Everyday Maths Made Easy: BMI, Tips, Averages, Ratios and Unit Conversions

Most everyday maths is a formula you half remember: weight over height squared, the tip on a bill, the middle value of a list. These calculators do the arithmetic and show the steps that matter, so you can trust the answer and explain it to someone else.

Which calculator for which question

BMI: a quick health screen

Choose Metric or Imperial, then enter your height and weight. Someone 175 cm tall weighing 70 kg has a BMI of about 22.9, in the healthy range of 18.5–24.9. If you also enter your age and sex, the calculator estimates body fat percentage.

• BMI cannot tell muscle from fat, so very muscular people can show as overweight.
• Some Asian health authorities treat 23 and above as overweight, because health risks start at a lower BMI.
• It is not meant for children, teenagers or pregnancy.

Tips and splitting the bill

Enter the Bill amount, the Tip (%) and the number of People. An 80 bill with a 20% tip is 16 in tip and 96 in total, or 24 each for four people.

• Check the bill for a service charge first. If one is included, there is no need to tip on top.
• Customs vary widely: 15–20% is expected in the United States, around 10% is common in the UK, Nepal and India, and tipping is not customary in Japan.
• For quick mental maths, 10% is the bill with the decimal point moved one place left; double it for 20%.

Averages that tell the truth

Paste numbers separated by commas or new lines, straight from a spreadsheet column if you like, and select Calculate. You get the count, mean, median, mode and range.

The mean is pulled up or down by extreme values; the median is not. If nine people earn 30,000 and one earns 1,000,000, the mean is 127,000, but the median of 30,000 describes the group far better. Use the median for salaries, house prices and anything with a few huge values, and the mean for marks, temperatures and other evenly spread data.

Ratios: scale, mix and share

Simplify Ratio reduces terms to their smallest whole numbers: 1920, 1080 becomes 16:9. Solve Proportion finds the missing value in A : B = C : ?.

• Recipes: 2 cups of flour to 3 cups of milk. For 5 cups of flour, 2 : 3 = 5 : ? gives 7.5 cups of milk.
• Mixes: 50:1 two-stroke fuel means 1 litre of oil for 50 litres of petrol, or 20 ml per litre.
• Sharing: to split 1,200 in the ratio 1:2:3, add the parts (6), divide (200 per part), then multiply: 200, 400 and 600.

Unit conversions

Choose Length, Weight or Temperature, enter a Value and pick the From and To units. The factors are the exact international definitions, such as 1 inch = 2.54 cm and 1 pound = 0.45359237 kg.

For feet and inches, convert the feet to inches first (5 ft = 60 in), add the inches, then convert to centimetres. Temperature cannot be converted by multiplying alone, because Celsius and Fahrenheit start at different zero points; the converter applies °F = °C × 9/5 + 32 for you.