Table of contents
Centimeters to inches. Kilograms to pounds. Celsius to Fahrenheit. These conversions come up dozens of times a week — in recipes, travel planning, science homework, and international emails. A reliable online unit converter handles them instantly without manual arithmetic or searching through tables.
This guide covers how the major measurement systems work, the most common conversions you will need, and how to handle the edge cases that trip people up.
Why unit conversion matters
The world uses two primary measurement systems: the metric system (SI) and the US customary system. Metric is used by the vast majority of countries and by all scientific disciplines. US customary is used in everyday life in the United States, with customary units also surviving in a few specific domains (nautical miles, screen sizes in inches, beer pints) in countries that otherwise use metric.
Every time data crosses this boundary — a US engineer reading a European spec sheet, a home cook following a recipe from a different country, a developer processing sensor data from an international source — unit conversion is required.
Beyond the metric/imperial divide, there are conversions within each system: kilometers to meters, pounds to ounces, degrees to radians. These are straightforward multiplications, but getting the factor slightly wrong produces meaningfully incorrect results.
Length conversion
Length is the most common conversion type. Use the FixTools Length Converter for instant results.
Metric length units
The metric system uses a base unit (meter) with standard prefixes:
| Unit | Symbol | Meters |
|---|---|---|
| Kilometer | km | 1,000 |
| Meter | m | 1 |
| Centimeter | cm | 0.01 |
| Millimeter | mm | 0.001 |
US customary length units
| Unit | Symbol | Inches |
|---|---|---|
| Mile | mi | 63,360 |
| Yard | yd | 36 |
| Foot | ft | 12 |
| Inch | in | 1 |
Key length conversions
| Convert | Formula |
|---|---|
| Inches to centimeters | × 2.54 |
| Centimeters to inches | ÷ 2.54 |
| Feet to meters | × 0.3048 |
| Meters to feet | × 3.28084 |
| Miles to kilometers | × 1.60934 |
| Kilometers to miles | × 0.621371 |
A practical shortcut: 1 inch = 2.54 cm exactly (this is the legal definition). Everything else follows from that single conversion factor.
Temperature conversion
Temperature is unique because units are on different scales — not just different sizes. Use the FixTools Temperature Converter for instant conversions.
Three scales are commonly used:
Celsius (°C): The metric standard. Water freezes at 0°C and boils at 100°C at sea level.
Fahrenheit (°F): Used in the United States for everyday temperature. Water freezes at 32°F and boils at 212°F.
Kelvin (K): The scientific absolute scale. 0 K is absolute zero (the coldest possible temperature). Kelvin uses the same step size as Celsius, just offset by 273.15.
Temperature formulas
°F to °C: (°F - 32) × 5/9
°C to °F: (°C × 9/5) + 32
°C to K: °C + 273.15
K to °C: K - 273.15
Reference points to memorize
| Celsius | Fahrenheit | Situation |
|---|---|---|
| -40°C | -40°F | The only temperature where both scales are equal |
| 0°C | 32°F | Water freezes |
| 20°C | 68°F | Comfortable room temperature |
| 37°C | 98.6°F | Normal human body temperature |
| 100°C | 212°F | Water boils at sea level |
Weight and mass conversion
Use the FixTools Weight Converter for mass and weight conversions.
Metric mass units
| Unit | Symbol | Grams |
|---|---|---|
| Tonne (metric ton) | t | 1,000,000 |
| Kilogram | kg | 1,000 |
| Gram | g | 1 |
| Milligram | mg | 0.001 |
US customary weight units
| Unit | Symbol | Pounds |
|---|---|---|
| Ton (short ton) | ton | 2,000 |
| Pound | lb | 1 |
| Ounce | oz | 0.0625 (1/16) |
Key weight conversions
| Convert | Formula |
|---|---|
| Pounds to kilograms | × 0.453592 |
| Kilograms to pounds | × 2.20462 |
| Ounces to grams | × 28.3495 |
| Grams to ounces | × 0.035274 |
| Stones to kilograms | × 6.35029 |
A practical shortcut: 1 kg ≈ 2.2 lb. For a quick estimate, multiply kilograms by 2 and add 10%: 70 kg → 140 + 14 = 154 lb (actual: 154.3 lb).
Area conversion
Area conversions involve squared units — the conversion factor is squared too. 1 meter = 3.28 feet, so 1 square meter = 3.28² = 10.76 square feet.
| Convert | Formula |
|---|---|
| Square feet to square meters | × 0.092903 |
| Square meters to square feet | × 10.7639 |
| Acres to hectares | × 0.404686 |
| Hectares to acres | × 2.47105 |
Volume conversion
Volume is often where confusion arises — especially between US and UK/metric measurements.
| Unit | Equivalent |
|---|---|
| 1 US gallon | 3.785 liters |
| 1 UK gallon | 4.546 liters |
| 1 liter | 33.814 US fl oz |
| 1 US cup | 236.6 ml |
| 1 tablespoon (US) | 14.79 ml |
Note that the US gallon and the UK (imperial) gallon are different volumes. A recipe from the UK that calls for a gallon uses a larger measure than the same recipe using US units.
Speed conversion
| Convert | Formula |
|---|---|
| mph to km/h | × 1.60934 |
| km/h to mph | × 0.621371 |
| Knots to km/h | × 1.852 |
| m/s to km/h | × 3.6 |
A practical shortcut for mph to km/h: multiply by 1.6. For km/h to mph: multiply by 0.6.
Common conversion mistakes to avoid
Confusing metric tons and short tons
A metric ton (tonne) is 1,000 kilograms (approximately 2,204 pounds). A US short ton is 2,000 pounds (approximately 907 kilograms). They are not the same. International shipping and trade specifications typically use metric tons; US domestic industry often uses short tons. Always check which is intended.
Using the wrong temperature formula
The most common error is applying the wrong formula. Remember: Celsius and Fahrenheit are on offset scales, not just different-sized steps. You cannot simply multiply; you must account for the offset (32°F = 0°C, not 0°F = 0°C).
Squaring or cubing the linear conversion factor
If 1 meter = 3.28 feet, then 1 cubic meter = 3.28³ = 35.3 cubic feet. Applying the linear factor to a volume or area is a common error that produces a wrong result by a large margin.
Confusing fluid ounces and weight ounces
A fluid ounce measures volume. A weight ounce measures mass. One fluid ounce of water weighs approximately one weight ounce — but one fluid ounce of olive oil, honey, or alcohol weighs a different amount. Never substitute fluid ounces for weight ounces in a recipe or formula.
Convert any unit now
Use the FixTools converters for instant, accurate unit conversion:
- Length Converter — inches, feet, meters, kilometers, miles
- Temperature Converter — Celsius, Fahrenheit, Kelvin
- Weight Converter — kilograms, pounds, ounces, grams
No sign-up required. Enter a value, pick the units, and get the result instantly.
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Frequently asked questions
6 questions answered
QWhy do the US and most countries use different measurement systems?
The United States primarily uses the US customary system (miles, pounds, Fahrenheit), which evolved from British imperial units. Most of the rest of the world uses the metric system (SI), standardized internationally in the late 18th century. The US has repeatedly attempted to switch to metric — Congress passed the Metric Conversion Act in 1975 — but the transition has been voluntary and largely incomplete outside of scientific and medical contexts. The result is that Americans must convert between systems routinely when working with international suppliers, scientific literature, or global products.
QWhat is the difference between mass and weight?
Mass is a measure of how much matter an object contains, measured in kilograms (kg) or pounds-mass (lbm). It does not change regardless of where the object is. Weight is the force gravity exerts on that mass, measured in newtons (N) or pounds-force (lbf). On Earth, the two are often used interchangeably in everyday language because gravity is approximately constant. On the moon, your mass stays the same but you weigh about one-sixth as much. In everyday unit conversion, when someone asks to convert kilograms to pounds, they are converting mass — the weight difference would only matter in different gravitational environments.
QHow do I convert Celsius to Fahrenheit in my head?
The exact formula is °F = (°C × 9/5) + 32. For a quick mental estimate: double the Celsius temperature and add 30. For example, 20°C → (20 × 2) + 30 = 70°F (actual: 68°F). This shortcut is off by a few degrees but works well for everyday estimates like checking the weather. Key reference points to memorize: 0°C = 32°F (freezing), 20°C = 68°F (comfortable room), 37°C = 98.6°F (body temperature), 100°C = 212°F (boiling).
QWhat is the difference between fluid ounces and ounces (weight)?
Fluid ounces (fl oz) measure volume — how much space a liquid occupies. Ounces (oz) measure weight or mass. They are completely different units that happen to share a similar name. One fluid ounce of water weighs approximately one ounce, which is why the confusion persists, but one fluid ounce of honey weighs significantly more than one ounce because honey is denser than water. When a recipe says "2 oz of chocolate," it means weight. When it says "2 fl oz of milk," it means volume. Never substitute one for the other.
QHow precise should my unit conversions be?
Precision depends on the context. For cooking, rounding to the nearest whole number is usually fine — a recipe calling for 2.5 cups does not need sub-milliliter precision. For engineering and manufacturing, you need full precision and should work with the exact conversion factors. For science, report as many significant figures as your measurement has — converting 1.234 km to miles as 0.77 miles loses precision; 0.7668 miles preserves it. When in doubt, retain more decimal places than you need and round only at the final step.
QWhy is a nautical mile different from a regular mile?
A nautical mile (1,852 meters) is based on the Earth's geometry: it equals one minute of arc of latitude (1/60 of one degree). This makes navigation calculations much simpler — moving one nautical mile north changes your latitude by exactly one arcminute. A standard (statute) mile (1,609.344 meters) is a land measurement unit with historical roots in Roman measurement. Aviation and maritime navigation use nautical miles because they relate directly to angular coordinates on charts. When you see a plane's speed in knots, one knot equals one nautical mile per hour.
O. Kimani
Software Developer & Founder, FixTools
Building FixTools — a single destination for free, browser-based productivity tools. Every tool runs client-side: your files never leave your device.
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