Conversion Formulas Handbook: Every Formula You Need

The metric system is based on the meter. The US customary system is anchored to the inch, which is defined as exactly 25.4 millimeters — an exact, legally defined relationship that makes all length conversions between systems perfectly precise. The core length formulas: feet to meters: m = ft × 0.3048. Meters to feet: ft = m ÷ 0.3048 (or × 3.28084). Inches to centimeters: cm = in × 2.54. Miles to kilometers: km = mi × 1.60934. Kilometers to miles: mi = km × 0.62137.

Quick mental shortcuts: to convert km to miles, multiply by 0.6 (within 3.5%); to convert feet to meters, multiply by 0.3. For inches to centimeters, multiply by 2.5 (within 1.6%). These are useful for estimation but use the exact factors for calculations requiring precision. Worked example: 100 feet = 100 × 0.3048 = 30.48 meters. A 400-meter athletic track = 400 ÷ 0.3048 = 1,312.3 feet = 437.4 yards.

The kilogram is the SI base unit of mass. The pound is defined as exactly 0.45359237 kilograms — another exact legal definition. This means all weight conversions between metric and US customary are precise to any number of decimal places you need. Core formulas: pounds to kilograms: kg = lb × 0.453592. Kilograms to pounds: lb = kg × 2.20462. Ounces to grams: g = oz × 28.3495. Grams to ounces: oz = g ÷ 28.3495.

Worked example: 175 lb = 175 × 0.453592 = 79.38 kg. Converting back: 79.38 × 2.20462 = 175.0 lb — the round-trip confirms accuracy. For cooking: 1 ounce = 28.35 grams, so a 16 oz (1 lb) block of cheese = 453.6 grams. Quick check: multiply pounds by 0.45 for a fast kilogram estimate (within 0.6%).

Temperature conversions are the one exception to simple multiplication — because the scales have different zero points, you need addition or subtraction too. Celsius to Fahrenheit: °F = (°C × 9/5) + 32. Fahrenheit to Celsius: °C = (°F − 32) × 5/9. Celsius to Kelvin: K = °C + 273.15. Fahrenheit to Kelvin: K = (°F + 459.67) × 5/9.

Worked examples: 100°C (boiling water) = (100 × 9/5) + 32 = 180 + 32 = 212°F. Body temperature 98.6°F = (98.6 − 32) × 5/9 = 66.6 × 5/9 = 37°C. Absolute zero = 0 K = −273.15°C = −459.67°F. Quick mental shortcut for weather: double the Celsius and add 30 gives a rough Fahrenheit estimate (e.g., 20°C → 40 + 30 = 70°F; actual: 68°F).

Area scales as the square of linear dimensions. One meter = 3.28084 feet, but one square meter = 3.28084² = 10.7639 square feet. This squared relationship is the most common source of area conversion errors. Core formulas: square feet to square meters: m² = ft² × 0.092903. Square meters to square feet: ft² = m² × 10.7639. Acres to hectares: ha = ac × 0.404686. Hectares to acres: ac = ha × 2.47105.

Worked example: a 2,000 sq ft house = 2,000 × 0.092903 = 185.8 m². A 5-acre property = 5 × 0.404686 = 2.023 hectares. One football field (including end zones) = about 1.32 acres = 0.535 hectares = 5,350 m². Land in the US is sold in acres; land in most other countries is sold in hectares — this conversion matters for international real estate.

Volume mixes two incompatible systems: metric (liters, milliliters) and US customary (gallons, quarts, pints, cups, fluid ounces). Key relationships: 1 US gallon = 3.78541 liters. 1 liter = 0.264172 gallons. 1 cup = 236.588 mL. 1 fluid ounce = 29.5735 mL. 1 tablespoon = 14.7868 mL (= 3 teaspoons). 1 teaspoon = 4.92892 mL.

Worked example: a 2-liter soda bottle = 2 × 0.264172 = 0.528 gallons = 2.11 quarts = 8.45 cups. A US recipe calls for 3 cups of flour: 3 × 236.588 = 710 mL. Note: UK cup = 250 mL and Australian cup = 250 mL; US cup = 236.6 mL — check your recipe's country of origin when precision matters.

Pressure: PSI to kPa: kPa = PSI × 6.89476. Bar to PSI: PSI = bar × 14.5038. Standard atmosphere (1 atm) = 101.325 kPa = 14.696 PSI = 1.01325 bar. Speed: mph to km/h: km/h = mph × 1.60934. Knots to km/h: km/h = kn × 1.852. Meters per second to mph: mph = m/s × 2.23694.

Data storage uses two parallel systems — SI decimal (1 KB = 1,000 bytes) and binary (1 KiB = 1,024 bytes). For storage: MB to GB: GB = MB ÷ 1,000. GB to TB: TB = GB ÷ 1,000. For binary equivalents: MiB to MB: MB = MiB × 1.048576. GiB to GB: GB = GiB × 1.073741824. Time: minutes to seconds: s = min × 60. Hours to seconds: s = hr × 3,600. Days to seconds: s = days × 86,400.