Metric and US Units Explained

1. Length

Since 1959, the inch has been defined as exactly 2.54 cm. A foot is 12 inches (30.48 cm), a yard 3 feet (91.44 cm) and a mile 1,760 yards (1.609 km). Rule of thumb: a meter is a little more than a yard, and 5 miles is about 8 km.

2. Weight

A pound is exactly 0.45359237 kg, and there are 16 ounces in a pound. Rule of thumb: 1 kg is about 2.2 lb, so halve the pounds and take off 10% to get kilograms. In the UK, body weight is often given in stone: 1 stone is 14 lb, about 6.35 kg.

3. Temperature

Water freezes at 0 °C (32 °F) and boils at 100 °C (212 °F) at sea level. Rule of thumb: double the Celsius value and add 30 for a rough Fahrenheit value; 20 °C is about 70 °F.

4. Volume and recipes

A US gallon is 3.785 liters and a UK gallon 4.546 liters, so check which one a recipe or fuel figure means. For baking, weigh dry ingredients: a cup of flour can weigh anything from 115 to 150 g depending on how it is filled.

5. Shoe and ring sizes

Size systems are not exact unit conversions, because each country and brand uses its own steps. Measuring your foot length or your finger and using a chart is more reliable than converting one size number to another.

Rules of thumb

Easy approximations for everyday use:

• 1 inch ≈ 2.5 cm, and 30 cm ≈ 1 foot.
• 1 meter ≈ 3.3 feet, and 1 km ≈ 0.6 miles.
• 1 kg ≈ 2.2 lb, and 100 g ≈ 3.5 oz.
• 1 liter ≈ 1 US quart, and 4 liters ≈ 1 US gallon.
• 1 tablespoon = 3 teaspoons ≈ 15 ml.
• °F ≈ °C × 2 + 30.