TAS=IAS/sqrt(delta)
where Delta=ratio of air density to ISA SL density
=288.15/(T+273.15) * (P/1013.25)
and P= Ambient pressure in HPa(mB)
T= ambient temperature in degrees celsius
This ignores the compressibility correction, which is very small at low Mach numbers up to about M0.3
The "rule of thumb" correction is actually nearer to 1.5% per 1000' at low altitudes