The Conversions You Will Search For Most
A handful of conversions covers the vast majority of real-world needs. Length: 1 inch is exactly 2.54 cm, 1 foot is 0.3048 m, and 1 mile is 1.609344 km. Weight: 1 pound is 0.45359237 kg (about 0.454), and 1 ounce is 28.35 g. Volume: 1 US gallon is 3.78541 litres. Temperature: 20°C is 68°F, and 37°C, normal body temperature, is 98.6°F.
People reach for these in predictable moments: converting an oven temperature from a US recipe, checking height or weight for an international form or fitness app, converting a road-trip distance before driving abroad, scaling DIY measurements from a plan drawn in the other system, or reading a weather forecast in an unfamiliar scale. Bookmark the factors above and most conversions become one multiplication.
- Celsius to Fahrenheit: 20°C = 68°F, 0°C = 32°F, 100°C = 212°F.
- Kilograms to pounds: 1 kg = 2.20462 lb; 70 kg = 154.3 lb.
- Inches to centimetres: 10 in = 25.4 cm; 6 ft = 182.9 cm.
- Miles to kilometres: 26.2 mi marathon = 42.16 km; 60 mph = 96.6 km/h.
Temperature: The Formula Everyone Gets Wrong
Length and weight conversions are single multiplications, but temperature needs an offset as well, because Celsius and Fahrenheit start at different zero points. Celsius to Fahrenheit: °F equals °C times 9/5 plus 32. Fahrenheit to Celsius: °C equals (°F minus 32) times 5/9. For 25°C: 25 times 9/5 is 45, plus 32 is 77°F. For 98.6°F: minus 32 is 66.6, times 5/9 is 37°C.
The classic mistake is skipping the offset: 20°C times 9/5 is 36, which is obviously not 36°F (freezing water cannot be above freezing). The correct answer adds 32 to get 68°F. Kelvin conversions are simpler: add 273.15 to Celsius. One exception: temperature differences need no offset. A 10°C rise is an 18°F rise (10 times 9/5), because you are converting an interval, not a point on the scale.
Weight and Length: Exact Factors Worth Memorizing
A few exact definitions are worth committing to memory because approximations compound. The inch is defined as exactly 2.54 cm (not 2.5), the foot as exactly 0.3048 m, and the international avoirdupois pound as exactly 0.45359237 kg since 1959. For mental math, 1 kg is about 2.2 lb (halve and add 10%), 1 km is about 0.62 miles, and 1 metre is roughly a yard plus 10%.
Watch for near-identical units that are not identical. A nautical mile (6,076 ft) is about 15% longer than a statute mile (5,280 ft): confusing them misplaces ships and planes. An acre is 4,047 m², roughly a football field without the end zones; a hectare is 10,000 m², about 2.47 acres. Mixing acres and hectares has derailed real land deals.
Three Mistakes That Cost Real Money
The most expensive error is using linear factors for area or volume. Area factors must be squared and volume factors cubed: 1 m² is not 3.28 sq ft but 10.76 sq ft, and 1 m³ is 35.31 cu ft, not 3.28. Ordering flooring with the linear factor leaves you about 70% short. Always convert in the correct category rather than multiplying a length factor by hand.
Second, US and UK units diverged centuries ago: the US gallon is 3.785 L while the UK imperial gallon is 4.546 L, a 20% gap that matters for fuel, chemicals, and recipes. The US ton (2,000 lb), UK long ton (2,240 lb), and metric tonne (2,205 lb) differ similarly. Third, fluid ounces measure volume while ounces measure weight; 1 litre of water weighs exactly 1 kg, a handy benchmark for sanity-checking kitchen conversions.
Cooking, Travel, and DIY: Conversions in Real Life
In the kitchen, precision beats speed. Spoon flour into the cup and level it rather than scooping, which packs in 25% extra, and remember that cups are volumetric approximations: for serious baking, weigh ingredients instead. Convert the total metric amount first when scaling recipes, then find the nearest practical imperial measure, so rounding errors do not accumulate across six repeated conversions.
For travel, convert distances and speed limits before you drive: 60 mph is 96.6 km/h, and a 50 lb airline baggage limit is 22.68 kg. For DIY, convert plan dimensions once into your working system and stick to it; mixing metric structure with imperial finishes is how walls end up in the wrong place. Our free Unit Converter covers length, weight, temperature, volume, and area with exact international factors, so the math is never the weak link.
