Ultimate Unit Conversion Guide: Length, Weight, Temperature & More

Unit conversion is one of those skills that surfaces constantly — in a foreign recipe, a science lab, an international flight, a construction project, or a medical form. Getting it wrong has real consequences: NASA lost a $125 million Mars orbiter in 1999 because one team used metric units and another used imperial.

This guide is a complete, bookmark-worthy reference. Every major unit category is covered with exact conversion factors, formulas, worked examples, and mental math shortcuts for when you need a quick estimate without a calculator.

How Unit Conversion Works

Every conversion follows the same principle: multiply by a conversion factor — the ratio between two units that equals exactly 1.

Result = Original Value × Conversion Factor

Example: Convert 5 miles to kilometres

• Conversion factor: 1 mile = 1.60934 km
• 5 × 1.60934 = 8.047 km

To convert in the reverse direction, divide by the same factor (or multiply by its reciprocal):

• 8.047 km ÷ 1.60934 = 5 miles

Temperature is the exception — it requires a formula rather than simple multiplication because its scales have different zero points.

Quick Reference: Most-Used Conversions at a Glance

1. Length Conversions

Metric ↔ Imperial Reference Table

Metric Length Scale

1 km = 1,000 m = 100,000 cm = 1,000,000 mm
1 m  = 100 cm = 1,000 mm
1 cm = 10 mm

Imperial Length Scale

1 mile = 1,760 yards = 5,280 feet = 63,360 inches
1 yard = 3 feet = 36 inches
1 foot = 12 inches

Height Conversion Chart (cm ↔ ft/in)

Mental Math Shortcuts

• km → miles: Multiply by 0.6 (exact: 0.621). 100 km ≈ 60 miles
• miles → km: Multiply by 1.6 (exact: 1.609). 50 miles ≈ 80 km
• cm → inches: Divide by 2.5 (exact: divide by 2.54). 30 cm ≈ 12 inches
• metres → feet: Multiply by 3 and add 10% (exact: × 3.281). 2 m ≈ 6.6 ft

2. Weight & Mass Conversions

Conversion Reference Table

Weight Scale Reference

Metric:   1 tonne = 1,000 kg = 1,000,000 g = 1,000,000,000 mg
Imperial: 1 stone = 14 lbs = 224 oz
          1 lb    = 16 oz

Body Weight Reference Chart (kg ↔ lbs ↔ stones)

Mental Math Shortcuts

• kg → lbs: Multiply by 2.2. 75 kg ≈ 165 lbs
• lbs → kg: Divide by 2.2. 180 lbs ≈ 81.8 kg
• g → oz: Divide by 28. 280 g ≈ 10 oz
• oz → g: Multiply by 28. 6 oz ≈ 168 g

3. Temperature Conversions

Temperature scales differ in both their zero point and their degree size — which is why a formula is required rather than a simple multiplication factor.

Conversion Formulas

Key Temperature Reference Points

Mental Math Shortcuts

• °C → °F (quick): Double the Celsius, then add 30. 20°C ≈ 40 + 30 = 70°F (exact: 68°F)
• °F → °C (quick): Subtract 30, then halve. 70°F → 40 ÷ 2 = 20°C (exact: 21.1°C)
• The crossover point: −40°C = −40°F (the two scales meet at this exact point)

4. Volume Conversions

Liquid Volume Reference Table

⚠️ US vs UK Gallon Warning: 1 US gallon = 3.785 L but 1 UK (Imperial) gallon = 4.546 L — a 20% difference. Always verify which system a recipe or specification uses. This applies to pints and fluid ounces too: a US pint = 473 ml, a UK pint = 568 ml.

Cooking Volume Conversions

Dry Volume / Cooking Weights

Baker's tip: Weighing ingredients in grams is always more accurate than volume measurements. A "cup of flour" can vary by 20–30% depending on how it's scooped.

5. Speed Conversions

Speed Reference Table

Common Speed Reference Points

6. Area Conversions

Area Reference Table

Area Size Reference

7. Pressure Conversions

Pressure Reference Table

Practical Pressure Reference Points

8. Energy Conversions

Energy Reference Table

Calorie confusion: In nutrition, "Calories" (capital C) on food labels are actually kilocalories (kcal). So a 200-Calorie snack contains 200 kcal = 836 kJ of energy.

9. Clothing Size Conversions

Women's Clothing Sizes

Men's Clothing Sizes

Shoe Sizes

10. Common Conversion Mistakes to Avoid

Frequently Asked Questions

Q: What is the easiest way to convert Celsius to Fahrenheit mentally? Double the Celsius temperature and add 30. It's not exact (the real formula adds 32, not 30, and multiplies by 1.8 not 2) but gives a result within 2–3°F for everyday temperatures. For 20°C: 20 × 2 + 30 = 70°F (exact: 68°F).

Q: Why do the US and UK use different gallon sizes? The US gallon is based on the Queen Anne wine gallon (1707), while the UK Imperial gallon was standardised later in 1824. They diverged historically and were never reconciled. The US also uses a different pint (473 ml) vs the UK pint (568 ml) — which is why a "pint" in a British pub is noticeably larger.

Q: What's the difference between mass and weight? Mass (measured in kg or lbs) is the amount of matter in an object — it doesn't change regardless of location. Weight (measured in Newtons) is the force of gravity acting on that mass — it would be different on the Moon. In everyday conversation, "weight" almost always means mass, and kg/lbs are used interchangeably.

Q: How do I convert a recipe from cups to grams? There is no single conversion factor — it depends on the ingredient's density. Use the dry volume cooking table in this guide, or weigh ingredients directly. Flour, sugar, and butter all have different weights per cup.

Q: Which countries still use imperial units? Only three countries have not officially adopted the metric system as their primary standard: the United States, Liberia, and Myanmar. The UK uses a mix — metric for most official purposes but miles for road distances and pints for beer.

Quick Reference Summary

Use our free Unit Converter to convert any value instantly across all categories — no mental math required.