Aviation Unit Converter

Why Unit Conversion Matters in Aviation

Aviation is an international industry built on standardized communication — but the units you encounter in the cockpit, on charts, and in weather briefings are far from uniform. A single cross-country flight can require you to think in nautical miles, statute miles, feet, meters, knots, miles per hour, inches of mercury, hectopascals, Celsius, Fahrenheit, gallons, liters, and pounds — sometimes within the same radio call or METAR.

Getting a conversion wrong has real consequences:

• Fuel planning — confusing gallons with liters, or misapplying fuel weight, can leave you short on range. A US gallon of avgas weighs 6.0 lbs; a liter weighs about 1.58 lbs. Mixing them up can produce a 26% error in your fuel load calculation.
• Altitude and pressure — ICAO transition altitudes use hectopascals (hPa or millibars), while the US uses inches of mercury (inHg). Setting 1013 when you meant 29.92 (or vice versa) will put your altimeter off by a dangerous margin.
• Speed limits and performance — ATC speed restrictions in the US are given in knots, but some older aircraft have MPH-only airspeed indicators. Exceeding a 250-knot speed restriction below 10,000 ft because you read 250 MPH is a violation — and 33 knots faster than intended.
• International operations — runway visual range may be reported in meters, visibility in statute miles, wind in knots, and cloud bases in feet or meters depending on the country. You must convert fluently or risk misinterpreting critical information.

A reliable unit converter eliminates mental math errors during preflight and in-flight decision-making.

Common Aviation Conversions

Here are the conversion factors pilots use most frequently:

Distance:

• 1 nautical mile (NM) = 1.15078 statute miles (SM) = 1.852 kilometers (km)
• 1 statute mile = 0.86898 NM = 1.60934 km
• 1 foot = 0.3048 meters; 1 meter = 3.28084 feet

Speed:

• 1 knot = 1 NM/hr = 1.15078 MPH = 1.852 km/h
• 1 MPH = 0.86898 knots
• Mach 1 ≈ 661.5 knots at sea level, standard day (decreases with altitude as temperature drops)

Pressure:

• 1 inHg = 33.8639 hPa (millibars)
• Standard sea-level pressure: 29.92 inHg = 1013.25 hPa
• 1 hPa = 0.02953 inHg

Weight and Fuel:

• 1 US gallon of avgas (100LL) ≈ 6.0 lbs
• 1 liter of avgas ≈ 1.58 lbs (0.72 kg)
• 1 US gallon = 3.785 liters
• 1 Imperial gallon = 4.546 liters = 1.201 US gallons
• 1 kilogram = 2.20462 lbs

Volume:

• 1 US gallon = 3.785 liters
• 1 liter = 0.2642 US gallons
• Jet-A weighs approximately 6.7 lbs per US gallon (varies with temperature)

Memorizing the most common conversions — 1 NM ≈ 1.15 SM, 1 knot ≈ 1.15 MPH, 1 inHg ≈ 33.86 hPa — will cover most situations. For everything else, use the converter above.

Temperature Conversions

Aviation weather is reported exclusively in Celsius, but many US-trained pilots still think intuitively in Fahrenheit. The conversion formulas are:

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

Key reference points every pilot should know:

• 0°C = 32°F — the freezing point. If the temperature or dewpoint is at or below 0°C, expect icing conditions in visible moisture.
• 15°C = 59°F — ISA standard temperature at sea level. Density altitude calculations start here.
• −40°C = −40°F — the one temperature where both scales meet.
• ISA lapse rate — standard temperature decreases by 2°C (3.6°F) per 1,000 feet of altitude gain.

Temperature directly affects density altitude, true airspeed, and engine performance. A hot day at a high-elevation airport can turn a routine departure into a performance-critical situation. Always convert and double-check when conditions deviate from standard.

Common Mistakes to Avoid

Confusing nautical and statute miles. VOR-based distances, airway segments, and RNAV waypoint separations are always in nautical miles. Visibility in a METAR is reported in statute miles. If you plan a 100 NM leg using statute miles, you will arrive 15% sooner than expected — a minor inconvenience on a good day, but a real problem if fuel is tight.

Mixing US and Imperial gallons. A US gallon is 3.785 liters; an Imperial gallon is 4.546 liters. Aircraft manufactured in countries using Imperial gallons (some older British or Canadian types) may have fuel quantity gauges calibrated in Imperial gallons. Filling with "50 gallons" in the wrong unit gives you 20% less fuel than planned.

Using the wrong fuel weight. Avgas (100LL) weighs 6.0 lbs/gal. Jet-A weighs approximately 6.7 lbs/gal but varies with temperature. Using the wrong fuel weight in your weight-and-balance calculation will shift your CG and may put you out of the approved envelope.

Forgetting that altimeter settings differ regionally. In the US, altimeter settings are in inches of mercury. Most of the rest of the world uses hectopascals. If you are flying internationally, confirm which unit ATC is giving you before setting your altimeter. A 1 hPa error corresponds to roughly 30 feet of altitude error.

Rounding too aggressively. For quick mental math, "1 NM is about 1.15 SM" is fine. But stacking multiple rounded conversions compounds the error. When precision matters — fuel planning, weight and balance, performance calculations — use the converter tool and carry the full values.

Frequently Asked Questions

Why do pilots use nautical miles instead of kilometers or statute miles?

One nautical mile equals one minute of latitude, which makes chart reading and navigation math straightforward. A degree of latitude is 60 NM, so distance and position translate directly. Knots (nautical miles per hour) follow from this convention. The system was inherited from maritime navigation and remains the global aviation standard under ICAO.

What is the difference between hPa and millibars?

They are identical. One hectopascal (hPa) equals one millibar (mb). The WMO adopted hPa as the preferred name, but you will still see "millibars" on older instruments and in some publications. Standard sea-level pressure is 1013.25 hPa (or mb), which equals 29.92 inHg.

How do I convert fuel volume to weight for weight-and-balance?

Multiply the volume in US gallons by the fuel's specific weight. For 100LL avgas, use 6.0 lbs per gallon. For Jet-A, use 6.7 lbs per gallon as a planning figure (actual weight varies from about 6.5 to 6.9 lbs/gal depending on temperature). If your fuel quantity is in liters, convert to gallons first (divide by 3.785) or use the metric weight directly: avgas is approximately 0.72 kg per liter.

My airspeed indicator reads in MPH. How do I comply with ATC speed restrictions given in knots?

Multiply the knot value by 1.15 to get the approximate MPH equivalent. For example, the 250-knot speed restriction below 10,000 ft MSL translates to approximately 288 MPH indicated. Better yet, print or save a small conversion table in your kneeboard. Some older Cessna 150s and Piper Cubs have MPH-only instruments, so this is a real consideration for vintage aircraft pilots.

Do I need to convert units for the FAA written exam?

Yes. The FAA knowledge tests include questions that require unit conversions — particularly temperature (for density altitude problems), distance (NM vs SM for fuel planning), and pressure (inHg to hPa). The provided E6B or calculator can handle the math, but you need to know which conversion to apply and when. Practice until the common factors are second nature.

What units does a METAR use?

In the US, a METAR reports wind in knots, visibility in statute miles, cloud heights in feet AGL (hundreds of feet), temperature and dewpoint in Celsius, and altimeter setting in inches of mercury. International METARs may report visibility in meters and pressure in hectopascals. Always check which format you are reading — the altimeter and visibility units are the most common sources of confusion.