Best Unit Converters for Students and Engineers

This is the DevOkk converter set, not an app-store ranking. Each page runs in the browser without an account. Open the family that matches the quantity.

Pick the family, then the page

Length, area, and volume look related because they are. Temperature is not a scaled length. Bytes are not a scaled mass. Fuel economy is not a linear “bigger number is better” scale once you leave US gallons. If you remember only one rule from this page: convert inside a family, and write the unit next to the number you copy.

The unit converters hub is the index. The pages below are the actual jobs.

Length, area, and volume

Length converter is metres, millimetres, inches, feet, yards, miles, and nautical miles. The constant that matters is exact: 1 inch = 25.4 mm by definition. That makes 1 foot = 0.3048 m and 1 statute mile = 1609.344 m. The sloppy “1 mile ≈ 1.6 km” shortcut is 0.6 percent low. On a 10 km run that is 62 m. On a survey it is a dispute.

Area converter is the square of those lengths, plus hectares and acres. 1 hectare is 10,000 m². 1 acre is 4046.8564224 m², not “about 4000.” You cannot convert a length to an area without a second length; the converter is for values that are already areas.

Volume converter is litres, cubic metres, cubic inches, and the two gallons. A US gallon is 3.785411784 L. An Imperial gallon is 4.54609 L. Recipes, fuel tanks, and lab glassware all lie if you treat “gallon” as one unit. See how to convert length units and how to convert area, volume, and density when the homework jumps dimensions.

Temperature is an affine scale

Temperature converter implements the three everyday formulas:

• °F = °C × 9/5 + 32
• °C = (°F − 32) × 5/9
• K = °C + 273.15

A 20 °C room is 68 °F and 293.15 K. A 20 °C difference is 36 °F and 20 K - the offset is not applied to deltas. That is the most common lab error after mixing °C and K in an ideal-gas problem. The long form is how to convert Celsius, Fahrenheit, and Kelvin.

Bytes, bits, and the file you just shipped

Bytes bits converter is 8 bits to a byte, then SI versus IEC prefixes. 1 kB = 1000 B. 1 KiB = 1024 B. A “500 GB” drive in the shop is 500 × 10⁹ bytes, which Windows reports as about 465.7 GiB while still saying GB. Bundle size estimator is the frontend cousin: gzip/brotli and transfer size, not a disk label. Pair them when a ticket says the JS “is 2 MB” and nobody knows whether that is decoded, gzipped, or SI.

The walkthrough is how to convert bytes, bits, KB, MB, and GB.

Unix time and civil clocks

Unix timestamp converter turns seconds or milliseconds since 1970-01-01 00:00:00 UTC into a date. Ten digits are seconds (until 2286). Thirteen digits are milliseconds. JavaScript Date.now() is milliseconds. Python time.time() is seconds. Treat 1700000000 as milliseconds and you get a date in January 1970; treat 1700000000000 as seconds and you get a date tens of millennia out. Timezone converter is the next page after you have a civil time and need another zone, including DST. Details: Unix timestamps to dates.

Speed versus velocity

Speed converter is m/s, km/h, mph, and knots. 1 m/s = 3.6 km/h exactly. 1 mph = 1.609344 km/h. 1 knot = 1.852 km/h. Speed is a scalar. Velocity calculator is for v = Δx / Δt when you have distance and time, not a posted speed. Use the converter when the radar already gave you 60 mph and the report wants 26.8224 m/s. Use the calculator when you timed a 100 m sprint.

Mass, weight, density

Mass converter is kg, g, tonnes, and pound-mass. Weight converter is force: newtons and pound-force. A 70 kg mass on Earth (standard g = 9.80665 m/s²) weighs about 686.5 N, which a US scale would also call about 154 lbf. The kilogram is not a unit of weight. The bathroom-scale “kg” is force divided by g. Density converter then takes mass/volume: 1 g/cm³ = 1000 kg/m³. Read mass vs weight before you convert a crate “weight” from a bill of lading into a density.

Pressure: four names for the same stress

Pressure converter is Pa, kPa, bar, atm, psi, and torr. Exact anchors: 1 bar = 10⁵ Pa; 1 atm = 101325 Pa; 1 psi = 6894.757 Pa. 32 psi in a tire is about 2.21 bar or 221 kPa - and that reading is usually gauge (above atmosphere), not absolute. Pressure calculator is P = F / A when you have a force and an area instead of a gauge. The traps are in how to convert pressure units.

Energy is not power

Energy converter is J, kJ, cal, kcal, kWh, BTU. Power converter is W, kW, and horsepower. 1 W = 1 J/s. 1 kWh = 3.6 MJ. A 1500 W kettle run for 4 minutes uses 0.1 kWh, not “1500 kW.” Mechanical horsepower is 745.7 W; metric horsepower is 735.5 W. How to convert energy and power is the longer split.

Fuel economy is inverted

Fuel converter is the page people skip and then get backwards. US mpg and L/100km are inverses: L/100km = 235.215 / (US mpg). 30 US mpg is 7.84 L/100km. Doubling mpg does not halve L/100km unless you start from that formula. UK mpg uses the Imperial gallon (4.54609 L), so the constant is 282.481. MPG vs L/100km is the dedicated note.

What this list is not

It will not solve a free-body diagram or override an exam phone ban. It converts the number you typed, in the unit pair you selected.

If the next step is a formula (F = ma, ideal gas, ρ = m/V), use the matching calculator. If it is still a unit, stay here: length, temperature, bytes, Unix time, pressure, or mass.