How to Convert Units of Measurement (Complete Guide)

Unit conversion is a skill you use more often than you think. Cooking a recipe from a British website, buying lumber measured in meters, checking your luggage weight before a flight, or reading a scientific paper that uses SI units—all of these require translating between measurement systems.

This guide walks through the two major systems, the most common conversion factors across length, weight, and volume, and mental math tricks that let you estimate conversions without a calculator. For instant results, use our Free Unit Converter.

The Two Systems: Metric vs. Imperial

Most of the world uses the metric system (officially the International System of Units, or SI). The United States, Liberia, and Myanmar are the only countries that still primarily use the imperial/US customary system for everyday measurements.

Metric is built on powers of 10. Every prefix shifts the decimal point:

Prefix	Symbol	Factor	Example
Kilo-	k	1,000	1 kilometer = 1,000 meters
Hecto-	h	100	1 hectoliter = 100 liters
Deca-	da	10	1 decagram = 10 grams
(base)	—	1	meter, gram, liter
Deci-	d	0.1	1 decimeter = 0.1 meters
Centi-	c	0.01	1 centimeter = 0.01 meters
Milli-	m	0.001	1 millimeter = 0.001 meters

Imperial uses irregular relationships. There are 12 inches in a foot, 3 feet in a yard, 1,760 yards in a mile, 16 ounces in a pound, and 8 pints in a gallon. There’s no consistent multiplier, which is why conversions within imperial are often just as tricky as converting between systems.

Length Conversions

Length is the most frequently converted measurement. Here are the key factors:

Conversion	Factor	Direction
1 inch	2.54 cm	Imperial to Metric
1 foot	30.48 cm (0.3048 m)	Imperial to Metric
1 yard	0.9144 m	Imperial to Metric
1 mile	1.609 km	Imperial to Metric
1 cm	0.3937 inches	Metric to Imperial
1 meter	3.281 feet	Metric to Imperial
1 km	0.6214 miles	Metric to Imperial

Example: Convert 5 feet 10 inches to centimeters.

Convert everything to inches: (5 x 12) + 10 = 70 inches
Multiply by 2.54: 70 x 2.54 = 177.8 cm

Example: Convert 100 meters to feet.

100 x 3.281 = 328.1 feet

Use the Length Converter for precise multi-unit results.

Weight and Mass Conversions

Conversion	Factor
1 ounce (oz)	28.35 grams
1 pound (lb)	453.6 grams (0.4536 kg)
1 stone	6.35 kg (14 lbs)
1 US ton	907.2 kg
1 metric ton	1,000 kg (2,204.6 lbs)
1 gram	0.03527 oz
1 kilogram	2.205 lbs

Example: Your suitcase weighs 23 kg. How many pounds is that?

23 x 2.205 = 50.7 lbs

Example: A recipe calls for 8 ounces of flour. How many grams?

8 x 28.35 = 226.8 grams

Check conversions instantly with the Weight Converter.

Volume Conversions

Volume conversions are tricky because US and UK gallons are different sizes.

Conversion	Factor
1 US fluid ounce	29.57 mL
1 US cup	236.6 mL
1 US pint	473.2 mL
1 US quart	946.4 mL
1 US gallon	3.785 liters
1 UK gallon	4.546 liters
1 liter	0.2642 US gallons
1 liter	33.81 US fluid ounces

Example: A European car has a 55-liter fuel tank. How many US gallons does it hold?

55 x 0.2642 = 14.5 US gallons

Watch out: UK pints (568 mL) are larger than US pints (473 mL). If a British recipe says “half a pint of cream,” that is about 284 mL, not 237 mL.

Mental Math Shortcuts for Quick Estimates

You don’t always need exact numbers. These shortcuts get you within 5-10% accuracy, which is close enough for most situations.

Kilometers to miles: Multiply by 0.6. Example: 80 km x 0.6 = 48 miles (exact: 49.7 miles).

The Fibonacci trick for km/miles: Consecutive Fibonacci numbers approximate the km-to-miles ratio. 5 km ≈ 3 miles. 8 km ≈ 5 miles. 13 km ≈ 8 miles.

Kilograms to pounds: Double the number and add 10%. Example: 70 kg → 140 + 14 = 154 lbs (exact: 154.3 lbs). This works because 2 + 0.2 = 2.2, which is the actual conversion factor.

Celsius to Fahrenheit (rough): Double the Celsius value and add 30. Example: 20C → 40 + 30 = 70F (exact: 68F). For precise temperature conversions, see our Temperature Converter.

Liters to US gallons: Divide by 4, then add a hair more. Example: 20 liters / 4 = 5 gallons (exact: 5.28 gallons).

Centimeters to inches: Divide by 2.5 for a quick estimate. Example: 30 cm / 2.5 = 12 inches (exact: 11.81 inches).

When to Use Which System

Situation	Preferred System
Scientific work	Metric (SI units required)
Medical dosages	Metric (mg, mL)
US construction	Imperial (feet, inches)
International shipping	Metric (kg, cm)
US cooking	Imperial (cups, tablespoons)
European/Asian cooking	Metric (grams, mL)
Running races	Metric (5K, 10K, marathon = 42.195 km)
US road distances	Imperial (miles)

If you work in engineering, medicine, or international trade, metric fluency is essential. If you live in the US, you will constantly bounce between both systems, so knowing the core conversion factors by heart saves time.

Common Conversion Mistakes

Confusing mass and weight: In everyday life, “weight” and “mass” are interchangeable. In physics, mass (kg) measures matter, while weight (Newtons) measures gravitational force. For unit conversions, you can safely treat kg and lbs as equivalent measures.

Mixing US and UK units: As noted above, US and UK gallons, pints, and fluid ounces differ. Always confirm which system a measurement uses before converting.

Forgetting to square or cube: Converting area or volume requires squaring or cubing the linear factor. 1 foot = 0.3048 m, but 1 square foot = 0.0929 square meters (0.3048 squared), and 1 cubic foot = 0.0283 cubic meters (0.3048 cubed).

Skip the Math

Our Unit Converter handles hundreds of unit pairs instantly. Enter your value, select the units, and get your answer. For specific categories, try the Length Converter, Weight Converter, or Temperature Converter.

Frequently Asked Questions

Why does the US still use the imperial system?

Congress passed the Metric Conversion Act in 1975, but it was voluntary. Without a mandate, industries that had already built infrastructure around imperial measurements (construction, manufacturing, road signage) stuck with what they knew. The cost of switching an entire country’s road signs, building codes, and consumer expectations has kept imperial entrenched for daily use, even though US science and military have largely adopted metric.

How do I convert square or cubic units?

Square the linear conversion factor for area, or cube it for volume. To convert 10 square feet to square meters: 10 x (0.3048)^2 = 10 x 0.0929 = 0.929 square meters. To convert 10 cubic feet to cubic meters: 10 x (0.3048)^3 = 10 x 0.0283 = 0.283 cubic meters.

What is the difference between a US ton and a metric ton?

A US ton (short ton) is 2,000 pounds or 907.2 kg. A metric ton (tonne) is 1,000 kg or 2,204.6 pounds. A metric ton is about 10% heavier than a US ton. There is also the UK long ton at 2,240 pounds (1,016 kg), which is rarely used today outside of older nautical contexts.

Is there a single conversion factor I should memorize above all others?

Memorize that 1 inch = 2.54 centimeters. From this single fact, you can derive feet to centimeters (multiply by 12 first), yards to meters, and miles to kilometers. It’s the foundation of length conversion between systems and the one factor engineers, tailors, and builders worldwide keep in their heads.