Metric & Imperial Conversion
Understand and use approximate equivalences between metric units and common imperial units (inches, pounds, pints)
What a learner can do afterwards
- State that 1 inch ≈ 2.5 cm and use this to estimate length in inches
- Know that 1 kg ≈ 2.2 pounds and estimate a person's weight in pounds
- Convert approximately between litres and pints (1 litre ≈ 1.75 pints)
1 · Read
Some places measure in metric units, like centimeters, kilograms, and litres. Other places also use imperial units, like inches, pounds, and pints. Both measure the same real-world sizes, just with different numbers. You don't need to be exact. A close estimate is enough.
A baby weighs 7 pounds. To estimate kilograms, divide by about 2.2: 7 divided by 2.2 is close to 3.2. So a 7-pound baby weighs a little over 3 kg.
Quick trick: to turn kilograms into pounds, double the number, then add a bit more. To turn litres into pints, add three quarters of the number you started with.
Remember the three swaps: about 2.5 cm to an inch, about 2.2 pounds to a kilogram, and about 1.75 pints to a litre.
More to read
Metric units of length review (mm, cm, m, & km)
Metric length is tidy, but many countries still use inches. About 2.5 cm make 1 inch, so multiply inches by 2.5 to guess centimetres. A 4 inch phone screen is about 10 cm wide. To go back, divide centimetres by 2.5. A 50 cm shelf is about 20 inches long. These are estimates, close enough for shopping and crafts but not for engineering.
US Customary units of length review (in, ft, yd, & mi)
Inches, feet, yards, and miles are the customary length units. There are 12 inches in a foot, 3 feet in a yard, and 1760 yards in a mile. They link in twelves and threes instead of tens. Convert inside the system with those links. 2 feet is 24 inches, and 36 inches is 1 yard. Metric and customary meet at 1 inch being about 2.5 cm.
US Customary units of weight review (oz & lb)
Weight comes in grams and kilograms on metric scales, and ounces and pounds on older scales. About 2.2 pounds make 1 kilogram, so multiply kilograms by 2.2 to estimate pounds. A 5 kg bag of potatoes is about 11 pounds. People still quote body weight in pounds in some countries. A 40 kg child weighs about 88 pounds. Divide pounds by 2.2 to return to kilograms.
US Customary units of volume review (c, pt, qt, & gal)
Volume splits into litres and millilitres in metric, and cups, pints, quarts, and gallons in customary. About 1.75 pints make 1 litre, so multiply litres by 1.75. A 4 litre jug holds about 7 pints. Recipes jump between systems all the time. Knowing 1 litre is near 1.75 pints saves a cake. Always check which pint a recipe means when precision matters.
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2 · Watch
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Where it sits
Learn first
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Where this leads
18 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.