What is the exact factor for miles to kilometers?

1 mile = 1.609344 kilometers exactly, and 1 kilometer = 0.621371 miles. The mile is defined as 1,609.344 meters, so the relationship is exact rather than approximate.

Is there a quick way to convert Celsius to Fahrenheit in my head?

Double the Celsius value and add 30 for a rough Fahrenheit estimate (e.g. 20°C → about 70°F; the exact answer is 68°F). For an exact result, multiply by 9/5 and add 32.

Why is a US gallon different from an imperial gallon?

They are simply two different historical definitions. A US liquid gallon is 3.785411784 liters; an imperial (UK) gallon is 4.54609 liters — about 20% larger. Always check which gallon a figure refers to.

How accurate are the mental-math shortcuts?

Shortcuts trade a little accuracy for speed and are usually within a few percent — fine for cooking, travel, and DIY. When precision matters, use the exact factor or the calculator, which keeps full precision internally.

Metric ↔ imperial cheat sheet

The metric and imperial systems describe the same physical world with different yardsticks, and moving between them is something most people do far more often than they expect — reading a recipe from another country, checking a weather forecast abroad, buying lumber, or working out a running pace. This cheat sheet gathers the conversions that actually come up day to day. For each one it gives the exact factor (the number to use when precision matters) and, where it helps, a mental-math shortcut that is close enough for cooking, travel, and DIY. Every factor here is the internationally defined value, not a rounded approximation.

Length and distance

Length is where most cross-system confusion happens, because both systems are in everyday use. The anchor facts to memorise are that 1 inch = 2.54 cm exactly and 1 mile = 1.609344 km exactly. From those two, everything else follows.

Inches ↔ centimetres: multiply inches by 2.54 to get cm; multiply cm by 0.3937 to get inches.
Feet ↔ metres: 1 foot = 0.3048 m. To go from metres to feet, multiply by 3.2808.
Miles ↔ kilometres: 1 mile = 1.609344 km; 1 km = 0.621371 miles.
Yards ↔ metres: 1 yard = 0.9144 m — close to a metre, but not equal to it.

Shortcut: to turn kilometres into miles quickly, multiply by 0.6 (or take the kilometre figure, halve it, and add a tenth). 100 km becomes roughly 60 miles; the exact answer is 62.1, so you are within a couple of percent. A neat coincidence helps here too: consecutive numbers in the Fibonacci sequence approximate the km-to-mile ratio, so 8 km is about 5 miles and 13 km is about 8 miles.

Weight and mass

For weight the headline fact is 1 pound = 0.45359237 kg exactly, which means 1 kilogram ≈ 2.2046 pounds. The ounce is one-sixteenth of a pound, so 1 oz = 28.3495 g.

Kilograms ↔ pounds: multiply kg by 2.2046 for pounds; multiply pounds by 0.4536 for kg.
Grams ↔ ounces: 1 oz = 28.35 g; 100 g ≈ 3.53 oz.
Stone: still common in the UK and Ireland for body weight — 1 stone = 14 pounds = 6.35029 kg.

Shortcut: to estimate pounds from kilograms, double the kilogram figure and add about ten percent. 70 kg → 140 + 14 = 154 pounds; the exact value is 154.3, so the trick is remarkably good. Going the other way, halving the pound figure gives a slightly heavy kilogram estimate that is fine for a quick gut check.

Temperature

Temperature is the one conversion that is not a simple multiplication, because the Celsius and Fahrenheit scales start from different zero points. The exact formulas are °F = (°C × 9/5) + 32 and °C = (°F − 32) × 5/9. Three reference points are worth committing to memory: water freezes at 0°C = 32°F, water boils at 100°C = 212°F, and the two scales cross at −40°C = −40°F. Normal human body temperature is about 37°C = 98.6°F.

Shortcut: for a fast Celsius-to-Fahrenheit estimate, double the Celsius number and add 30. So 20°C → 70°F (exact is 68°F) and 30°C → 90°F (exact is 86°F). The error grows as you move away from room temperature, so fall back to the full formula for anything where a few degrees matter, such as cooking or fever readings. Kelvin, used in science, is just Celsius shifted by 273.15: K = °C + 273.15, and 0 K is absolute zero.

Volume and capacity

Volume carries a hidden trap: the word "gallon" means two different things. A US gallon is 3.785411784 litres, while an imperial (UK) gallon is 4.54609 litres — roughly 20% larger. The same applies to pints and fluid ounces, which are defined differently on each side of the Atlantic. When a figure could be either, check the source.

Litres ↔ US gallons: 1 US gallon = 3.785 L; 1 L = 0.264 US gallons.
Litres ↔ imperial gallons: 1 imperial gallon = 4.546 L; 1 L = 0.220 imperial gallons.
Millilitres ↔ US fluid ounces: 1 US fl oz = 29.5735 ml.
Cooking measures (US): 1 cup = 236.588 ml, 1 tablespoon = 14.787 ml, 1 teaspoon = 4.929 ml.

Shortcut: to convert litres to US gallons, divide by four and you will be in the right ballpark (1 L ≈ 0.26 gallons, so dividing by four slightly underestimates). For cooking, it helps to remember that a US cup is just under a quarter-litre, so 250 ml is a touch more than one cup.

How to use this with the calculator

The factors above are exactly what the converters on this site use internally — they carry full precision through the calculation and round only the number they display, so you never accumulate rounding error. Use this page to build intuition and to sanity-check a result, and use the calculator when you want an exact figure without doing the arithmetic. If a number ever looks off, compare it against one of the reference points listed here: water freezing, a kilogram being a bit over two pounds, or a mile being about 1.6 kilometres.

These conversions are for everyday and educational use. For trade, engineering, or legal contexts where a defined tolerance applies, follow the relevant standard for that field.