed
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Free Unit Converter — Length, Weight, Temperature, Volume & Speed
Convert between the units you actually use every day. Pick a category, choose your units, and type a value in either box — the result appears instantly, in both directions, with the exact conversion formula shown underneath. Nothing is uploaded; every conversion runs in your browser.
📏 Unit Converter
Factors from NIST SP 811 — exact where internationally defined
How to Use This Unit Converter
- Choose a category — Length, Weight, Temperature, Volume, or Speed. The unit menus update automatically.
- Select your two units — the unit you are converting from, and the unit you want the answer in.
- Type a value in either box — this converter is bidirectional. Type on the left to convert forward, or on the right to convert backward. There is no submit button.
- Use the ⇄ swap button to instantly reverse the direction of the conversion.
- Read the formula strip below the result — it shows the exact arithmetic with the factor written out, so you can verify the answer by hand.
Conversion Formulas — Explained
Every unit conversion is one of two operations. Length, weight, volume, and speed are ratio conversions: you multiply by a fixed factor. Temperature is different — it is an affine conversion, because the scales do not share a zero point, so an offset must be applied as well as scaling.
Ratio conversions (length, weight, volume, speed)
Every unit in a category is defined relative to one base unit. Length uses the metre, weight the kilogram, volume the litre, and speed metres per second. A value is first converted to the base unit, then to the target unit:
Worked example — 5 miles to kilometres. One mile is 1,609.344 metres and one kilometre is 1,000 metres, so 5 × (1609.344 ÷ 1000) = 8.04672 km. This two-step approach is why the converter handles any pair of units without needing a separate factor for every combination.
Temperature conversions
Celsius, Fahrenheit, and Kelvin measure the same quantity but start counting from different points, so plain multiplication is not enough. Each conversion needs both a scale factor and an offset:
📐 °C = (°F − 32) × 5/9
📐 K = °C + 273.15
📐 °C = K − 273.15
📐 °F = (K − 273.15) × 9/5 + 32
Worked example — 25 °C to Fahrenheit: (25 × 9/5) + 32 = 45 + 32 = 77 °F. Note that a difference of 1 °C equals a difference of 1.8 °F, but 1 °C does not equal 1.8 °F as an absolute reading — this is the most common mistake in temperature conversion.
Why 9/5 and not something rounder
Water freezes at 0 °C and boils at 100 °C — a 100-degree span. On the Fahrenheit scale those same two points sit at 32 °F and 212 °F — a 180-degree span. The ratio 180 ÷ 100 = 1.8 = 9/5 is therefore fixed by the definitions of the two scales, and the offset of 32 exists because Fahrenheit's zero was originally set well below water's freezing point.
Kelvin has no negative values
Kelvin starts at absolute zero — the temperature at which molecular motion stops. That is why 0 K = −273.15 °C, and why a Kelvin reading is never negative. Kelvin is also written without a degree symbol: 300 K, not 300 °K.
Common Conversion Reference Table
| Conversion | Exact factor | Quick mental estimate |
|---|---|---|
| Miles → Kilometres | × 1.609344 | Add about 60% |
| Kilometres → Miles | × 0.621371 | Multiply by 0.6 |
| Inches → Centimetres | × 2.54 (exact) | Multiply by 2.5 |
| Feet → Metres | × 0.3048 (exact) | Divide by 3.3 |
| Pounds → Kilograms | × 0.45359237 (exact) | Halve it, then subtract 10% |
| Kilograms → Pounds | × 2.20462 | Double it, then add 10% |
| Ounces → Grams | × 28.349523125 (exact) | Multiply by 28 |
| US Gallons → Litres | × 3.785411784 (exact) | Multiply by 3.8 |
| Litres → US Gallons | × 0.264172 | Divide by 3.8 |
| mph → km/h | × 1.609344 | Add about 60% |
| Celsius → Fahrenheit | × 9/5, then + 32 | Double it, then add 30 |
| Fahrenheit → Celsius | − 32, then × 5/9 | Subtract 30, then halve |
The factors marked exact are defined by international agreement rather than measured — the inch has been exactly 2.54 cm and the pound exactly 0.45359237 kg since the 1959 international yard and pound agreement.
📡 About These Conversion Factors
All factors follow NIST Special Publication 811, the official guide to the International System of Units (SI), which lists the internationally agreed factors between SI and US customary units.
- Results are computed at full precision and displayed to 9 significant figures — round yourself if you need fewer.
- This converter uses the US liquid gallon by default; the imperial (UK) gallon is listed separately.
- Temperature uses the affine formulas above, not a single multiplier.
- Everything runs in your browser — no values are sent to any server.
Real-World Examples
🌍 Travel — reading a speed limit sign
A European motorway sign reads 120 km/h. In mph that is 120 × 0.621371 = 74.6 mph. The reverse case: a US limit of 65 mph is 65 × 1.609344 = 104.6 km/h.
🍳 Cooking — an American recipe in a metric kitchen
A recipe calls for 2 lb of flour and an oven at 350 °F. That is 2 × 0.45359237 = 0.907 kg of flour, and (350 − 32) × 5/9 = 176.7 °C — which is why recipe books round it to 180 °C.
🏥 Health — body weight for a medical form
A form asks for weight in kilograms but your scale reads 165 lb. That is 165 × 0.45359237 = 74.8 kg. A height of 5 ft 9 in = 69 × 2.54 = 175.3 cm. Both figures are exactly what a BMI calculation needs.
⛽ Driving — fuel volume abroad
A tank holds 14 US gallons. In litres that is 14 × 3.785411784 = 53 litres, which is what a pump outside the US will display.
Frequently Asked Questions
How many kilometres are in a mile?
Exactly 1.609344 kilometres, because the international mile is defined as 1,609.344 metres. For mental arithmetic, adding roughly 60% gets you within one percent — 50 miles is about 80 km, and the exact answer is 80.47 km.
How do I convert Celsius to Fahrenheit in my head?
Double the Celsius figure and add 30. For 20 °C that gives 70, and the exact answer is 68 °F. The shortcut is accurate to within a few degrees across normal weather temperatures, but use the calculator above for oven, medical, or scientific figures.
Is a US gallon the same as a UK gallon?
No, and confusing the two is a common error. A US liquid gallon is 3.785411784 litres, while an imperial (UK) gallon is 4.54609 litres — about 20% larger. Both are listed in the Volume category above, so check which one your source means before converting.
Why does the converter show so many decimal places?
Because rounding early is where conversion errors come from. The result is calculated at full precision and displayed to a sensible number of significant figures. For everyday use you can round the answer yourself; for engineering or dosage work, keep the precision shown.
Does typing in the right-hand box work too?
Yes. The converter is bidirectional — type in either field and the other updates. This is useful when you know the target value and want the equivalent in the original unit.
Is anything I type stored or sent anywhere?
No. Every conversion is performed by JavaScript inside your own browser. No values are transmitted to a server, written to a database, or shared with any third party, and everything disappears when you close the tab.
Which standard are these factors taken from?
NIST Special Publication 811, Guide for the Use of the International System of Units (SI). See our Sources page for the full reference list.
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