23 Free Calculators
Everyday calculations made simple. Age, unit conversions, percentages, tips, and time zones. 23 free utility calculators — no sign-up required.
Unit conversion looks like the most trivial arithmetic there is, right up until a rounded factor propagates through a real project. One inch is exactly 25.4 millimetres by international agreement — that is a definition, not a measurement. But 1 kilogram is 2.20462 pounds, and people round it to 2.2. Over a 200 kg load that shortcut loses nearly a full pound, which matters for shipping tiers, dosing, and material orders.
Every converter here carries the full precision factor internally and rounds only for display. The reference values below are the ones worth committing to memory:
| Conversion | Exact factor | Common rounding | Error |
|---|---|---|---|
| Inches → millimetres | 25.4 (exact) | 25.4 | None |
| Kilograms → pounds | 2.204622622 | 2.2 | 0.21% low |
| Miles → kilometres | 1.609344 (exact) | 1.6 | 0.58% low |
| Litres → US gallons | 0.264172052 | 0.26 | 1.58% low |
| Feet → metres | 0.3048 (exact) | 0.3 | 1.6% low |
| Ounces → grams | 28.349523125 (exact) | 28 | 1.23% low |
Length, mass, and volume scales all start at zero, so converting them is one multiplication. Fahrenheit and Celsius do not share a zero point, which is why the formula needs an offset: °F = (°C × 9/5) + 32. Skipping the offset is the single most common conversion error there is. Kelvin and Celsius do share a scale spacing, so that one is just an offset: K = °C + 273.15.
A US gallon is 3.785 litres. An imperial (UK) gallon is 4.546 litres — about 20% larger. Fuel economy figures quoted in "miles per gallon" are therefore not comparable between US and UK sources unless you know which gallon is meant. The same trap applies to pints and fluid ounces.
If an interest rate moves from 4% to 5%, that is a rise of one percentage point but a 25% increase. Both statements are correct and they describe very different things. Our percentage calculator handles increase, decrease, difference, and "X is what percent of Y" as separate operations for exactly this reason.
Before trusting any conversion, ask whether the result should be bigger or smaller than what you started with. Converting metres to feet should give a larger number, because a foot is smaller than a metre. Getting the direction backwards is far more common than getting the factor wrong — and much easier to catch.
Why do converters sometimes disagree in the last decimal place?
Rounding strategy. Some tools round the conversion factor before multiplying; others carry full precision and round only the displayed result. The second approach is correct and is what these calculators do. For everyday use the difference is irrelevant, but in engineering, dosing, or manufacturing tolerances it can matter.
How do I convert Celsius to Fahrenheit in my head?
Double the Celsius figure and add 30. It is not exact, but it is close enough for weather: 20°C estimates to 70°F against a true 68°F. The error grows at extremes, so use the temperature converter when precision matters.
What is the difference between weight and mass in these converters?
Strictly, mass is the amount of matter (kilograms) and weight is a force (newtons). In everyday use — and in every converter on this page — pounds and kilograms are both treated as units of mass, which is what people actually mean. The distinction only matters in physics contexts.
How many days are between two dates, and are both days counted?
The days calculator returns the count of days elapsed, so 1 January to 2 January is one day. If you need both endpoints included — for billing periods or leave requests, for example — add one to the result. Leap years are handled automatically.
Does the square footage calculator handle irregular rooms?
Yes. Break the space into rectangles, triangles, or circles, calculate each separately, and add the results. The square footage calculator supports each shape individually, which covers most real rooms including L-shaped ones and bay windows.
Why do paint and concrete calculators add extra to the total?
Because real jobs waste material. Paint estimates typically add roughly 10% for a second coat and surface absorption on porous walls, and concrete estimates add 5–10% for spillage and uneven subgrade. Ordering the exact calculated amount reliably leaves you short.
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