Customary Units: Inches, Feet, Cups and Pounds That Finally Make Sense

Metric units are easy — everything is a power of ten. US customary units are not, and pretending otherwise is why children guess. Twelve inches in a foot, three feet in a yard, sixteen ounces in a pound, eight ounces in a cup, two cups in a pint, two pints in a quart, four quarts in a gallon. These have to be known, and there is no pattern to lean on.

So attach them to the child's body and kitchen. An inch is the top joint of a thumb. A foot is roughly a school ruler. A yard is one big stride. A pound is a block of butter. A gallon is the big milk jug. Estimation before measurement is not a warm-up, it is the thing that makes a wrong answer visible: a child who thinks the table is 3 inches long has not been careless, they have no mental picture of an inch.

The one rule for converting. Going to a smaller unit, the number gets bigger — 2 feet is 24 inches. Going to a bigger unit, the number gets smaller. Children who check the direction first stop dividing when they should multiply.

Reading a ruler is a fractions lesson hiding in plain sight. The marks between the inches are halves, quarters, eighths, sixteenths — and a child who can read 2 3/8 on a ruler understands fractions better than one who has only shaded circles.

Do the metric side too. Millimetres, centimetres, metres, grams, kilograms, millilitres, litres — all powers of ten, all a matter of moving the decimal point. Meeting both systems side by side makes each one clearer.

Related: types of angles, reading a clock, ratios and proportions and learning games for 4th graders.