Tank Factor Calculator: Convert PSI to Cubic Feet

A tank factor is the cubic feet of gas in every 100 psi. Pick your cylinder, single or doubles, and enter a pressure or a volume. The calculator converts one to the other, gives you the exact factor and a rounded-down one for mental math, and shows each step so you can check yours.

By Jeremiah Gardner · Free. Runs in your browser. Nothing you enter is saved or sent.

Your cylinder

Cylinder

Pressure (psi)

Gas (cu ft)

Type in either box. The other one converts.

Check my math. Hide the answers until I’ve worked them out.

1,800 psi is 46.4 cu ft

Tank factor: 2.58cu ft per 100 psi
For mental math: 2.5rounded down
Full cylinder: 77.4 cu ftat 3,000 psi

The work

Tank factor77.4 cu ft ÷ 3,000 psi × 100 = 2.58 cu ft per 100 psi
Pressure to gas1,800 psi ÷ 100 × 2.58 = 46.4 cu ft
In your head18 × 2.5 = 45.0 cu ft, a little under the real figure, never over

A training aid, not a substitute for training or your team’s plan. Imperial units, salt water.

What is a tank factor?

The cubic feet of gas in every 100 psi of a cylinder. Divide the rated volume by the rated pressure and multiply by 100. For doubles, multiply by 2. An AL80 holds 77.4 cubic feet at 3,000 psi, so its factor is 77.4 ÷ 3,000 × 100 = 2.58.

How do you use it?

Pressure to gas. Drop the last two zeros and multiply by the factor. 1,800 psi in an AL80 is 18 × 2.58 = 46.4 cubic feet.
Gas to pressure. Divide by the factor and add two zeros. A 20 cubic foot rock bottom in an AL80 is 20 ÷ 2.58 × 100 = 775 psi.
Gas per minute. Breathing rate × ATA ÷ factor × 100 is how fast your gauge moves. The SAC rate calculator does that one.

Why round down for mental math?

UTD rounds an AL80’s 2.58 down to 2.5. The math gets easy, and the error lands on the safe side: you count a little less gas than you have, and a reserve in psi comes out a little higher. The calculator shows the rounded-down factor next to the exact one.

Tank factors for common cylinders

Cylinder	Rated volume	Rated pressure	Single	Doubles
AL40	40 cu ft	3,000 psi	1.33	—
AL80	77.4 cu ft	3,000 psi	2.58	5.16
HP80	80 cu ft	3,442 psi	2.32	4.65
HP100	100 cu ft	3,442 psi	2.91	5.81
HP117	117 cu ft	3,442 psi	3.40	6.80
HP120	120 cu ft	3,442 psi	3.49	6.97
HP133	133 cu ft	3,442 psi	3.86	7.73
LP50	50 cu ft	2,640 psi	1.89	3.79
LP85	85 cu ft	2,640 psi	3.22	6.44
LP95	95 cu ft	2,640 psi	3.60	7.20
LP104	104 cu ft	2,640 psi	3.94	7.88
LP108	108 cu ft	2,640 psi	4.09	8.18

Many low-pressure steel cylinders reach their nominal volume only with an overfill, so check your tank’s spec sheet. The download below lists specs for 88 steel and aluminum cylinders. For more on using tank factors with a team, read Tank Factors.

Frequently asked questions

What is the tank factor of an AL80?

About 2.58, from 77.4 cubic feet at 3,000 psi. UTD rounds it down to 2.5 for mental math. Double AL80s are about 5.16, which UTD rounds to 5.

How do I convert psi to cubic feet?

Divide the pressure by 100 and multiply by the tank factor. 1,800 psi in an AL80 is 18 × 2.58, or about 46 cubic feet.

How do I convert cubic feet to psi?

Divide the volume by the tank factor and multiply by 100. 30 cubic feet in an AL80 is 30 ÷ 2.58 × 100, or about 1,160 psi.

Why round the tank factor down?

So the rounding errs on the safe side. A smaller factor means you count a little less gas than you have, and a reserve converted to psi comes out a little higher.

Why does my low-pressure steel tank hold less than its label?

Many low-pressure steel cylinders reach their nominal volume only with an overfill. Check your tank’s spec sheet before you trust the number on the label.