Why does Mach change with altitude?

The speed of sound depends on air temperature, not pressure. In the ISA troposphere, temperature drops with altitude, so the speed of sound decreases - meaning the same TAS is a higher Mach number at higher altitudes.

What is the flight level field used for?

The flight level is used to compute the local speed of sound for the Mach conversion. Setting it accurately ensures the displayed Mach number matches what you would see on an airspeed indicator at that altitude.

Navigation Tool

Time / Speed / Distance

Solve any leg of the TSD triangle. Enter two known values to find the third - supports knots, Mach numbers, and fuel estimates.

CalculatorInteractive

Distance

Speed

kts

Flight Level

Used for the Mach conversion and readout

Fuel Burn Rate

kg/h

Result

Flight Time1h 00m

60 minutes total

Mach 0.78 at FL350 · 200 kg/h burn: 200 kg

Rule of thumb: at 450 kts, 1 nm per 8 seconds.

How to Use & Reference

About

Solves any side of the time-speed-distance triangle used in flight planning. Enter two known values to instantly compute the third. Supports knots and Mach numbers, and includes a fuel burn estimate based on a typical jet burn rate.

How to Use

1Select the value you want to find - Time, Speed, or Distance.
2Enter the two known values in the fields that appear.
3For speed, tap the KTS/MCH toggle to switch between knots and Mach number.
4Set the flight level for accurate Mach-to-knots conversion at altitude.
5Read the result and secondary Mach or fuel note below the output panel.

Tips

At 450 kts groundspeed, a useful rule of thumb is 1 nm per 8 seconds.
Mach number varies with altitude - at FL350 Mach 0.78 is roughly 460 kts TAS, but at FL200 it would be about 450 kts.
The fuel estimate uses a fixed 200 kg/h burn rate - useful for relative comparison but not for actual fuel planning.
To find the groundspeed needed to make a time restriction, use Find Speed and enter the distance and required time.
Distance in this tool uses nautical miles - the standard unit for en-route aviation navigation.

Formula

TimeD / S x 60 (minutes)

SpeedD / (T / 60) (kts)

DistanceS x (T / 60) (nm)

MachTAS / speed of sound

Speed of sound varies with temperature. ISA standard at FL350: ~574 kts.

FAQ

Why does Mach change with altitude?: The speed of sound depends on air temperature, not pressure. In the ISA troposphere, temperature drops with altitude, so the speed of sound decreases - meaning the same TAS is a higher Mach number at higher altitudes.
What is the flight level field used for?: The flight level is used to compute the local speed of sound for the Mach conversion. Setting it accurately ensures the displayed Mach number matches what you would see on an airspeed indicator at that altitude.