Unit Conversion Reference: Length, Weight, Temperature, and Speed

Whether you're traveling abroad, following an international recipe, reading scientific papers, or working with international colleagues, unit conversion is a daily necessity. This guide covers the most common conversions you'll need.

Temperature

Temperature is the most confusing conversion because the scales don't share a zero point.

Key formulas:

• Celsius to Fahrenheit: °F = (°C × 9/5) + 32
• Fahrenheit to Celsius: °C = (°F - 32) × 5/9
• Celsius to Kelvin: K = °C + 273.15

Useful reference points:

• Water freezes: 0°C = 32°F
• Room temperature: 20-22°C = 68-72°F
• Body temperature: 37°C = 98.6°F
• Water boils: 100°C = 212°F
• Both scales meet: -40°C = -40°F

Quick mental math: For a rough Celsius to Fahrenheit estimate, double the Celsius value and add 30. Example: 20°C → 40 + 30 = 70°F (actual: 68°F). Close enough for daily use.

Use our Temperature Converter for precise conversions.

Length

The metric system is base-10 (easy to convert within), but converting to imperial requires memorizing key factors.

Essential factors:

• 1 inch = 2.54 cm (exactly)
• 1 foot = 30.48 cm = 12 inches
• 1 yard = 91.44 cm = 3 feet
• 1 mile = 1.609 km = 5,280 feet
• 1 km = 0.621 miles

Quick estimates:

• Inches to cm: multiply by 2.5
• Feet to meters: multiply by 0.3
• Miles to km: multiply by 1.6
• Km to miles: multiply by 0.6

Use our Length Converter for exact calculations.

Weight (Mass)

Essential factors:

• 1 ounce (oz) = 28.35 grams
• 1 pound (lb) = 453.6 grams = 16 ounces
• 1 kilogram = 2.205 pounds
• 1 stone = 14 pounds = 6.35 kg
• 1 US ton (short ton) = 2,000 lbs = 907.2 kg
• 1 metric tonne = 1,000 kg = 2,204.6 lbs

Quick estimates:

• Pounds to kg: divide by 2.2
• Kg to pounds: multiply by 2.2
• Ounces to grams: multiply by 28

Cooking conversions:

• 1 cup flour ≈ 120g
• 1 cup sugar ≈ 200g
• 1 cup butter ≈ 227g (2 sticks)
• 1 tablespoon ≈ 15g

Use our Weight Converter for precision.

Speed

Essential factors:

• 1 km/h = 0.621 mph
• 1 mph = 1.609 km/h
• 1 knot = 1.852 km/h = 1.151 mph
• 1 m/s = 3.6 km/h

Quick estimates:

• Km/h to mph: multiply by 0.6
• Mph to km/h: multiply by 1.6

Common speed references:

• Walking: 5 km/h = 3.1 mph
• Cycling: 20 km/h = 12.4 mph
• City driving: 50 km/h = 31 mph
• Highway: 100 km/h = 62 mph = 54 knots

Use our Speed Converter for all velocity conversions.

Why Two Systems Exist

The metric system (SI) was developed in France in the 1790s as a rational, base-10 system. Most of the world adopted it during the 19th and 20th centuries. The United States, Liberia, and Myanmar still primarily use imperial/customary units for everyday measurements, though US science and military use metric.

The practical result: anyone working internationally needs to convert regularly, which is why we built these conversion tools to work instantly in your browser.