Aviation unit conversions: a practical reference
Flight planning combines nautical miles and knots with feet, pounds, gallons, Celsius, and pressure units such as inches of mercury or hectopascals. Most conversion errors happen when the arithmetic is applied to the wrong quantity or units are dropped midway through the calculation. Write the source unit beside each value, convert before combining unlike units, and carry units through the result.
Speed: convert units without changing the kind of speed
One knot is one nautical mile per hour. Multiply knots by 1.15078 for statute miles per hour, or by 1.852 for kilometers per hour. For example, 120 kt is about 138 mph or 222 km/h. These are unit conversions only: converting 120 KIAS to mph does not turn indicated airspeed into true airspeed, and converting a true airspeed does not make it groundspeed. The speed converter handles the units; the airspeed and groundspeed guide explains the distinct quantities.
Distance: nautical miles, feet, and statute miles
One nautical mile is exactly 1,852 meters and approximately 6,076.12 feet or 1.15078 statute miles. A nautical mile corresponds to one minute of latitude, which is why it is used in navigation. For a rough mental check, 10 NM is about 11.5 statute miles; use unrounded factors where precision matters. Open the distance converter or review distance-unit details.
Temperature: aviation weather and performance inputs
Convert Celsius to Fahrenheit with °F = (°C × 9/5) + 32. Convert Fahrenheit to Celsius with °C = (°F − 32) × 5/9. Convert Celsius to kelvin by adding 273.15. For example, 20°C is 68°F and 293.15 K. Temperature differences use a different relationship than absolute readings: a change of 1°C equals a change of 1.8°F. Use the temperature converter. Confirm whether the aircraft performance chart asks for outside air temperature in Celsius or another unit.
Pressure: keep setting units explicit
Altimeter settings are commonly expressed in inches of mercury in the United States and hectopascals in many other regions. One inHg is approximately 33.8639 hPa; 1013.25 hPa is the standard pressure, approximately 29.92 inHg. These are setting conversions, not an instruction to alter an altimeter setting. Copy the current setting from the appropriate official report and use the unit expected by the instrument or procedure. Open the pressure converter and read the altimeter-pressure reference.
Fuel volume and fuel mass are different quantities
One U.S. liquid gallon is about 3.78541 liters; one imperial gallon is about 4.54609 liters. They are not interchangeable. A volume-to-mass conversion needs a fuel density. A commonly used planning approximation for avgas is about 6 lb per U.S. gallon, but actual fuel mass changes with grade and temperature; use the value and method required by the aircraft's approved loading data or operating procedure. Do not treat a volume conversion as a weight-and-balance answer. The volume converter converts volume units, while fuel weight discusses the separate mass calculation.
Mass and loading math
One pound is approximately 0.453592 kilograms, and one kilogram is approximately 2.20462 pounds. Convert every aircraft, occupant, baggage, and fuel entry into the units used by the approved weight-and-balance data before calculating moments. Moment is weight multiplied by arm, so changing the weight unit without consistently changing the moment units produces a plausible-looking but invalid center of gravity. Use the weight converter for units, then check the calculation against the aircraft's current loading records and limits.
Vertical speed and climb gradient are not interchangeable
One foot per minute is 0.3048 meters per minute, or 0.00508 meters per second. Thus 500 fpm is 2.54 m/s. A climb gradient in feet per nautical mile also depends on groundspeed: gradient (ft/NM) = vertical speed (ft/min) × 60 ÷ groundspeed (kt). At 500 fpm and 90 kt, that is about 333 ft/NM. Do not compare FPM directly with ft/NM. See the vertical-speed converter and the guide to climb rate versus climb gradient.
A short unit-check routine
- Write down the quantity and its unit: for example, 120 kt groundspeed or 25 US gal fuel.
- Confirm the destination unit expected by the chart, formula, or instrument.
- Convert the units before combining values, and label each intermediate result.
- Check whether the conversion changes only units or also requires a different physical input, such as fuel density or an airspeed correction.
- Round only at the point and precision appropriate to the source material; preserve published aircraft values as directed.
Common traps to catch
- Using imperial gallons where U.S. gallons are expected.
- Entering Fahrenheit into a Celsius input field.
- Confusing KIAS, KTAS, and groundspeed because they share the same unit label.
- Mixing pounds and kilograms within one weight-and-balance calculation.
- Comparing feet per minute directly with feet per nautical mile.
- Converting fuel volume to mass without stating a density assumption.
For the detailed factor tables, use speed, distance, temperature, volume, mass, pressure, and vertical speed. Each page links to its calculator.
References
The exact definitions for U.S. customary and SI units are maintained by the U.S. National Institute of Standards and Technology in its SI units reference. Aircraft-specific performance, pressure settings, and loading procedures must come from current approved sources for the operation.