Shipping a package abroad, following a recipe with grams and cups, or reading a weather forecast in Fahrenheit — unit conversion shows up constantly. The math itself is simple, but remembering the factors (and doing them under pressure) is where mistakes happen. This guide covers the most common conversions, the factors worth memorizing, and how to convert anything instantly online.
Common Conversion Factors Worth Knowing
| Type | Conversion | Factor |
|---|---|---|
| Length | 1 inch → cm | 2.54 cm |
| Length | 1 mile → km | 1.609 km |
| Weight | 1 kg → lb | 2.2046 lb |
| Weight | 1 oz → g | 28.35 g |
| Volume | 1 gallon → liter | 3.785 L |
| Temperature | °C → °F | °F = °C × 9/5 + 32 |
Why Factors Differ (and Why It Matters)
Some conversions are exact (1 inch = 2.54 cm by definition), while others are rounded for everyday use (1 mile ≈ 1.61 km). A converter uses the full precision factors, then rounds the display — so "120 kg = 264.55 lb" is more accurate than the "264 lb" you might round mentally. For safety-critical or commercial use, always confirm the standard (US vs. imperial gallon, for example, differ by about 20%).
How to Convert Units Online
- Open the tool: go to the unit converter.
- Pick a category: length, area, volume, weight, temperature or currency.
- Enter a value: results for every unit in that category appear instantly.
- Switch directions: the tool converts both ways — no need to flip the formula.
- Copy the result: grab the value you need for your document or label.
Tip: temperature is not a linear ratio — you can't "multiply" Celsius to Fahrenheit. Use the formula °F = °C × 9/5 + 32, or let the converter handle it.
Frequently Asked Questions
How many centimeters are in an inch?
Exactly 2.54 cm. It's one of the few imperial–metric conversions defined precisely, so it never varies.
What's the difference between US and imperial gallons?
The US gallon is about 3.785 L; the imperial (UK) gallon is about 4.546 L — a 20% difference. Always confirm which standard you're converting.
Are the conversions in the tool accurate?
Yes — they use full-precision standard factors and round only the displayed result. Currency rates are reference values, not live market quotes (see the currency converter guide).
Is my input uploaded when I convert?
No. The unit converter computes everything locally in your browser.
References
- SI units and conversion factors — NIST: https://www.nist.gov/pml/owm/metric-si/si-units
- BIPM — International System of Units: https://www.bipm.org/en/measurement-units