About the Nitrox MOD and Best Mix Calculator
This nitrox calculator does the three gas checks every enriched-air diver runs before a dive. It gives the maximum operating depth (MOD) of a blend at your chosen oxygen partial pressure limit, the best mix — the richest oxygen percentage you can safely breathe at a planned depth — and the equivalent air depth (EAD) you can use with air dive tables or to compare nitrogen loading.
Enter the analysed oxygen percentage of your cylinder, pick a PO2 limit and enter your planned depth in metres or feet. MOD is shown in both metres of seawater (10 m per bar) and feet of seawater (33 ft per atmosphere), along with a chart of oxygen partial pressure against depth and the contingency MOD at 1.6.
Recreational agencies generally plan dives to a maximum PO2 of 1.4, with 1.6 as the absolute ceiling for contingencies; DAN notes PADI’s 1.4 ata limit for open-circuit nitrox, and NOAA’s single-dive oxygen exposure limits run from 210 minutes at 1.2 down to just 45 minutes at 1.6. This tool is a planning aid for certified nitrox divers — always analyse your own gas, label the cylinder with its MOD and follow your training agency’s limits and your dive computer.
How to use the nitrox mod and best mix calculator
- 1Analyse your cylinder and enter the oxygen percentage.
- 2Choose the maximum PO2 your training and dive plan allow (1.4 is typical).
- 3Pick metres or feet and enter the planned maximum depth.
- 4Read the MOD and make sure the planned depth is shallower.
- 5Use the best mix to order a fill and the EAD with air tables.
Formula and method
All three checks come from Dalton’s law: the partial pressure of oxygen is its fraction in the mix times the absolute pressure. In seawater, pressure rises by about one atmosphere every 10 m (33 ft), so absolute pressure P = depth ÷ k + 1, with k = 10 for metres and 33 for feet. Setting FO2 × P equal to your PO2 limit and solving for depth gives the maximum operating depth; solving for FO2 at a fixed depth gives the best mix, rounded down so the limit is never exceeded.
Equivalent air depth is the depth at which air (79% nitrogen) would give the same nitrogen partial pressure as your nitrox does at the planned depth. It is used with air tables and shows why nitrox extends no-decompression limits. Fresh water is slightly less dense (about 10.3 m or 34 ft per atmosphere), so seawater figures are a little conservative in lakes. Values here are planning figures; use an analysed gas and your dive computer.
- FO2
- Fraction of oxygen in the mix (32% = 0.32)
- FN2
- Fraction of nitrogen (1 − FO2)
- PO2max
- Maximum oxygen partial pressure, in ata (usually 1.4)
- P
- Absolute pressure at depth, in ata
- k
- 10 m or 33 ft of seawater per atmosphere
- D
- Depth in metres or feet of seawater
Worked examples
EAN32 at PO2 1.4, planned 30 m dive
MOD = 10 × (1.4 ÷ 0.32 − 1) = 33.75 m (111 ft). At 30 m the pressure is 4 ata, so PO2 = 0.32 × 4 = 1.28 and the best mix is 1.4 ÷ 4 = 35%. EAD = 10 × (4 × 0.68 ÷ 0.79 − 1) = 24.4 m.
EAN36 for a 100 ft wreck dive
EAN36 has a MOD of 33 × (1.4 ÷ 0.36 − 1) = 95.3 ft, so 100 ft exceeds it: PO2 there is 0.36 × 4.03 = 1.45. The best mix for 100 ft is 1.4 ÷ 4.03 = 34.7%, rounded down to 34%.
EAN40 at the 1.6 contingency limit
At PO2 1.6, EAN40 can go to 10 × (1.6 ÷ 0.4 − 1) = 30 m or 99 ft. At 25 m (3.5 ata) its PO2 is exactly 1.4, and nitrogen loading equals air at about 16.6 m.
Frequently asked questions
What is the MOD of EAN32?+
At a PO2 of 1.4, EAN32 has a maximum operating depth of about 33.8 m or 111 ft of seawater. At the 1.6 contingency limit it is 40 m or 132 ft. Most divers label EAN32 cylinders with a 1.4 MOD of 33 m or 110 ft.
Why is 1.4 the usual PO2 limit for nitrox?+
Breathing oxygen at high partial pressure can cause central nervous system oxygen toxicity, which may trigger convulsions underwater. DAN calls a PO2 of 1.4 ata or less the “green light” zone for open-circuit scuba and notes PADI’s 1.4 limit for nitrox. At 1.6 the risk is still low but the margin for error is much slimmer, so it is kept for contingencies only.
How do you calculate best mix for a dive?+
Divide your PO2 limit by the absolute pressure at the planned maximum depth and round down. For 30 m (4 ata) at PO2 1.4, the best mix is 1.4 ÷ 4 = 0.35, or EAN35.
What is equivalent air depth used for?+
EAD converts a nitrox dive into the air depth with the same nitrogen partial pressure, so you can use air dive tables for no-stop limits. A 30 m dive on EAN32 has an EAD of about 24 m, which is why nitrox gives longer bottom times.
Does nitrox let you dive deeper?+
No — nitrox actually has a shallower maximum depth than air because of its higher oxygen content. Its benefit is reduced nitrogen absorption, which extends no-decompression time and shortens surface intervals within recreational depths.
Is the calculation different in fresh water?+
Fresh water is less dense, so each atmosphere takes about 10.3 m or 34 ft. Using the seawater constants of 10 m and 33 ft gives slightly shallower, more conservative MODs in lakes and quarries.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.