When converting from IAS to TAS the rule of thumb is is that TAS increases by 1.5% over IAS for every thousand feet up to 20,000 feet.
For example, if you are flying at 4000 feet and your IAS is 100 KTs your TAS will be roughly 106 KT TAS.
100 * 0.015 = 1.5
1.5 * 4 = 6
100 + 6 = 106 KT TAS