Nitrox Calculator | MOD (Maximum Operating Depth at PPO2 1.4/1.6) and Oxygen Partial Pressure for EAN32 and EAN36, EAD (Equivalent Air Depth), Best Mix, CNS% and OTU (NOAA Oxygen Exposure Limits and Surface Interval), Air Consumption (SAC and RMV) and How Long You Can Dive, MOD Chart and Glossary
The Nitrox Calculator brings together the nitrox (enriched air) and scuba gas calculations behind questions such as “how deep can I go on EAN32?”, “what is the best mix for 30 m?”, “what is my CNS% for 40 minutes at 30 m on EAN32?” and “what is my air consumption in litres per minute?”. In the MOD tab, choose the oxygen fraction (with buttons for EAN28 to EAN40) and the PPO2 limit (1.2 to 1.6) to get the MOD (EAN32 is 33.7 m at 1.4 and 40 m at 1.6), plus the oxygen partial pressure and EAD (equivalent air depth; 24.5 m for EAN32 at 30 m) at your planned depth, with a warning when you go past the MOD. The best mix tab gives the oxygen fraction that reaches the PPO2 limit at your planned maximum depth (EAN35 for 30 m at 1.4) and a chart by depth. The CNS% tab uses the NOAA oxygen exposure limits to give the CNS% used on a dive (21.5% for EAN32 at 30 m for 40 minutes) and OTU, and adds what is left of the previous dive's CNS%, which halves over the surface interval. The air consumption tab takes the cylinder size (10 L, 12 L, etc.), start and end pressure, dive time and average depth to give your SAC (bar/min) and RMV (L/min), and uses the RMV to tell you how many minutes you can stay at a planned depth before reaching 50 bar. It also includes a MOD chart, the NOAA table and a glossary of EANx, PPO2, EAD, CNS%, OTU, SAC and RMV. Depths can be switched between metres and feet, and between seawater and fresh water.
Nitrox MOD (maximum depth), oxygen partial pressure and EAD
From the oxygen fraction of your gas, this gives the maximum depth you may dive to (MOD), plus the oxygen partial pressure at your planned depth and the depth on air with the same nitrogen load (EAD).
PPO2 limit
MOD (PPO2 1.4)
33.7 m
4.38 ata
MOD (PPO2 1.4)
33.7 m
Recreational limit
MOD (PPO2 1.6)
40.0 m
Contingency depth
PPO2 at 30.0 m
1.28
4.00 ata
EAD at 30.0 m
24.5 m
Same nitrogen as diving 5.5 m shallower on air
Formulas: MOD = (PPO2 limit ÷ oxygen fraction − 1) × 10 m (seawater), EAD = (depth + 10) × nitrogen fraction ÷ 0.79 − 10. The MOD is rounded shallower and the EAD deeper, to 0.1.
| PPO2 limit | MOD | Absolute pressure |
|---|---|---|
| 1.2 | 27.5 m | 3.75 ata |
| 1.3 | 30.6 m | 4.06 ata |
| 1.4 | 33.7 m | 4.38 ata |
| 1.5 | 36.8 m | 4.69 ata |
| 1.6 | 40.0 m | 5.00 ata |
This tool is a planning guide. Only use nitrox after training (such as an Enriched Air Diver course), analyse your cylinder's oxygen fraction yourself before the dive and set your dive computer to match. No correction is made for altitude (such as mountain lakes).
ABOUT
About Nitrox Calculator | MOD (Maximum Operating Depth at PPO2 1.4/1.6) and Oxygen Partial Pressure for EAN32 and EAN36, EAD (Equivalent Air Depth), Best Mix, CNS% and OTU (NOAA Oxygen Exposure Limits and Surface Interval), Air Consumption (SAC and RMV) and How Long You Can Dive, MOD Chart and Glossary
The Nitrox Calculator works out the MOD (maximum operating depth), oxygen partial pressure, EAD (equivalent air depth), best mix, CNS% and OTU for nitrox (enriched air), and your scuba air consumption (SAC and RMV). Depths can be switched between metres and feet, and between seawater and fresh water.
How to use Nitrox Calculator | MOD (Maximum Operating Depth at PPO2 1.4/1.6) and Oxygen Partial Pressure for EAN32 and EAN36, EAD (Equivalent Air Depth), Best Mix, CNS% and OTU (NOAA Oxygen Exposure Limits and Surface Interval), Air Consumption (SAC and RMV) and How Long You Can Dive, MOD Chart and Glossary
- 1At the top, choose the depth unit (m or ft) and the water (seawater or fresh water)
- 2In “MOD, PPO2 & EAD”, choose the oxygen fraction (EAN32, etc.) and the PPO2 limit (usually 1.4), enter the planned depth and check the MOD, oxygen partial pressure and EAD
- 3In “Best mix”, enter the planned maximum depth and check the oxygen fraction that suits it
- 4In “CNS% & OTU”, enter the oxygen fraction, depth and time; for a second dive, also enter the previous dive's CNS% and the surface interval
- 5In “Air consumption”, enter a dive you have done (cylinder, pressures, time and average depth) to get your RMV, then check how long you can stay on the next dive
Great for
- Checking the MOD of a nitrox cylinder you have rented
- Practising or checking the calculations (MOD, EAD, best mix) for an enriched air course or exam
- Checking how CNS% builds up over several dives in a day
- Finding your air consumption (RMV) and whether you need a bigger cylinder
- Comparing depths in feet at an overseas dive centre with your MOD
- Checking how the MOD changes when diving in fresh water such as a lake or reservoir
Good to know
- MOD is (PPO2 limit ÷ oxygen fraction − 1) × 10 m (seawater) and EAD is (depth + 10) × nitrogen fraction ÷ 0.79 − 10. One atmosphere is taken as 10.3 m in fresh water, and 33 ft in seawater and 34 ft in fresh water. The MOD is rounded shallower and the EAD deeper, to 0.1.
- The best mix is PPO2 limit ÷ absolute pressure, rounded down to a whole percent. Standard nitrox courses cover 22–40% oxygen, and dive centres most often fill 32% or 36%.
- CNS% is the share of the NOAA oxygen exposure limit (allowed time for a single dive: 150 minutes at 1.4 and 45 minutes at 1.6), with straight-line interpolation between table values. The previous dive's CNS% is assumed to halve every 90 minutes of surface interval, and what is left is added.
- OTU is time × ((PPO2 − 0.5) ÷ 0.5)^0.83, with 850 OTU per day (REPEX) as a guide. It is rarely an issue for recreational diving.
- This tool is a planning guide. Only use nitrox after training, analyse your cylinder's oxygen fraction yourself before the dive and set your dive computer to match. No correction is made for altitude (such as mountain lakes).
Questions this tool answers
Use Nitrox Calculator | MOD (Maximum Operating Depth at PPO2 1.4/1.6) and Oxygen Partial Pressure for EAN32 and EAN36, EAD (Equivalent Air Depth), Best Mix, CNS% and OTU (NOAA Oxygen Exposure Limits and Surface Interval), Air Consumption (SAC and RMV) and How Long You Can Dive, MOD Chart and Glossary for searches like these:
- Calculate the MOD for nitrox
- Find the maximum depth for EAN32
- Calculate the EAD (equivalent air depth)
- Calculate the best mix
- Calculate CNS% for a dive
- Calculate scuba air consumption (SAC and RMV)
FAQ
- How deep can I go on EAN32?
- 33.7 m (about 34 m, 111 ft) with a PPO2 limit of 1.4, and 40 m with a limit of 1.6. Recreational diving usually uses 1.4 as the limit, so remembering EAN32 as “to about 33 m” keeps you on the safe side.
- What is the MOD of EAN36?
- 28.8 m at a PPO2 of 1.4 and 34.4 m at 1.6. EAN36 suits plans shallower than 30 m, and the lower nitrogen lets you extend your no-decompression limit.
- What is the EAD for EAN32 at 30 m?
- (30 + 10) × 0.68 ÷ 0.79 − 10 = about 24.4 m, or 24.5 m rounded up to be safe. The nitrogen load is the same as diving on air at about 24.5 m, so on an air dive table you read the no-decompression limit at this depth. The oxygen partial pressure is 1.28.
- What is the best mix for 30 m?
- With a PPO2 limit of 1.4, 1.4 ÷ 4.0 ata = 35%, so EAN35. It is 40% for 25 m and 28% for 40 m. Dive centres most often fill 32% or 36%, so for 30 m EAN32 is the usual choice.
- What is my CNS% for 40 minutes at 30 m on EAN32?
- The oxygen partial pressure is 1.28 and the allowed time interpolated from the NOAA table is 186 minutes, so CNS is about 21.5% and OTU about 58. After a one-hour surface interval, about 13.5% is left.
- How do I calculate my air consumption (RMV)?
- If you used a 12 L cylinder from 200 bar to 80 bar (120 bar) over 40 minutes at an average depth of 15 m (2.5 ata), your SAC is 120 ÷ 40 ÷ 2.5 = 1.2 bar/min and your RMV is 1.2 × 12 = 14.4 L/min. With an RMV of 15 L/min at 20 m you use 45 L a minute, so a 12 L, 200 bar cylinder lasts about 40 minutes before you reach 50 bar.
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