Unit Converter: The Complete Guide to Converting Length, Weight, and Temperature Online
You need to convert kilometres to miles before a road trip. A recipe calls for ounces but your scale reads grams. A weather app shows Fahrenheit and you think in Celsius. Unit conversion is one of those small frictions that appears dozens of times a week — and it always happens at the worst moment.
This guide covers the most common unit conversions you’ll actually need: length, weight, temperature, volume, and speed. All formulas included, so you can do it by hand or verify any result.
Why Unit Conversion Is Still Confusing in 2026
You would think this problem would be solved by now. It isn’t — for a few reasons.
The world runs on two incompatible measurement systems. The metric system (used by most of the world) is built on powers of ten: kilometres, kilograms, litres. The imperial system (dominant in the United States, and partially in the UK) uses miles, pounds, and gallons — units with no clean mathematical relationship to each other.
On top of that, some fields have their own conventions. Scientists use Kelvin for temperature. Aviation uses feet for altitude even in metric countries. Cooking recipes mix cups, tablespoons, and millilitres depending on the country of origin. Context matters as much as the formula.
Length Conversion
Length is the most common conversion need. The key relationships:
| From | To | Multiply by |
|---|---|---|
| Kilometres | Miles | × 0.621371 |
| Miles | Kilometres | × 1.60934 |
| Metres | Feet | × 3.28084 |
| Feet | Metres | × 0.3048 |
| Centimetres | Inches | × 0.393701 |
| Inches | Centimetres | × 2.54 |
A practical shortcut worth memorising: 1 mile ≈ 1.6 km. It’s not exact (1.60934), but for quick mental estimates — “that’s about 50 km, so roughly 31 miles” — it works perfectly.
For height, most people in the US describe themselves in feet and inches. Converting 5 feet 10 inches to centimetres: first convert everything to inches (5 × 12 + 10 = 70 inches), then multiply by 2.54 to get 177.8 cm.
Weight Conversion
Weight is where the imperial system gets genuinely confusing, because it has two pound systems (avoirdupois and troy), and the relationship between pounds, ounces, and stones is non-obvious.
| From | To | Multiply by |
|---|---|---|
| Kilograms | Pounds | × 2.20462 |
| Pounds | Kilograms | × 0.453592 |
| Grams | Ounces | × 0.035274 |
| Ounces | Grams | × 28.3495 |
| Kilograms | Stones | × 0.157473 |
| Stones | Kilograms | × 6.35029 |
Quick mental shortcut: 1 kg ≈ 2.2 lbs. For cooking, remember that 1 ounce ≈ 28 grams — this makes recipe scaling much faster than looking up the exact conversion each time.
Stones are used primarily in the UK for body weight. One stone equals 14 pounds, so a person who weighs 11 stone 4 pounds weighs (11 × 14) + 4 = 158 pounds, or about 71.7 kg.
Temperature Conversion
Temperature is unique because it uses additive offsets, not just multiplication. You cannot simply multiply to convert — you must account for the different zero points of each scale.
Celsius → Fahrenheit: °F = (°C × 9/5) + 32
Fahrenheit → Celsius: °C = (°F − 32) × 5/9
Celsius → Kelvin: K = °C + 273.15
Kelvin → Celsius: °C = K − 273.15
A few temperature anchors worth knowing by heart:
- 0°C = 32°F — water freezes
- 20°C = 68°F — comfortable room temperature
- 37°C = 98.6°F — normal human body temperature
- 100°C = 212°F — water boils at sea level
There’s also a convenient coincidence: −40°C = −40°F. The two scales cross at that point, so if someone mentions −40° without a unit, it doesn’t matter which system they’re using.
Kelvin is used in physics and chemistry because it starts at absolute zero (the coldest theoretically possible temperature, −273.15°C). There are no negative Kelvin values, which makes mathematical operations on temperature much cleaner.
Volume Conversion
Volume conversions are especially relevant in cooking, where recipes from different countries mix metric and imperial units freely.
| From | To | Multiply by |
|---|---|---|
| Litres | US Gallons | × 0.264172 |
| US Gallons | Litres | × 3.78541 |
| Millilitres | US Fluid Ounces | × 0.033814 |
| US Fluid Ounces | Millilitres | × 29.5735 |
| Litres | US Cups | × 4.22675 |
| US Cups | Millilitres | × 236.588 |
Note: US and UK (imperial) gallons are different. One US gallon = 3.785 litres; one imperial gallon = 4.546 litres. If a recipe specifies “gallon,” check which system it uses.
Tablespoons and teaspoons vary slightly between US and Australian/UK conventions. If precision matters for baking, always measure by weight (grams) rather than volume — it’s more accurate.
Speed Conversion
Speed conversions matter for international travel, weather, and anything involving vehicles.
| From | To | Multiply by |
|---|---|---|
| km/h | mph | × 0.621371 |
| mph | km/h | × 1.60934 |
| m/s | km/h | × 3.6 |
| km/h | m/s | × 0.27778 |
| Knots | km/h | × 1.852 |
| Knots | mph | × 1.15078 |
A speed limit of 100 km/h is approximately 62 mph — close enough that a simple “divide by 1.6” works fine in practice. Aviation uses knots (nautical miles per hour), where one knot = 1.852 km/h.
The Metric System: Why It’s Easier
If you grew up with imperial, metric can feel foreign. But its design is genuinely simpler:
- Every unit scales by a factor of 10, 100, or 1000
- Prefixes are consistent: kilo- (×1000), centi- (÷100), milli- (÷1000)
- 1 litre of water weighs exactly 1 kilogram — mass and volume are linked by definition
Converting 1.5 km to metres is trivial: multiply by 1000, get 1500 m. Converting 1.5 miles to feet requires knowing that there are 5280 feet in a mile — an arbitrary number with no mathematical logic behind it.
This is why science, medicine, and international trade use metric almost exclusively. The cognitive overhead of imperial is real, even if familiarity masks it.
When Precision Actually Matters
For everyday purposes — cooking, travel, fitness — the quick shortcuts above are perfectly adequate. But some contexts demand precision:
Engineering and manufacturing — tolerances can be in thousandths of an inch or tenths of a millimetre. Rounding introduces error that compounds.
Pharmaceuticals — medication dosing is calculated by weight (mg/kg). Using an approximate conversion for body weight when calculating a drug dose is a genuine safety issue.
Aviation — altitude in feet, fuel in kg or lbs depending on the aircraft, distance in nautical miles. Mixed systems in the same environment have caused accidents.
For casual use, round freely. For technical use, use exact conversion factors and carry more decimal places than you think you need.
Convert Any Unit Instantly
You can convert between all the units in this guide — and many more — with our free online unit converter. It supports length, weight, temperature, volume, speed, area, data storage, and more. No account, no tracking, no data sent to any server.
→ Open the free Unit Converter
Type your value, select the units, and get the result instantly. Bookmark it — you’ll use it more than you expect.