Scuba Tank Factors: How to Turn PSI Into Cubic Feet in Your Head

A tank factor is the cubic feet of gas in every 100 psi of a cylinder. Divide the rated capacity by the rated pressure and multiply by 100. An AL80 works out to about 2.6, which UTD rounds down to 2.5. Doubles are twice the single. Multiply pressure by the factor to get the gas you actually have.

By Jeremiah Gardner · Last reviewed October 17, 2026

Two divers check gas at the turn. Both show 1,500 psi.

One is in an AL80. The other is in double HP100s. Same number on the gauge. One of them has more than twice as much gas as the other.

So who's carrying the team? And who's about to run short?

That's what tank factors answer. Psi is a pressure. Gas is cubic feet. You need a way to get from one to the other, in your head, underwater.

What is a tank factor?

The cubic feet of gas in every 100 psi of a cylinder.

Tank factor = (rated capacity ÷ rated pressure) × 100

For doubles, multiply by 2.

An AL80 holds about 77.4 cubic feet at 3,000 psi. So 77.4 ÷ 3,000 × 100 = 2.58. UTD rounds that down to 2.5 for planning, which makes the math easy and keeps the rounding on the safe side. Double AL80s are about 5.

Tank factors for common cylinders

Cylinder	Rated capacity	Rated pressure	Single	Doubles
AL40	40 cu ft	3,000 psi	1.3	2.7
AL80	77.4 cu ft	3,000 psi	2.6 (UTD uses 2.5)	5.2 (UTD uses 5)
HP100	100 cu ft	3,442 psi	2.9	5.8
HP120	120 cu ft	3,442 psi	3.5	7.0
HP133	133 cu ft	3,442 psi	3.9	7.7
LP85	85 cu ft	2,640 psi	3.2	6.4
LP104	104 cu ft	2,640 psi	3.9	7.9

One catch with low-pressure steel. Many reach their nominal capacity only with an overfill, so check your tank's spec sheet before you trust the label. The download below lists specs for 88 steel and aluminum cylinders.

How do you use a tank factor?

Three ways. All of them in your head.

Pressure to gas. Drop the last two zeros, multiply by the factor.

1,500 psi in an AL80: 15 × 2.5 = 37.5 cubic feet.
1,500 psi in double HP100s: 15 × 5.8 = 87 cubic feet.

That's the two divers at the turn. Same pressure. More than twice the gas.

Gas to pressure. Divide by the factor, add two zeros.

Your rock bottom is 30 cubic feet. In an AL80: 30 ÷ 2.5 = 12, so 1,200 psi. In double AL80s: 30 ÷ 5 = 6, so 600 psi.

Gas per minute. Breathing rate × ATA ÷ tank factor × 100.

At 99 feet you're at 4 ATA. At UTD's swimming planning rate of 0.75 cubic feet a minute, you breathe 3 cubic feet a minute.
In an AL80: 3 ÷ 2.5 × 100 = 120 psi a minute.
In double AL80s: 60 psi a minute.

If you know that before the dive, your gauge stops being a surprise. That's the whole point of Instruments Should Confirm What You Already Know.

Why does this matter for a team?

Because teams share gas. And you can't share what you can't measure.

If your teammate is on a smaller cylinder, their turn pressure isn't yours. If one diver's rock bottom is 1,200 psi and another's is 600, "turn at 1,500" means two completely different things.

Tank factors put everyone on the same unit: cubic feet. Plan the dive in volume. Then convert to each diver's pressure.

Here's the reality check.

The comfortable belief: we all turn at the same pressure, so we're all on the same plan.

The truth: you're on the same number. Not the same gas.

Put it to work

For the next steps in gas planning, read How to Calculate Your SAC Rate and the Rock Bottom Calculator. Then plan your next dive in cubic feet, not psi.

What's your tank factor? Your teammate's? Do you know the difference between your gas and theirs at the turn?

If you want to make this automatic, gas planning and team gas management are core to Essentials of Tech.

Frequently asked questions

What is the tank factor of an AL80?

About 2.6, from 77.4 cubic feet at 3,000 psi. UTD rounds it down to 2.5 for planning. Double AL80s are about 5.

How do I calculate a tank factor?

Rated capacity in cubic feet, divided by rated pressure in psi, times 100. For doubles, multiply by 2.

Why do tank factors matter?

Because psi isn't gas. Two divers at the same pressure on different cylinders can have very different amounts of gas. Tank factors let you plan, compare and share gas in cubic feet.

How do I use a tank factor to get psi per minute?

Multiply your breathing rate by the pressure in ATA, divide by the tank factor and multiply by 100.