Length: metres, feet and inches
The exact relationship is 1 inch = 2.54 centimetres, by definition. From that, 1 metre is about 3.281 feet, and 1 foot is about 0.305 metres. For everyday estimates, remember that a metre is a little longer than a yard (about 1.094 yards), and a kilometre is about 0.621 miles — so 5 kilometres is roughly 3.1 miles, and 10 kilometres is about 6.2 miles.
Converting a person's height is where most people slip. Five feet six inches is not 5.6 feet: it is 5 feet plus 6 inches, which is 66 inches, or about 167.6 centimetres. Convert feet and inches separately — feet to centimetres, inches to centimetres — and add them, rather than treating the inches as a decimal. The same caution applies to any measurement written as feet and inches.
Weight: kilograms and pounds
One kilogram is about 2.205 pounds, and one pound is about 0.454 kilograms. For a quick estimate, doubling the kilograms and adding ten percent gets you close: 70 kilograms doubled is 140, plus 14 is 154 pounds, against the exact 154.3. For the reverse, halving the pounds and subtracting ten percent works similarly: 154 pounds halved is 77, minus 7.7 is about 69.3 kilograms.
Watch out for stones, still used for body weight in the UK and Ireland: one stone is 14 pounds, or about 6.35 kilograms. Twelve stone is therefore 168 pounds, or about 76.2 kilograms — not 120 pounds, which is the mistake people make when they read “12” as a decimal. For cooking, grams and ounces are the usual pair: one ounce is about 28.35 grams, so 100 grams is roughly 3.5 ounces.
Temperature: Celsius and Fahrenheit
Temperature is the one conversion you cannot do with a simple factor, because the two scales start at different points. The formula is: Fahrenheit = Celsius × 9/5 + 32, and Celsius = (Fahrenheit − 32) × 5/9. A few landmarks are worth memorising: 0 °C is 32 °F (freezing), 20 °C is 68 °F (a comfortable room), 37 °C is about 98.6 °F (body temperature), and 100 °C is 212 °F (boiling).
A useful shortcut near room temperature: double the Celsius value and add 30 to get roughly Fahrenheit — 20 °C doubled is 40, plus 30 is 70 °F, close to the exact 68 °F. The shortcut drifts further out at extremes, so use the exact formula for ovens, weather extremes, and anything safety-related. And never confuse a temperature difference with an absolute temperature: a rise of 10 Celsius degrees is a rise of 18 Fahrenheit degrees, with no +32 involved.
Volume and cooking measures
One US fluid ounce is about 29.57 millilitres, while one UK fluid ounce is about 28.41 millilitres — a small difference that matters in baking. One US cup is 8 US fluid ounces, about 237 millilitres; a UK cup is not a standard measure in the same way, so recipes that say “cup” usually mean the US cup. One litre is about 0.264 US gallons, and one US gallon is about 3.785 litres — note that the UK gallon, at about 4.546 litres, is larger, which is one reason fuel-economy figures differ between countries.
For cooking, weight is more reliable than volume for dry ingredients: a cup of flour can vary by 20 percent depending on how it is packed, while 120 grams is always 120 grams. If a recipe gives both, trust the weight. If it gives only cups, spoon the ingredient in lightly rather than scooping, and level it off.
Avoiding the classic mistakes
The three classic errors are: treating feet-and-inches as a decimal, forgetting that temperature needs an offset as well as a factor, and mixing US and UK gallons or fluid ounces. A fourth is rounding too early: keep at least one extra digit through the calculation and round only the final answer, especially when converting several values that will be added together.
When precision matters — medication, engineering, trade, or safety — do not rely on a remembered shortcut at all. Use a converter, check the result against a second source, and write down the units with the number. A bare number without its unit is how most conversion accidents happen.
Try the related tools
Try the Unit Converter on Daily Tools. Try the Percentage Calculator on Daily Tools. Try the EMI Calculator on Daily Tools.
Frequently asked questions
How many feet are in a metre?
One metre is about 3.281 feet. For a quick estimate, a metre is a little over three feet.
How do I convert kilograms to pounds?
Multiply kilograms by 2.205. For a rough estimate, double the kilograms and add ten percent.
What is 20 degrees Celsius in Fahrenheit?
20 °C is 68 °F, using the formula Fahrenheit = Celsius × 9/5 + 32.