Tank size and runtime
How long the tank covers the gap between what the tool takes and the pump makes.
A tank stores compressed air, and the useful quantity is how much free air it gives back as it falls from cut-out to cut-in pressure.
Free air stored = tank volume in cubic feet x (cut-out minus cut-in) / 14.696
Runtime in minutes = free air stored / (demand minus supply)
A US gallon is 0.133681 cubic feet, so a 60 gallon tank is 8.0 cubic feet of steel holding compressed air.
Runtime by tank size
Covering a 5 CFM shortfall, cycling between 150 and 100 PSI:
| Tank | Free air stored | Runtime |
|---|---|---|
| 6 gal | 2.7 cu ft | 0.6 min |
| 8 gal | 3.6 cu ft | 0.7 min |
| 20 gal | 9.1 cu ft | 1.8 min |
| 26 gal | 11.8 cu ft | 2.4 min |
| 30 gal | 13.6 cu ft | 2.7 min |
| 60 gal | 27.3 cu ft | 5.5 min |
| 80 gal | 36.4 cu ft | 7.3 min |
| 120 gal | 54.6 cu ft | 10.9 min |
Why the pressure band matters as much as the size
A tank that cycles over a wide pressure range stores more usable air than the same tank cycling over a narrow one. For a 60 gallon tank against the same 5 CFM deficit:
| Band | Free air | Runtime |
|---|---|---|
| 125 to 100 PSI | 13.6 cu ft | 2.7 min |
| 150 to 100 PSI | 27.3 cu ft | 5.5 min |
| 175 to 100 PSI | 40.9 cu ft | 8.2 min |
| 150 to 90 PSI | 32.8 cu ft | 6.6 min |
When the tank stops mattering
If the compressor delivers at least what the tool takes, the tank never empties and runtime is unlimited. The tank then only smooths the cycling. This is the case worth aiming for with any tool you use for more than a minute at a time, and it is why sizing the pump correctly matters more than buying tank capacity.