Most of the world uses metric. The US uses imperial. If you deal with American recipes, British road signs, or international anything, you need quick conversion instincts. Forget memorizing exact factors. Learn the approximations.

Length: The Key Anchors

  • 1 inch = 2.5 cm (actually 2.54, but 2.5 works)
  • 1 foot = 30 cm
  • 1 mile = 1.6 km
  • 1 meter = 3.3 feet (just over a yard)

Quick tricks:

  • To convert miles to km, multiply by 1.6. Or just add 60% (50 miles = 80 km).
  • km to miles: multiply by 0.6. Or take 60% (100 km = 60 miles).
  • Your height: 5’10” = 5 x 30 + 10 x 2.5 = 150 + 25 = 175 cm.

Weight: The Essentials

  • 1 kg = 2.2 pounds
  • 1 pound = 454 grams (roughly half a kg)
  • 1 ounce = 28 grams

Quick tricks:

  • kg to pounds: double and add 10%. (70 kg = 140 + 14 = 154 lbs)
  • Pounds to kg: halve and subtract 10%. (150 lbs = 75 - 7.5 = 67.5 kg)

Volume: Kitchen Math

  • 1 liter = about 4 cups (4.2 technically)
  • 1 gallon = 3.8 liters (call it 4)
  • 1 cup = 240 ml
  • 1 tablespoon = 15 ml
  • 1 teaspoon = 5 ml

Quick tricks:

  • Liters to gallons: divide by 4. (8 liters = 2 gallons)
  • American recipes in ml: 1 cup = 240, half cup = 120, quarter cup = 60

Temperature: The Tricky One

  • Celsius to Fahrenheit: multiply by 1.8, add 32
  • Fahrenheit to Celsius: subtract 32, divide by 1.8

The exact formula is annoying for mental math. Use these anchors instead:

Celsius Fahrenheit What it means
0 32 Freezing
10 50 Cool jacket weather
20 68 Room temperature
30 86 Hot day
37 98.6 Body temperature
40 104 Very hot / fever
100 212 Boiling

Rough trick: Double Celsius, add 30. It’s off by a few degrees but close enough for weather.

  • 25°C: 50 + 30 = 80°F (actual: 77°F). Close enough to know it’s warm.

Area

  • 1 acre = about 4,000 sq meters (4,047 exactly)
  • 1 hectare = 2.5 acres
  • 1 sq foot = 0.09 sq meters (roughly 1/11)
  • 1,000 sq feet = 93 sq meters

The One Rule

Don’t memorize everything. Memorize the anchors (1 inch = 2.5 cm, 1 kg = 2.2 lbs, 1 mile = 1.6 km, 0°C = 32°F) and derive the rest. Five anchors cover 90% of situations.