Unit Converter

The metric system (officially the SI — Système International d'Unités) is the global standard for science, medicine, and trade. Every country except the United States, Liberia, and Myanmar uses metric for everyday measurements. The US has been 'about to switch' since the Metric Conversion Act of 1975, but the cost of relabeling road signs, retooling factories, and retraining workers has stalled progress for half a century.

In practice you'll need both. American recipes, lumber, and home-improvement projects use feet, inches, pounds, and Fahrenheit. International products, scientific papers, automotive specs, and most consumer electronics use meters, grams, and Celsius. A converter is the cheapest way to bridge the two without making expensive mistakes.

Length: 1 inch = 2.54 cm (exact). 1 foot ≈ 30 cm. 1 meter ≈ 3.28 ft. 1 mile ≈ 1.61 km. For quick estimates, double a metric distance and add 25% to convert km to miles roughly.

Weight: 1 lb ≈ 0.45 kg. 1 kg ≈ 2.2 lb. 1 oz ≈ 28 g. A standard letter-mail envelope under 1 oz corresponds to under ~28 g — useful when international postage uses metric.

Volume: 1 US gallon ≈ 3.785 L. 1 L ≈ 33.8 fl oz. 1 cup (US) ≈ 237 mL. Note that the US gallon is ~17% smaller than the imperial (UK) gallon — a frequent source of recipe and fuel-economy confusion.

Area: 1 sq m ≈ 10.76 sq ft. 1 acre ≈ 4,047 sq m ≈ 0.405 hectares. Real estate listings in most countries use square meters; in the US they use square feet, so a quick mental ×10 gets you close.

Most conversion factors are now defined as exact numbers rather than measured. The inch was redefined in 1959 as exactly 25.4 mm by international agreement, ending decades of slight differences between US and British inches. The pound is defined as exactly 0.453 592 37 kg.

When converting, keep the precision appropriate to your input. Converting '6 feet' to meters and reporting '1.8288 m' implies more accuracy than '6 ft' actually carries — '1.83 m' or '1.8 m' is more honest. For engineering or scientific work, follow significant-figure rules: the result shouldn't claim more precision than the least precise input. For everyday use, 2–3 decimal places is usually plenty.

The most expensive conversion mistake in cooking is confusing volume and weight. A cup of flour weighs roughly 120 g, but a cup of water is 237 g. Recipes that specify '1 cup' without weight assume a standard scoop-and-level method — international bakers should convert to grams for repeatability.

Construction lumber in the US is sold in 'nominal' dimensions: a 2×4 is actually 1.5×3.5 inches. When converting to metric for architectural drawings or international shipping, use the actual finished size, not the nominal name, or your tolerances will be off by half an inch.

Fuel economy is miles per gallon in the US and liters per 100 km in Europe. The two scales are inverses — 20 mpg ≈ 11.8 L/100km. Comparing the raw numbers directly without inverting the relationship is a common source of confusion for travelers comparing rental-car efficiency.