Nitrox and Trimix Calculator: MOD, Best Mix, END and PPO2

Enter your mix and depth. The calculator works out the oxygen partial pressure at that depth, the maximum operating depth at your PPO2 limit, the best nitrox for the depth, the equivalent narcotic and air depths, the helium needed for a target END and your CNS oxygen clock, with the formula for each.

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Your mix and depth

Oxygen (%)

Helium (%)

Depth (ft)

PPO2 limit 1.4 bottom max, 1.6 deco

Target END (ft)

Minutes at this depth

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

Nitrox 32 at 100 ft: PPO2 1.29

PPO2 at depth: 1.29
MOD at 1.4: 111 ft
END: 100 ftoxygen narcotic
EAD: 81 ft
Best mix here: 34% O₂at 1.4
Helium for 100 ft END: None
Oxygen clock: 22% CNS40 min

The work

ATA100 ft ÷ 33 + 1 = 4.03 ATA
PPO2 at depth4.03 ATA × 0.32 = 1.29
MOD at 1.4(1.40 ÷ 0.32 × 33) − 33 = 111.4 ft → round down: 111 ft
Best mix for this depth1.40 ÷ 4.03 ATA = 0.347 → round down: 34% oxygen
END, oxygen counted as narcotic(1 − 0.00) × (100 + 33) − 33 = 100.0 ft
EAD0.68 × (100 + 33) ÷ 0.79 − 33 = 81.5 ft
Helium for an END of 100 ft1 − (100 + 33) ÷ (100 + 33) = 0.000 → none needed
Oxygen clock40 min × 100 ÷ 180 min (NOAA limit at 1.3) = 22% CNS

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

The formulas

Depth is in feet of seawater. Fractions are percentages ÷ 100.

ATA = depth ÷ 33 + 1
PPO2 = ATA × oxygen fraction
MOD = (PPO2 limit ÷ oxygen fraction × 33) − 33, rounded down
Best mix = PPO2 limit ÷ ATA, rounded down
END = (1 − helium fraction) × (depth + 33) − 33. Oxygen counts as narcotic, as UTD calculates it.
EAD = nitrogen fraction × (depth + 33) ÷ 0.79 − 33
Helium for a target END = 1 − (target END + 33) ÷ (depth + 33), rounded up
Oxygen clock = minutes × 100 ÷ NOAA’s single-exposure limit at that PPO2

Why do DIR teams dive standard gases?

Because a short list of mixes means fewer numbers to get wrong. The buttons above load the common ones. UTD keeps END at 100 feet or less, so anything deeper than 100 feet gets helium. Read Standard Gases for the full list and the depths each one covers.

Analyze every mix

You breathe what’s in the cylinder, not what you ordered. Analyze every fill, label it, and run these numbers on the mix you measured. Diving nitrox or trimix takes training for that gas, which starts with Nitrox Diver.

Frequently asked questions

What’s the MOD of nitrox 32?

111 feet at a PPO2 of 1.4: (1.4 ÷ 0.32 × 33) − 33 = 111.4, rounded down. UTD still keeps air and nitrox dives to 100 feet.

Why does END count oxygen as narcotic?

UTD treats oxygen as narcotic, like nitrogen, so only helium lowers your equivalent narcotic depth. It’s the conservative choice, and it’s why UTD keeps END to 100 feet or less with helium.

What PPO2 limit should I use?

UTD chooses bottom mixes for a working PPO2 of 1.2 and a maximum of 1.4, and uses decompression gases up to 1.6. Use the limits your training sets.

How is the oxygen clock calculated?

Divide your minutes at a PPO2 by NOAA’s single-exposure limit for that PPO2 and multiply by 100. The calculator uses the NOAA row at or above your PPO2, so it never undercounts.

Do I need training to dive nitrox or trimix?

Yes. These numbers help you plan and check your math, but each gas needs its own certification, and every mix must be analyzed before you breathe it.