Scuba Air Consumption (SAC and RMV) Calculator

SAC, RMV and gas at depth

Surface air consumption, or SAC, is how fast you use gas measured at surface pressure. Divers quote it two ways. SAC in psi per minute depends on the cylinder. Respiratory minute volume, or RMV, is in cubic feet per minute and works for any cylinder. To find either, take the gas you used on a past dive and divide by the time and by the average pressure in atmospheres.

Pressure at depth is the key. Every 33 feet of salt water adds one atmosphere to the 1 at the surface, and fresh water uses about 34 feet. At 45 feet you are at 2.36 atmospheres, so every breath holds 2.36 times as much gas. Use 1,800 psi from an aluminum 80, which holds 77.4 cubic feet at 3,000 psi. That is 46.4 cubic feet used, over 40 minutes at 2.36 atmospheres, giving an RMV of 0.49 and a SAC near 19 psi per minute.

Plan from that rate. At 60 feet the pressure is 2.82 atmospheres, so you use about 1.38 cubic feet per minute. A 30-minute stay at that depth takes 41 cubic feet, about 1,600 psi, and leaves 1,400 psi from a 3,000 psi fill. If you hold aside 500 psi, the cylinder lasts about 47 minutes at that depth. You set the held-aside pressure, and this tool makes no judgment on it.

Your rate is a personal figure. It changes with work, water temperature, nerves and equipment. Use an average from several dives for a fair number. This page is gear and gas math only. It is not a dive plan or a safety check, so follow your training agency, instructor and dive computer. To budget a trip, see the skydiving cost calculator for a comparison of adventure costs and the pounds to kilograms converter for weight units.

Where these numbers come from

The formulas and default values in this calculator are based on:

• standard gas-law arithmetic (one atmosphere per 33 ft of sea water, 34 ft fresh) and published cylinder ratings for the Aluminum 80 and steel HP100, HP120 and LP85. Note: cylinder capacities are nominal, and this page was not checked against a single manufacturer chart

This is a planning estimate, not medical, legal or immigration advice. Fees, allowances, limits and entry rules change, so confirm the figures with your airline, cruise line, customs agency or the official government source before you book or travel, and follow their rules if they differ from this page.