単位変換

How unit conversion actually works

Almost every everyday unit conversion comes down to one simple idea: pick a base unit for the quantity, express each unit as a fixed multiple of that base, then convert by dividing. This tool keeps a base unit for each category — metres for length, kilograms for weight, litres for volume, square metres for area, and metres-per-second for speed — and stores every other unit as how many base units it equals. To convert any value, it multiplies your input by the source unit's factor to reach the base, then divides by the target unit's factor.

So the core formula is:

result = value × (factor_from ÷ factor_to)

where factor is each unit's size relative to the base. Because both factors are exact constants, the conversion is a single multiply-and-divide with no rounding until the final display. This tool shows the answer to eight significant figures, which is more than enough precision for engineering, cooking, or travel.

A worked example: miles to kilometres

Suppose you enter 5 miles and ask for kilometres. The length base is the metre. A mile is defined as exactly 1609.344 metres, and a kilometre is 1000 metres. The tool computes:

• Convert to base: 5 × 1609.344 = 8046.72 metres.
• Convert to target: 8046.72 ÷ 1000 = 8.04672 kilometres.

That single chained operation is why you can switch the "from" and "to" units freely without the tool needing a separate rule for every possible pair. With 9 length units, a pairwise table would need 72 entries; the base-unit method needs only 9 factors.

Temperature is the exception

Temperature does not use a simple multiplier because the scales have different zero points. You cannot say "1 °F equals X °C" the way you can with metres and feet — the relationship is a line with both a slope and an offset. This tool handles temperature separately by converting everything through Celsius:

• Fahrenheit to Celsius: (°F − 32) × 5/9
• Kelvin to Celsius: K − 273.15
• then Celsius back out to the target scale.

For example, 98.6 °F becomes (98.6 − 32) × 5/9 = 37 °C, and on to 310.15 K. Forgetting the offset is the single most common temperature-conversion mistake — multiplying 98.6 by 5/9 alone gives a nonsense answer.

Watch out for US vs UK gallons

Volume hides a famous trap. A US gallon is about 3.785 litres, but a UK (imperial) gallon is about 4.546 litres — roughly 20% larger. Recipes, fuel-economy figures, and shipping volumes are frequently quoted in one and read as the other. This tool lists both (gal_us and gal_uk) so you can pick the right one deliberately rather than assuming. The same caution applies to fluid ounces, pints, and quarts, which also differ between the two systems.

Practical tips

• Round only at the end. If you need an intermediate value, copy the full eight-figure result rather than a rounded one — rounding twice compounds error.
• Match the category to the dimension. You can convert area to area and speed to speed, but not length to area; that would require knowing a second dimension. If a "conversion" feels impossible, you are probably mixing dimensions.
• Nautical units are distinct. A nautical mile (1852 m) and a knot (one nautical mile per hour) are used in aviation and shipping and are noticeably larger than their land cousins.
• Significant figures, not decimal places. Converting a measurement you only know to two figures into eight figures does not make it more accurate — keep your real precision in mind.

Common use cases

Travellers convert temperatures, distances, and fuel volumes between metric and imperial. Cooks scale recipes between cups, millilitres, and ounces. Students check physics homework where speeds must move between m/s and km/h. Tradespeople convert areas between square feet and square metres for flooring or paint estimates. Because the tool runs entirely in your browser using fixed conversion factors, nothing you type is sent anywhere — it works offline and returns the same exact answer every time.