Countries and industries use mixtures of unit systems. U.S. customary units are not identical to British imperial units: a U.S. liquid gallon and an imperial gallon, for example, have different volumes. The NIST Metric (SI) Program explains the International System of Units and its use in U.S. trade and commerce. When cooking, traveling or comparing specifications, first identify the exact unit definition, then apply the appropriate conversion.
Why U.S. Customary Units Remain Common
U.S. customary measurements developed from older English units; the British imperial system was defined later and differs in important volume measures. Congress authorized metric use in 1866, and the Metric Conversion Act of 1975 established a voluntary conversion policy. NIST describes SI as the preferred system for U.S. trade and commerce. Everyday road signs, recipes and measurements still commonly use customary units, while science and many industrial applications use SI. This mixed usage is why checking a unit label matters.
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
Recipes may measure ingredients by volume or mass. Cups and grams are not interchangeable without an ingredient's density: packing, moisture and the ingredient itself can change the mass in a cup. For precise recipes, use the specified unit and a scale where mass is requested. NIST's International System of Units reference defines measurement units; it does not supply a universal cup-to-gram factor for foods.
| 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.