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📏 Metric and Imperial Units

An inch is about 2.5 cm and a kilogram is about 2.2 pounds. Learn the handy bridges between metric and the older English-derived units, and estimate any measurement across both.

5
lessons
~12 min
to learn
🔢 Math
subject
Adults
level
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What you’ll learn

  1. Two Families of UnitsUnderstand that a measurement is a number plus a unit, and that the world uses two unit families — metric and the older English-derived units.A number means nothing without a unit. Metric (metres, grams, litres) counts in tens and is used almost everywhere. The older English-derived family (inches, pounds, gallons) uses mixed steps. The popular 'only three countries still use imperial' map is a myth: the United States is a hybrid, and so is the United Kingdom.
  2. The Length BridgeConvert lengths between inches/centimetres and miles/kilometres using simple bridges.One inch is exactly 2.54 cm and about 2.5 cm in your head. One mile is about 1.6 km. Because a unit and its bridge are just two labels for one real length, converting is multiplying — and the smaller unit always gives the bigger number.
  3. The Weight BridgeConvert masses between kilograms/pounds and grams/ounces.One kilogram is about 2.2 pounds; one ounce is about 28 grams. Metric steps by tens and thousands while the older family uses 16 ounces to a pound, but both name the same mass, joined by a small multiplier.
  4. Liquids and TemperatureConvert liquid volumes (litres/U.S. gallons) and understand why temperature conversion needs a shift as well as a stretch.A U.S. gallon is about 3.8 litres; a UK imperial gallon is about 4.5 litres. Temperature is trickier: Celsius and Fahrenheit start at different zeros (0°C vs 32°F freezing), so converting requires shifting and stretching, best handled with memorised NIST anchor points.
  5. Estimating Like a ProEstimate across measurement systems quickly using rounded bridges, then go read a real label.Everyday life needs good estimates, not exact figures. Rounding the bridges (inch is about 2.5 cm, mile is about 1.6 km, kg is about 2.2 lb) lets you guess within about ten percent. Tonight, try one label in the other family.

Questions this course answers

Your cat's weight is written as just "5." Why is that not a real measurement?

Every measurement is a number PLUS a unit. Without the unit, "5" could mean wildly different amounts, so the number alone tells you nothing.

Why do scientists and most countries prefer the metric system?

Metric counts in tens, so you jump between sizes by sliding a decimal point. The older family uses mixed steps (12, 3, 16, 1760) that must each be memorised.

A centimetre is smaller than an inch. So when you measure a desk, the number of centimetres will be...

The smaller the unit, the more of them it takes to cover the same length — so the centimetre count is larger than the inch count for the same desk.

A bag of flour is labelled 1 kilogram. About how many pounds is that?

The weight bridge is 1 kg is about 2.2 lb. A kilogram is the bigger unit, so it equals more than one pound.

Why can't you convert Celsius to Fahrenheit just by multiplying, like you do with length?

Water freezes at 0°C but 32°F — the scales don't share a zero. So converting needs both a shift (the 32) and a stretch, not a single multiply.

In everyday life, what is usually the most useful skill when you meet a measurement in the other system?

Daily life rarely needs exact figures. Rounding the bridge (inch is about 2.5 cm, mile is about 1.6 km, kg is about 2.2 lb) lets you estimate fast and confidently.

Grounded in trusted sources

  • NIST Guide to the SI, Appendix B.9 — exact factors: 1 in = 2.54 cm; 1 mi = 1.609344 km; 1 lb (avoirdupois) = 0.45359237 kg, https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b9
  • NIST SP 365 Metric Conversion Card — approximate everyday bridges: 1 in × 2.54 = cm; 1 mi × 1.61 = km; 1 lb × 0.45 = kg; 1 kg × 2.20 = lb; 1 U.S. gal × 3.79 = L, https://www.nist.gov/pml/owm/metric-si/unit-conversion/approximate-conversions-us-customary-measures-metric and https://www.nist.gov/pml/owm/metric-si/unit-conversion/approximate-conversions-metric-us-customary-measures
  • NIST — Busting Myths about the Metric System (Elizabeth Benham, 2020) and U.S. Metrication FAQs: the United States / Liberia / Myanmar 'only three countries' story is untrue; every country has adopted SI, including the U.S.; metric legal since 1866 and preferred for U.S. trade since 1988; U.S. practice is a hybrid continuum, https://www.nist.gov/blogs/taking-measure/busting-myths-about-metric-system and https://www.nist.gov/pml/owm/faqs/us-metrication-faqs
  • NIST — SI Units Temperature: water freezes at 0 °C and boils at about 100 °C; touchstones 0/32 ice, 20/68 room, 30/86 hot day, 37/99 body, 100/212 boiling; exact convert is ×1.8 then +32, https://www.nist.gov/pml/owm/si-units-temperature
  • NIST — Metric (SI) Prefixes: milli, centi, and kilo mean thousandth, hundredth, and thousand, so metric steps by tens, hundreds, and thousands, https://www.nist.gov/pml/owm/metric-si-prefixes
  • GOV.UK — Weights and measures: the law: most packaged and loose goods in England, Scotland, or Wales must be sold in metric units; imperial-only exceptions are draught beer or cider by the pint, milk in returnable containers by the pint, and precious metals by the troy ounce, https://www.gov.uk/weights-measures-and-packaging-the-law

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