IAS, CAS and TAS: the airspeeds pilots use
How indicated, calibrated and true airspeed differ, why altitude and temperature drive the gap, and when the 2 percent rule is good enough.
Why one aircraft has several airspeeds
A pitot-static system measures airspeed indirectly, by sensing the dynamic pressure of air ramming into the pitot tube. That pressure depends on both how fast you are moving and how dense the air is. Because density falls with altitude, the same needle reading represents a faster real speed the higher you climb. Pilots therefore work with a family of airspeeds rather than a single number, each one correcting the raw reading for a different effect.
From indicated to calibrated to true
Indicated airspeed is the raw needle value. Calibrated airspeed corrects it for installation and instrument error, which is small on most light aircraft but grows at high angles of attack. True airspeed then corrects calibrated airspeed for air density, accounting for the fact that thin high-altitude air produces less dynamic pressure than dense sea-level air. This calculator skips the small calibration step and estimates true airspeed straight from the indicated value, which is why it is labelled a rule of thumb.
The 2 percent per 1000 feet shortcut
The classic mental-math rule says add roughly 2 percent to indicated airspeed for every 1000 feet of pressure altitude. At 5000 feet that is about 10 percent, and at 10000 feet about 20 percent. The tool implements exactly this rule, multiplying your speed by one plus 0.02 times the altitude in thousands of feet. It is easy to do in your head on the ground and lines up closely with a flight computer on a standard day.
Where temperature bends the numbers
The shortcut assumes a standard atmosphere, where temperature falls predictably with height. On a hotter than standard day the air is even thinner, so true airspeed runs a little above what the 2 percent rule predicts, and on a cold day it runs a little below. For cross-country fuel planning that margin rarely matters, but for performance work near gross weight or high density altitude you should switch to an E6B or a glass-cockpit computer that reads outside air temperature.