IAS is the indicated airspeed, with no corrections at all. If you correct the IAS for pressure (position) and instrument error, you will obtain the CAS.
Above a certain speed, the air molecules are hitting the pitot probe too fast to normally flow through the probe. This build-up creates a higher pressure and will result in an erroneous airspeed reading due to a higher dynamic pressure. This is the compressibility error, and it will induce the ASI to over-read. For instance, the IAS (or CAS) will be too high than it is in reality, and a correction needs to be applied to account for the compressibility error to obtain the Equivalent Air Speed (EAS). The higher the airspeed, the higher the compressibility error.
Besides the airspeed, the compressibility error is also influenced by the altitude. In fact, the less dense air at high altitudes is more easily compressed than at lower levels. The ASI’s calibration formulae include a factor addressing the compressibility of the air, so that a correct dynamic pressure is measured.
Up to about 200 kt CAS and an altitude of 10 000 ft, the compressibility error is not significant and the difference between the CAS and EAS is negligible.
In order to find the TAS from the given IAS, you just need to divide your current altitude in feet (ft) by 600 to obtain the percentage difference:
Altitude (in thousands) ÷ 600 = % increase