Units that make sense
Every section explains the logic behind the units first — where the numbers come from, why the formulas work — then lets you calculate instantly.
The metric system is built on powers of 10. Every prefix multiplies or divides by 10, 100, or 1000. The base unit is the meter (m). A kilometer is literally "kilo" = 1000 meters. A centimeter is "centi" = 1/100 of a meter. Once you know the prefixes, you know every metric unit.
Time units are not decimal — they come from ancient astronomy and base-60 counting (Babylonian math). A day has 24 hours because the sky was divided into 12 daytime + 12 nighttime sections. Each hour has 60 minutes and each minute has 60 seconds from the Babylonian sexagesimal system. The year is 365.25 days on average (accounting for leap years).
The gram (g) is the metric base unit of mass. Like distance, it uses powers of 10 with prefixes. A kilogram (kilo = 1000) is the most common unit in daily life and in science. Imperial units like pounds and ounces come from older, inconsistent systems — they're based on historical trade standards, not math.
Temperature is special — you can't just multiply by a constant to convert between °C, °F, and K. Each scale has a different zero point. Celsius sets 0° = water freezes. Fahrenheit sets 0° = coldest brine solution in 1724 (historical quirk). Kelvin sets 0 K = absolute zero (−273.15°C) — the coldest anything can ever be. This is why Kelvin is used in science: it starts at the actual physical minimum.
The liter (L) was defined as the volume of 1 kilogram of water. This links mass and volume together elegantly in the metric system. 1 liter = 1 cubic decimeter (dm³) = 1000 cm³. So volume and distance scales are directly connected through cubing the prefix factor.
Speed is distance ÷ time. So any speed unit is made of a distance unit and a time unit. km/h means "how many kilometers in one hour." The SI base unit is meters per second (m/s). To convert between any two speed units, you convert their distance parts and time parts separately. For example: 1 km/h = 1000 m / 3600 s = 1/3.6 m/s ≈ 0.2778 m/s.
The SI unit of energy is the Joule (J) = 1 kg·m²/s². The calorie (cal) was originally defined as the energy to heat 1 gram of water by 1°C — that's why food uses kilocalories (kcal), often confusingly called "Calories" (capital C). In physics and chemistry, the electron volt (eV) is used at the atomic scale, because Joules would give absurdly tiny numbers.
Pressure = Force ÷ Area. The SI unit is the Pascal (Pa) = 1 N/m². In chemistry you'll see atm (atmosphere) constantly — it's defined as the average air pressure at sea level. mmHg (millimeters of mercury) comes from old barometers where pressure literally pushed a column of liquid mercury up a tube. 1 atm ≈ 101,325 Pa = 760 mmHg = 1.01325 bar.