How Long Does a Scuba Tank Last? Minutes by Depth and SAC Rate

What decides how long a scuba tank lasts?

Your breathing rate and your depth. Tank size and fill pressure set how much gas you start with; your SAC rate and the absolute pressure at depth set how fast it goes. Gas used per minute at depth is SAC × (depth ÷ 10 + 1), so the same diver empties a tank twice as fast at 10 m (33 ft) as at the surface.

The US Navy Diving Manual gives a worked example. A diver swimming at 0.5 knot inhales 20 L/min, and a cylinder holding about 2,000 standard litres lasts that diver roughly 100 minutes at the surface, 50 minutes at 33 ft, where the pressure doubles, and a third as long at three atmospheres.

If you don't know your SAC, work it out from a logged dive with our SAC calculator, and plan with a figure from dives like the one ahead, rounded up. DAN Europe's example treats 13.4 L/min as 14. The tables below use 15, 20 and 25 L/min to show the spread, and our tank duration calculator runs the same sum with your own SAC, tank and reserve. The Navy lists what moves a diver's rate, including water temperature, suit thickness, current, the task and how recently the diver has been in the water.

How long does a 12 L, 200 bar tank last at 10, 18 and 30 m?

Between 18 and 60 minutes, if you keep a 50 bar reserve and spend the whole dive at one depth.

Breathing rate moves the answer as much as depth does. At 18 m, a SAC 25 diver gets 17 minutes less than a SAC 15 diver. Taking a SAC 20 diver down to 18 m instead of 10 m costs 13 minutes.

Real dives are multilevel, so our gas planner works from average depth. A dive that touches 18 m but spends most of its time at 12 m lasts longer than the 18 m row. One that stays at 18 m and swims into a current lasts less, because current is on the Navy's list of factors that influence work rate.

How the table is calculated

Each cell is usable gas divided by gas per minute at depth, rounded down to a whole minute. Rounding down keeps the table in line with the planner, which rounds pressure up to the next 5 bar. Enter 32 minutes at 18 m and SAC 20 in its Required Gas tab and it returns 1,792 L, which is 149.3 bar, rounded up to 150 bar: exactly the gap between a 200 bar fill and a 50 bar reserve. At 33 minutes it asks for 155 bar, which eats into the reserve.

The 18 m row at SAC 20, step by step:

• Usable gas: (200 - 50) × 12 = 1,800 L.
• Absolute pressure at 18 m: 18 ÷ 10 + 1 = 2.8 bar.
• Gas per minute at 18 m: 20 × 2.8 = 56 L/min.
• Time: 1,800 ÷ 56 = 32.1, rounded down to 32 minutes.

Why the reserve comes off the top

Because the minutes in the table are all the gas there is to spend. The Navy manual counts air supply duration from initial descent to surfacing, so the descent, the ascent and any stop draw on the same budget. It also warns that relying solely on a reserve may leave a diver without enough air to reach the surface, and that this matters more the deeper the dive.

In a DAN study of 947 recreational open-circuit scuba deaths from 1992-2003, insufficient gas was the most frequent trigger, at 41%, ahead of entrapment (20%) and equipment problems (15%).

DAN's gas management advice is to build a limit into the plan, for example turning back when the first diver in a pair has used a third or half of the gas, not counting the reserve. The planner's Rule of Thirds tab does the arithmetic for the first option.

How much gas does the ascent need?

For a calm solo ascent, little. For two divers sharing one tank from 30 m, more than a 50 bar reserve. Shearwater's manual works a 3-minute safety stop at 4.5 m (15 ft) with a SAC of 1.4 bar/min: it uses 6.1 bar.

The planner's rock bottom figure models the bad case: one minute sorting out a problem at maximum depth, an ascent to 5 m at 9 m/min (the US Navy's limit is 30 ft/min), a 3-minute stop at 5 m, the last 5 m to the surface at the same rate, and two divers breathing from one tank at a stressed SAC, taken here as double the normal rate. For 30 m at SAC 20 that is 1 minute at 4 bar (4), 2.78 minutes of ascent at an average of 2.75 bar (7.64), 3 minutes at 1.5 bar (4.5) and 0.56 minutes to the surface at an average of 1.25 bar (0.69), or 16.83 bar-minutes. Two divers at 40 L/min each breathe 80 L per bar-minute, so they need 1,347 L. That is 112.2 bar in a 12 L tank, rounded up to 115 bar.

At 10 m with SAC 15, rock bottom works out to 45 bar, so the 50 bar floor applies. Deeper, the table changes a lot: at 30 m a SAC 20 diver has 12 usable minutes instead of 22, and a SAC 25 diver has 6.

How long does an aluminium 80 last?

One to three minutes less than the 12 L tank, because an aluminium 80 has a smaller internal volume and holds less than 80 cubic feet. Catalina's spec sheet lists its S80 at 11.1 L (678 cu in) water capacity and 3,000 psi (207 bar) service pressure, and notes that the industry-standard S80 has an actual capacity of 77.4 cu ft.

Our calculators take the water capacity, 11.1 L. Filled to its 207 bar service pressure with a 50 bar (725 psi) reserve, that leaves 157 bar. The planner rounds pressure up to the next 5 bar, so the largest result that stays inside 157 bar is 155 bar, or 155 × 11.1 = 1,720.5 L. The table uses that figure.

In imperial units, 20 L/min is about 0.71 cu ft/min, so at 60 ft an AL80 gives that diver about 30 minutes before a 725 psi reserve. Rock bottom in bar also rises with the smaller tank: at 30 m and SAC 20 the planner gives 125 bar for the AL80, against 115 bar for the 12 L.

One caveat for a full tank. The rated 77.4 cu ft is about 2,192 L, while 11.1 L × 207 bar comes to 2,298 L, roughly 5% more. Our calculators multiply bar by litres, and Shearwater's manual says its gas calculations make the same ideal-gas assumption, which it calls a good approximation up to about 207 bar. For a full AL80, read these minutes as a ceiling.

Can your dive computer tell you how long your tank will last?

An air-integrated computer can estimate it live, as long as you know what the estimate leaves out. Shearwater's gas time remaining (GTR) on the Perdix AI is the time you can stay at the current depth before a direct ascent at 10 m/min (33 ft/min) would bring you up with your set reserve. It uses your SAC averaged over the last two minutes and does not include safety or decompression stops.

Two things follow. GTR moves with your breathing, so a hard swim into current shortens it straight away. And it answers a solo question: it does not model sharing gas with a buddy. Use it as a live check against the rock bottom pressure you worked out before the dive.