How Long For An Air Compressor To Fill? Fill Time Calculation Based On Size

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How long does it take for an air compressor to fill an air tank?

I have been asked that question many times on this website, and since there appears to be a good deal of interest, I have added a page to this site dealing with how long it takes to fill an air tank.

How long to fill an air tank - image of air tank

How long to fill an air tank?

You need to gather some information about your air compressor first, and then plug the information into the formula shown.

  • V = Tank size
  • Final pressure (cut-out pressure)
  • Initial pressure (cut-in pressure)
  • 7.48 – conversion from gallons to CFM
  • Atmospheric pressure = 14.7
  • Compressor Pump Delivery in CFM
Fill An Air Tank Formula
Fill An Air Tank Formula


  • My tank size = 100 gallons
  • Cut-out pressure = 160 PSI
  • Cut-in pressure = 80 PSI
  • Conversion = 7.48
  • Atmospheric pressure = 14.7
  • Compressor Pump capacity = 4 CFM

That gives us:

compressed air formula
Example of populated formula for the time to fill an air tank.


100 x 80 = 8,000

Divided by:

440 (7.48 x 14.7 x 4 rounded up)

= 18.2 minutes

Folks, you are welcome to use these calculations yet be a bit cautious. While the information is sourced from a number of reputable locations I cannot be 100% sure of the accuracy, and therefore, you use this information at your own risk.

If you need to be 100% sure, get an air engineer to help you.

If you have a question about how long it takes to Fill An Air Tank, feel free to post it here.

Maybe an engineering type will see this page and offer some help!

Please add your question here, along with photos of the compressor and tank in question.

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What’s that 7.48 is can someone tell me, conversion of Gallons to CFM I don’t get it… Can someone mention it to me please?

CFM should say cubic feet, not per minute
Look it up – cubic feet to to gallons

Can you say in detail how they got it 7:8?.. I converted like you said but I didn’t get it…… What Gallons they actually converted to get 7:8 and why?..

7.48 – conversion from gallons to cubic feet (not CFM)
It’s a measured equivalence, like feet to meters and vice versa.
When you tried, you should have gotten one or both of the results shown in the attached screenshots. It’s the number in the box at right, or the *formula* at the bottom, depending on which way you’re converting.

Tank size in litres can u tell me how to do conversion?

Search “volume calculator”
Pick one. Fill in the blanks for the shape of your tank. If it’s the usual cylinder with rounded ends, just measure between the welds and add the depth of one end..
If you know the volume in other units, just type (number & units) in (units) and you’ll probably get the answer.

Can you transpose the equation for me so the known’s are the tank volume 60 gallons and the time to fill 0 psi to 120 psi is 6 minutes to get the CFM of the compressor pump?

Nope, I can’t. But maybe someone reading this can. Good luck.

It was a long shot anyway.
I knew it would be difficult as the pressure increases the CFM decreases.
It would probably involve calculus way over my head.

Yeah, well, the calculus would mean we had a curve of the pumps output as a function of pressure against it – in which case we’d know the answer already. But we can approximate it, anyway. I use google to get equivalencies: First off, how many cubic feet in a gallon? 0.133681. So 60 gallons is 8.02086 cuft. We’ll call it 8. A bar is atmospheric pressure, and by a gauge, an “empty” tank is zero bar. Now, 120 PSI is 8.27371 bar. We’ll call it 8.27. So, the pump must push 8.(02086) cuft of air for the tank pressure… Read more »

That sure makes my compressor sound pretty small.
Thanks for the answer.

Or maybe sick. Have you got any specs or a model number? Maybe a HP rating?

running 3.2hp on the motor and the pump is new. the box says [email protected] [email protected] 1200 RPM.

“the box says [email protected] [email protected] 1200 RPM.” The pump box?
Have you checked the motor and pulleys to verify the pump is turning at 1200?
Is the motor 240V? Or if 120V, what amperage is the circuit?
How about leaks? Belt(s) tension correct?
This should be filling your tank in around one minute, not 6, if it’s set up right and the pump specs aren’t vastly overstated.
Maybe you could put up a couple or four pix of it including the motor plate?

Yes the pump box and I have verified the pump is turning 1200 RPM at least it is when it starts. The belt is pretty tight I don’t suspect it is slipping. If it were I’d be smelling burning rubber. The motor is 240 volts only says 3.2 running HP on it. I have no measured the amps. (Note: link did not work and has been removed. Please add another comment and use the image uploader to add the photo(s) here. Thanks. – Moderator) The pump was failed when I bought it the rods were knocking. I bought a replacement… Read more »

No need to measure amps, that was only for reference if the motor was 120V or rated “peak” or some other BS.
“3.2 running HP” is something I can trust. Usually. Anyway, I guess it’s not bogging, is it?

Anyway, sounds like possibly a bad tank check valve. I presume you’ve checked for other leaks, including head gaskets.
And if not the check, then normally I’d guess bad reed valves (if that’s what it has) or rings.
But this is a *new* replacement pump? Is it single or two stage?

I found 1 small leak at the pipe from the pump to the tank check/unlloader valve. compression fitting. still took 5:40 to go from zero to 120 psi. checked speed at load 100 psi 1256 or so hard to test through the guards.
Would it be safe to put the oem pelt and pulley on it and run at 1550 rpm?
As i said everything looks identical but the new flywheel has 4 spokes/fan blades and the old pump has 8.

The tank’s holding pressure, right?
And you got the 1256 using a tach?
I’d maybe try 1550, but that would only increase output at most maybe 30%, maybe not even that, depending on the valves.
I think something else is going on.
What pump are you (trying to) using, anyway? Could it be mislabeled?

no leaks on the tank and used a second pressure gauge to check the one on the pressure switch. RPM checked with a photo tach. I initially tested it with the guards off so there was already reflective tape on it. That time was just to make sure my pulleys were ok. I found a really cool site with pulley formulas just plug in the numbers, NICE. so reduced the motor pully and got a 1 inch shorter belt. the 2 flywheels are the same diameter. I tried to pop the oem one off with a few hammer taps but… Read more »

The calculator at, probably. Yeah, it’s really handy.
Shipping to AK HI is a pain.
Maybe at some point you can guesstimate the bore and stroke on that so we can see if the specs on the box are in the realm of possible…
Let us know if you discover anything interesting.