Convert between units of length, weight, temperature, and more. Free and instant.
The world runs on two measurement systems that trace their origins to the French Revolution. The metric system was established in 1795 by the French National Convention, designed from scratch to be rational and universal: every unit is a power-of-10 multiple of a base unit (meter for length, gram for weight, liter for volume), and all derived units are defined by physical constants rather than arbitrary objects. Today, the metric system — officially the International System of Units (SI) — is used by all countries except three: the United States, Liberia, and Myanmar. Even these exceptions use metric in specific domains: US scientific research, medicine, and military operations are all metric.
The imperial system (or more precisely the US customary system) evolved organically from medieval English units based on human-scale references: an inch was three barleycorns laid end-to-end, a foot was, well, a foot, and a mile was a thousand Roman paces. These units persist because they are deeply embedded in American infrastructure — road signs, construction standards, recipes, and everyday language — and because transition costs are enormous. The UK officially adopted metric in 1965 but retains miles on road signs and pints for beer, making it a partial-metric nation. Canada is officially metric but trades extensively with the US, creating a practical hybrid environment where people may know their height in feet and their outdoor temperature in Celsius.
These are the conversions that appear most frequently in daily life — from travel to cooking to fitness to weather. Bookmark or memorize the bolded values; they cover 90% of practical needs.
Subtract 30, then halve. 86°F: 86 − 30 = 56, 56 ÷ 2 = ~28°C (actual: 30°C). Close enough for weather conversations. For precision, use the formula: C = (F − 32) × 5/9. But nobody does that in their head at a dinner party.
Double, add 30. 20°C: 20 × 2 = 40, 40 + 30 = ~70°F (actual: 68°F). The error is under 2 degrees for typical weather ranges. For scientific work, use F = C × 9/5 + 32.
Double, then add 10%. 70 kg: 70 × 2 = 140, plus 14 = 154 lb. The exact factor is 2.205, so this 2.2x shortcut is within 0.25% of the precise value.
A Fibonacci-like shortcut: each mile is roughly 1.6 km. For quick estimation, use 1.5× and add 10%. 60 miles: 60 × 1.5 = 90, plus 6 = ~96.5 km. Or simply multiply by 8/5.
American recipes use volume (cups, tablespoons, fluid ounces) rather than weight (grams), which is standard in most other countries. This creates a problem because volume measures depend on ingredient density. One cup of flour weighs approximately 120-130 grams, but one cup of sugar weighs 200 grams, and one cup of butter weighs 227 grams. When converting a recipe, you cannot use a single cup-to-gram multiplier — you must look up the specific ingredient, or better yet, use the weight-based version of the recipe if one exists. Most serious baking cookbooks now provide both systems, because weight is intrinsically more precise and reproducible than volume.
Key cooking conversions to know: 1 stick of butter = 113 g = 8 tbsp = 1/2 cup. 1 US cup = 237 ml (often rounded to 240 ml or 250 ml depending on the country). 1 tablespoon = 3 teaspoons = 15 ml. Oven temperatures: 350°F = ~175°C (moderate), 400°F = ~200°C (hot), 450°F = ~230°C (very hot). When in doubt, use this converter tool rather than estimating — especially for baked goods where precision determines whether you get a cake or a brick.
On September 23, 1999, NASA's Mars Climate Orbiter — a $327.6 million spacecraft — disintegrated in the Martian atmosphere after a 10-month journey from Earth. The official failure review identified the root cause with devastating clarity: the spacecraft's navigation software, developed by Lockheed Martin, calculated thruster impulse in pound-seconds (imperial), while NASA's ground software expected the values in newton-seconds (metric). The mismatch caused the orbiter to enter the Martian atmosphere at 57 km instead of the planned 140-150 km — well below the altitude at which the spacecraft could survive aerodynamic heating. Every calculation, every line of code, every simulation was correct except for one uncaught unit mismatch.
The Mars Climate Orbiter is not an isolated incident. In 1983, an Air Canada flight ran out of fuel mid-flight because ground crew loaded fuel in pounds instead of kilograms, forcing an emergency landing (the "Gimli Glider"). In 1999, a Korean Air cargo flight crashed because the altimeter was calibrated in feet but the crew used meters. These disasters share the same thread: a unit conversion error, made in a mundane, routine step, that cascaded into catastrophic failure. The lesson applies far beyond aerospace: whether you are mixing medication dosages, ordering construction materials, or configuring a server's memory allocation, verify your units. Double-check. Then check again.