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#1 - Take half of your altitude and add it to your indicated air speed.

example: 250 KIAS @ FL240 => 240/2 = 120 -> 250+120 = 370 kts (TAS)

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#2 - Divide your altitude by 1,000 then multiply by 5. Add the outcome to your indicated airspeed.

example: 35,000 ft / 1000 = 35 -> 35*5 = 175 -> 175+280 = 455 kts (TAS)

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#3 - Increase your indicated airspeed by 2% per thousand feet of altitude.

example: IAS 180 kts @ 10,000 ft => TAS = 180 + 20% (2*10) = 216 kts (TAS)

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#4 - Subtract TAT (total air temperature) from 444 = TAS at M.72.
For every .01 Mach above/below .72 add/subtract 5 kts.

example: Mach .75 & TAT=10 °C => 444 - 10 = 434, then add 3 (75-72) * 5 kts => TAS = 449 kts

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#5 - Take altitude in thousands of feet and muliply by three, add this to indicated airspeed, and add seven knots for true airspeed. This will lose accuracy with very low temperature or high pressure. Normally accurate to within five knots.

example: 24,000 ft & IAS=200 kts => 24*3 = 72 -> 72+200+7 = 279 kts (TAS)