Smurfjet,
Not so much a formula as a 'rule of thumb'. Works when conditions are close to ISA and at altitudes not flight levels.
The TAS and IAS will vary by 1.8 kts/1000'.
eg. If we're cruising at 10'000 and indicating 145kts then TAS will be 163kts.
ie 1.8(kts/1000')X 10(lots of 1000')= 18kts
145kts (IAS)
+
18kts = 163kts (TAS)
Also works just as well the other way around!!