The problem: "just multiply by the ratio" works until it doesn't
Most unit conversions are the same operation: pick a base unit, express everything as a multiplier of that base, and multiply. One meter is 3.28084 feet, so ten meters is 31.0084 feet. A kilometer is 1,000 meters, a megabyte is 1,048,576 bytes, a knot is 0.514444 meters per second. You pick the pair, the tool looks up the ratio, and the math is one multiplication.
Then you try to convert 100°C to Fahrenheit and the answer isn't 100 × 1.8. It's (100 × 1.8) + 32 = 212. The +32 is the part people forget, and it's there because Celsius and Fahrenheit don't agree on where zero is. That's not a ratio conversion. It's an affine conversion — multiply, then add an offset — and it's the first of three categories where the "just multiply" approach gives you a wrong answer that looks right.
Fastest path
Open the Unit Converter, pick the category, type the value, read the result. For 15 categories the tool handles the ratio, the offset, and the inverse internally, so you don't have to remember which type each one is. The rest of this guide is about why three categories break the pattern, because that's where wrong answers come from.
The three conversion types
Type 1: Multiplicative (most units)
Length, weight, area, volume, speed, pressure, energy, power, time, angle, frequency, force — all of these convert by a single ratio. The tool picks a base unit (meters for length, pascals for pressure, joules for energy) and expresses every other unit as a multiplier of that base. One atmosphere is 101,325 pascals. One horsepower is 745.7 watts. One knot is 0.514444 m/s. You multiply to get to the base, divide to leave it. No offsets, no surprises.
The only thing to get wrong here is using the wrong ratio — confusing US gallons (3.785 L) with UK gallons (4.546 L), or using statute miles when you meant nautical miles. The tool lists both. Pick the right one.
Type 2: Affine (temperature)
Temperature is the affine case because the scales don't share a zero point. Celsius sets zero at water's freezing point. Fahrenheit sets zero at the freezing point of a brine solution. Kelvin sets zero at absolute zero. Rankine sets zero at absolute zero but uses Fahrenheit-sized degrees.
The conversion is always two steps: scale, then shift.
°C → °F: (°C × 9/5) + 32
°F → °C: (°F − 32) × 5/9
°C → K: °C + 273.15
K → °C: K − 273.15
The shift is why you can't chain temperature conversions the way you chain length conversions. To go from meters to inches you multiply by 39.3701. To go from Celsius to Fahrenheit you multiply by 1.8 and add 32, and if you forget the +32 you get 180°F instead of 212°F — wrong by 32 degrees, which is the difference between boiling water and hot tap water.
The tool handles this by using offset functions instead of ratios for temperature. Every other category uses value × multiplier. Temperature uses (value − 32) × 5/9 and value + 273.15. That's why temperature gets its own code path in any converter worth using — including this one.
Type 3: Inverse (fuel economy)
Fuel economy is the weird one. Most units go in the same direction: more of the base unit means more of the target unit. Fuel economy goes the other way: a lower number in L/100km means better efficiency, and a higher number in MPG means better efficiency. The two scales are inversely related.
Converting 8 L/100km to MPG isn't 8 × something. It's 235.214 / 8 = 29.4 MPG. The tool divides, not multiplies:
L/100km → MPG (US): 235.214 / L/100km
MPG (US) → L/100km: 235.214 / MPG
km/L → L/100km: 100 / km/L
The 235.214 constant comes from converting liters to gallons (3.785) and 100 km to miles (62.137), then dividing: 62.137 / 3.785 × 100 ≈ 235.214. The UK version uses 282.481 because the imperial gallon is bigger. If you try to convert fuel economy with a simple ratio, you'll get a number that's off by a factor of 2 or 3, and you won't notice because it's in a plausible range.
The KB-vs-KiB trap
The tool's data storage category includes both kilobytes (KB) and kibibytes (KiB), and both use the 1,024 multiplier. Technically, they shouldn't: KB is supposed to be 1,000 bytes (the SI prefix "kilo" means 1,000), and KiB is supposed to be 1,024 bytes (the binary prefix "kibi" means 2^10). But for decades, operating systems and storage tools used "KB" to mean 1,024, and the IEC's binary prefixes (KiB, MiB, GiB) were introduced in 1998 to fix the ambiguity. Most of the computing world ignored them.
The practical consequence: a "1 TB" hard drive from a manufacturer is 1,000,000,000,000 bytes (decimal), but Windows reports it as 931 GB (binary, dividing by 1,024 three times). The drive isn't smaller than advertised. The units are different. The tool uses the 1,024 convention for both KB and KiB, matching what most people expect from a converter — but if you're writing specifications or reading a disk utility that distinguishes the two, know which one you're looking at.
Energy vs power: different categories, not the same thing
The tool has separate categories for energy and power because they're different quantities. Energy is the total amount of work. Power is the rate at which that work happens. One joule is a unit of energy. One watt is one joule per second — a unit of power.
The confusion comes from units that look similar. A kilowatt-hour (kWh, energy) is not a kilowatt (kW, power). A kWh is the energy delivered by 1 kW of power sustained for one hour: 1 kW × 3,600 s = 3,600,000 J. Your electric bill is in kWh because it measures total energy consumed, not the rate at which you consumed it. A BTU is energy (1,055.06 J). A BTU/hour is power (0.293 W). The tool keeps them in separate categories to prevent you from "converting" between a stock and a flow.
Gotchas
- Mach is a fixed approximation. The tool converts Mach at 340.29 m/s, which is the speed of sound at sea level at 15°C. At 40,000 feet where airliners cruise, the speed of sound drops to about 295 m/s because the air is colder and thinner. Mach 0.85 at cruise altitude is 251 m/s, not 289. The tool's Mach conversion is fine for back-of-envelope work but not for flight planning.
- US vs UK gallons are different. A US gallon is 3.785 liters. A UK (imperial) gallon is 4.546 liters — 20% larger. The tool lists both. Using the wrong one is the most common volume conversion error, and it carries into fuel economy: MPG (US) and MPG (UK) are not the same number.
- The cooking units are US-standard. The tool's tablespoon is 14.787 mL (US). An Australian tablespoon is 20 mL, a UK tablespoon is 15 mL. If you're converting a recipe from an Australian source, the tablespoon conversion will be off by 35%.
- Calorie vs kilocalorie. The tool lists both "calorie" (4.184 J) and "kilocalorie" (4,184 J). What food labels call "Calories" (with a capital C) are actually kilocalories. A 200-Calorie snack is 200 kcal = 836,800 J, not 836.8 J. The naming is a historical accident that the tool can't fix for you.
- NATO mil is not a milliradian. A NATO mil is 1/6,400 of a circle (3.375 arcminutes). A true milliradian is 1/6,283 of a circle (3.438 arcminutes). The difference is about 1.8%, which matters for artillery and long-range shooting but not for most surveying work.
Summary
- Most unit conversions are multiply-by-a-ratio: length, weight, volume, speed, pressure, energy, power, time, angle, frequency, force. Pick the right pair and the tool does the math.
- Temperature is affine: multiply by the scale ratio, then add or subtract an offset. The offset is why 0°C is 32°F, not 0°F.
- Fuel economy is inverse: L/100km and MPG are reciprocals, not ratios. Convert by dividing a constant, not multiplying.
- Data storage has the KB-vs-KiB ambiguity: the tool uses 1,024 for both, matching common convention, but the SI standard says KB is 1,000.
- Energy and power are different quantities in different categories. Don't convert joules to watts. Use the Unit Converter for the 15-category scope, Length Converter for the dedicated length-only job, and Epoch to Date for Unix timestamp conversions.