Scuba: SAC Rate From Tank Pressure Used, Dive Time And Depth

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Surface air consumption — SAC — is the number a diver needs to plan gas for any dive. It comes from a dive you already did: pressure used divided by minutes gives your rate at that depth, and dividing by the absolute pressure in atmospheres corrects it back to the surface. Once you have SAC, multiply by any depth's atmospheres and any time to see how much gas that dive will take.

Quick formula: Pressure used per minute, divided by absolute atmospheres at the average depth:

=(B2-C2)/D2/(E2/33+1)

3000 to 1000 psi in 40 minutes at 66 ft: 50 psi/min at depth, and 66 ft is 3 atmospheres absolute, so SAC = 16.7 psi/min.

Functions used (tap for the full reference guide):

SUM ROUND

The example

Three logged dives on the same 80 cu ft aluminum tank. The deep dive burns far more per minute at depth but comes back to a lower surface rate — the diver was relaxed.

	A	B	C	D	E	F	G
1	Dive	Start psi	End psi	Minutes	Avg depth ft	PSI/min at depth	SAC psi/min
2	Reef dive	3,000	1,000	40	66	50.0	16.7
3	Shallow training	3,000	800	45	33	48.9	24.4
4	Wreck	3,000	1,200	50	99	36.0	9.0

The formula

Consumption at depth first, then corrected to the surface:

=(B2-C2)/D2 then =F2/(E2/33+1) // psi per minute at depth, then surface rate

How it works

The correction to the surface is the step people skip, and it is why the same diver logs wildly different rates on different dives:

B2-C2 is pressure used: 3000 minus 1000 is 2000 psi.
/D2 divides by dive minutes. 2000 over 40 is 50 psi per minute — at 66 feet.
E2/33+1 is absolute pressure in atmospheres. Every 33 ft of seawater adds one atmosphere to the one you started with, so 66 ft is 3 ata.
F2/3 brings the rate back to the surface: 16.7 psi/min. That number is yours regardless of depth.

To plan a dive, run it forward: SAC * (Depth/33+1) * Minutes. At 16.7 psi/min, 30 minutes at 99 ft (4 ata) needs about 2,000 psi — before reserve.

Try it: interactive demo

Interactive

Enter start and end pressure, dive time and average depth in feet of seawater.

Start psi

End psi

Minutes

Average depth (ft)

Variations

RMV in cubic feet per minute

SAC in psi is tied to one tank. Convert to volume using the tank's rated size and working pressure so the number transfers to any cylinder.

=G2*TankCuFt/RatedPsi

Metres and bar

Same formula with 10 m per atmosphere.

=(StartBar-EndBar)/Minutes/(DepthM/10+1)

Pitfalls & errors

Use average depth, not maximum. A dive computer's average-depth reading is ideal; using max depth on a multilevel dive understates your SAC and you will plan too little gas next time.

SAC changes with workload and temperature. Log it on several dives and plan with the highest of your recent normal values, not the best one.

Forgetting the +1 — using E2/33 instead of E2/33+1 — makes the 33 ft dive divide by 1 instead of 2 and doubles the SAC. The +1 is the atmosphere you are already under at the surface.

Practice workbook

📊

Download the free Scuba: SAC Rate From Tank Pressure Used, Dive Time And Depth practice workbook

Edit the yellow pressure, time and depth cells; consumption at depth and SAC recalculate.

Frequently asked questions

Why 33 feet?

Because 33 feet of seawater exerts roughly one atmosphere of pressure. In fresh water it is about 34 feet, and in metric it is 10 metres. The formula just needs the depth divided by whatever one atmosphere is in your units.

What is a typical SAC rate?

Relaxed recreational divers often log 15–25 psi/min on an aluminum 80; new or working divers can be 30 or more. Because psi depends on the tank, instructors compare divers using RMV in cubic feet or litres per minute instead — see the variation.

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Function references: SUM ROUND