By the iCalculateFast Editorial Team · Utilities · Published June 23, 2026 · 6 min read
The United States, Myanmar, and Liberia are the only three countries that have not officially adopted the International System of Units — the metric system — as their primary standard. For anyone who cooks from international recipes, travels abroad, works with non-US colleagues, or reads product specifications from overseas manufacturers, unit conversion is a practical daily requirement. Here is a complete reference for the conversions that come up most often.
Why the US Still Uses Imperial Units
The US adopted British imperial measurements at its founding. Congress authorized metric use as early as 1866, and the Metric Conversion Act of 1975 made metrication voluntary — but never mandatory. Infrastructure, tradition, and the cost of transition have kept imperial units dominant in everyday US life: construction, road signage, cooking measurements, and body measurements remain imperial. Meanwhile, science, medicine, pharmaceuticals, the military, and international trade largely use metric. The result is a hybrid system that most Americans navigate daily without fully realizing it.
Temperature
To convert Fahrenheit to Celsius: C = (F − 32) × 5/9. To convert Celsius to Fahrenheit: F = (C × 9/5) + 32. A few anchor points make mental estimation practical without the formula:
| °Fahrenheit | °Celsius | Reference |
|---|---|---|
| 32°F | 0°C | Water freezes |
| 50°F | 10°C | Cool weather — light jacket needed |
| 68°F | 20°C | Comfortable room temperature |
| 98.6°F | 37°C | Normal human body temperature |
| 212°F | 100°C | Water boils at sea level |
| 325°F | 163°C | Low oven (baking delicate items) |
| 375°F | 190°C | Moderate oven (most baking) |
| 450°F | 232°C | Hot oven (roasting, pizza, searing) |
Distance and Length
The key conversion is 1 mile = 1.60934 km, and 1 km = 0.62137 miles. For practical navigation: 60 mph = 96.6 km/h; 100 km/h = 62.1 mph. A quick mental shortcut — multiply km/h by 0.6 to approximate mph.
| Imperial | Metric Equivalent |
|---|---|
| 1 inch | 2.54 cm |
| 1 foot (12 inches) | 30.48 cm |
| 1 yard (3 feet) | 91.44 cm / 0.914 m |
| 1 mile (5,280 feet) | 1.609 km |
| 5 miles | 8.047 km |
| 26.2 miles (marathon) | 42.195 km |
Weight and Mass
The key conversion is 1 kilogram = 2.20462 pounds, commonly rounded to 2.2 lbs/kg. To convert body weight: divide pounds by 2.2 to get kilograms. A 180-pound person weighs approximately 81.6 kg; a 70 kg person weighs approximately 154 pounds.
| Imperial | Metric Equivalent |
|---|---|
| 1 ounce (oz) | 28.35 grams |
| 1 pound (lb) | 453.6 grams / 0.4536 kg |
| 10 pounds | 4.54 kg |
| 100 pounds | 45.4 kg |
| 150 pounds | 68.0 kg |
| 200 pounds | 90.7 kg |
Cooking Measurements
US recipes use a volume-based system (cups, tablespoons, teaspoons) while most international and professional recipes use weight (grams). Volume measurements are less precise because density varies by ingredient — one cup of all-purpose flour weighs approximately 120–130 grams, while one cup of almond flour weighs about 96 grams. For precision baking, use a scale and grams. NIST's official unit conversion tables provide authoritative reference values for all standard measurements.
| US Volume | Metric Equivalent |
|---|---|
| 1 teaspoon (tsp) | 4.93 mL |
| 1 tablespoon (tbsp) | 14.79 mL |
| 1 fluid ounce | 29.57 mL |
| ¼ cup | 59.15 mL |
| ½ cup | 118.3 mL |
| 1 cup | 236.6 mL |
| 1 pint (2 cups) | 473.2 mL |
| 1 quart (4 cups) | 946.4 mL |
| 1 gallon | 3.785 liters |
Area
For real estate and home measurement: 1 square foot = 0.0929 square meters. To convert square feet to square meters, divide by 10.764. A 1,000 sq ft apartment is approximately 93 square meters. For land: 1 acre = 4,047 square meters = 0.4047 hectares. One hectare = 2.471 acres.
Fuel Economy
The US measures fuel efficiency in miles per gallon (MPG); most other countries use liters per 100 kilometers (L/100km). The two systems have an inverse relationship — higher MPG and lower L/100km both indicate better efficiency. To convert: L/100km = 235.21 ÷ MPG.
| US MPG | Liters per 100 km |
|---|---|
| 20 MPG | 11.76 L/100km |
| 25 MPG | 9.41 L/100km |
| 30 MPG | 7.84 L/100km |
| 35 MPG | 6.72 L/100km |
| 40 MPG | 5.88 L/100km |
| 50 MPG (hybrid) | 4.70 L/100km |
Convert Any Unit Instantly
The conversions above cover the most common everyday cases, but dozens of unit types — speed, pressure, energy, data storage, and more — come up in real use. Our Unit Converter handles all of them instantly in both directions, without requiring any formula memorization. Enter your value, select the unit type, and get the result.
Conversion Questions People Ask Most
Why is a US gallon different from a UK gallon?
The two countries standardized on different base measures in the 1800s. A US gallon is 128 US fluid ounces (about 3.785 liters), while a UK imperial gallon is 160 imperial fluid ounces (about 4.546 liters) — roughly 20% larger. That's why a car rated at 40 miles per gallon in a British review delivers only about 33 mpg by American math. Recipes are affected too: a UK pint is 568 ml against the US pint's 473 ml.
Is a kilogram a weight or a mass?
Strictly, a kilogram measures mass and a pound measures force — but on Earth's surface the distinction rarely matters for daily life, which is why scales freely display either. The everyday factor to memorize is that 1 kilogram equals about 2.205 pounds, and the exact legal definition runs the other way: one pound is exactly 0.45359237 kilograms, fixed by international agreement in 1959.
What's the fastest mental math for Celsius to Fahrenheit?
Double the Celsius figure and add 30. It's not exact — the true formula multiplies by 1.8 and adds 32 — but it lands within a couple of degrees across the everyday range. So 20°C estimates to 70°F (actual: 68°F) and 35°C estimates to 100°F (actual: 95°F). For anything where precision matters, like an oven temperature or a medical reading, skip the shortcut and use the exact conversion.
Which countries still use imperial units?
Only three countries have never officially adopted the metric system: the United States, Liberia, and Myanmar. Even so, the US is quietly hybrid — soda comes in 2-liter bottles, medicine is dosed in milligrams, and science and the military run fully metric. The UK is the reverse hybrid: officially metric, but road signs remain in miles and pubs still pour imperial pints.
Sources & References
- NIST — U.S. Metric Program
- NIST — The International System of Units (SI)
- Metric Conversion Act of 1975 (Public Law 94-168)
- Bureau International des Poids et Mesures — The SI Brochure
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