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The tank does not increase the air the compressor delivers: it stores air while the tool is idle and gives it back while the tool works. It helps with tools used intermittently, such as an impact wrench that tightens for a few seconds.

How much air it holds

Usable air is the air between the tank’s maximum pressure and the minimum pressure the tool needs. In litres of free air it is roughly: tank volume × pressure difference in bar ÷ 1.013.

From 8 bar to 6.3 bar, that is, with an impact wrench asking for about 6.2 bar plus a little loss in the hose. The run time applies to a 360 l/min tool with a compressor that delivers 90 l/min; the refill, from 6.3 to 8 bar, applies to the same compressor with the tool idle:

The 360 l/min consumption is the figure declared for a 1/2-inch impact wrench [31]. The 90 l/min is an example value, in the range of the delivered air of hobby compressors that declare it.

What it means

• For an impact wrench working in bursts, 9–19 seconds of air and a minute of refill may be enough to change a tyre, not for continuous work.
• For a grinder or a sander, which work for long stretches, delivered air is what counts: the tank empties and then the tool is left without pressure.
• Doubling the tank doubles both the working time and the refill time.

The calculation assumes constant temperature: during refilling the air heats up and then cools, so real times are somewhat different. The calculator gives the time from a full tank as a span, from the tank alone to the tank plus each compressor’s declared delivered air; it shows the refill time only when the manufacturer’s data cover all the pressures of the refill.