Conversions Calculators

Unit conversion is one of the most common everyday math operations and one of the most error-prone — kilometer-to-mile and Celsius-to-Fahrenheit confusion has caused real-world disasters from misdosed medications to lost spacecraft. Our conversion calculators are designed to be unambiguous and to show both the formula and the result so you can verify the math.

Length conversions cover metric (mm, cm, m, km) and imperial (inch, foot, yard, mile) with the nautical mile included for marine and aviation use. The exact conversion factor (1 inch = 25.4 mm exactly, by international agreement since 1959) is preserved in our calculations.

Weight and mass conversions handle the metric (mg, g, kg, tonne) and imperial (ounce, pound, stone, ton) systems. We use the standard distinction between mass and force where it matters; for most everyday use, the difference between weight (force) and mass is negligible at Earth's surface gravity.

Speed conversions cover km/h, mph, m/s, knots, and the less-common ft/s for engineering applications. The cups-to-ounces calculator handles the U.S./UK cup-volume difference (240 mL vs 250 mL respectively), which is a frequent source of recipe failures.

Number-base conversions include binary-to-decimal and hexadecimal-to-decimal — useful for programming, networking, and digital electronics work. Color converters handle RGB/HSL/Hex transformations for design work (also available in the technology category).

Temperature conversions use the standard formulas: F = (C × 9/5) + 32 and C = (F - 32) × 5/9. The Kelvin scale is supported for scientific use (K = C + 273.15).

All conversion factors follow NIST SP 811 (Guide for the Use of the International System of Units) which is the authoritative U.S. reference. Where unit definitions have changed historically (e.g., the meter being redefined in terms of the speed of light), we use the current definition. Conversion results are exact to the precision shown; for engineering applications requiring extreme precision, always verify the conversion factor against the most recent NIST publication.