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Mental Math Tricks for Converting Units Without a Calculator

Updated · 2 min read

You do not always have a calculator at hand, and sometimes you only need a good estimate: Is that road sign in kilometers or miles? Is a suitcase near the weight limit? Will a 3-liter tank fit your fuel need? A handful of mental shortcuts will get you within a few percent of the exact answer.

Distance

  • Kilometers to miles: multiply by 0.6. 100 km is about 60 miles (exact: 62.1). For a better answer, add 3 percent.
  • Miles to kilometers: multiply by 1.6, or use the Fibonacci sequence (1, 2, 3, 5, 8, 13, 21): 5 miles is about 8 km, 8 miles about 13 km and 13 miles about 21 km.
  • Inches to centimeters: multiply by 2.5. Ten inches is about 25 cm.
  • Meters to feet: multiply by 3 and add 10 percent. 10 meters is about 33 feet.

Weight

  • Kilograms to pounds: double it and add 10 percent. 50 kg is 100 + 10 = 110 lb (exact: 110.2).
  • Pounds to kilograms: halve it and subtract 10 percent of the result. 150 lb is 75 − 7.5 = 67.5 kg (exact: 68.0).
  • Ounces to grams: multiply by 28 (or 30 for a rough guess).

Volume

  • Liters to US gallons: divide by 4 and add a little. 40 liters is about 10.6 gallons.
  • US gallons to liters: multiply by 4 and subtract 5 percent.
  • Cups to milliliters: multiply by 240. One cup is about 240 ml.
  • Fluid ounces to milliliters: multiply by 30.

Temperature

  • Celsius to Fahrenheit: double it and add 30. 20 °C gives 70 °F (exact: 68).
  • Fahrenheit to Celsius: subtract 30 and halve. 80 °F gives 25 °C (exact: 26.7).
  • Better accuracy: every 10 °C is 18 °F, starting from 0 °C = 32 °F.

Speed

  • km/h to mph: multiply by 0.6. 100 km/h is about 60 mph (exact 62).
  • m/s to km/h: multiply by 3.6. A speed of 10 m/s is 36 km/h.
  • Knots to mph: add 15 percent.

How accurate are these?

Most of these shortcuts land within 1 to 5 percent of the true value, which is fine for planning, shopping or checking a figure. For anything that needs precision, such as medicine, engineering, baking or customs forms, use an exact converter: our miles to kilometers, kilograms to pounds and Celsius to Fahrenheit converters give you the full result along with the formula.

A good habit is to estimate first and then calculate. If the two disagree wildly, you have probably divided when you should have multiplied.

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