What is MOD in scuba diving?

MOD (maximum operating depth) is the deepest you can dive a given breathing gas before the partial pressure of oxygen exceeds your chosen safe limit. Past the MOD, the risk of oxygen toxicity climbs sharply.

What is the formula for MOD?

In metres: MOD = ((PO2 limit / FO2) - 1) x 10, where PO2 limit is your maximum oxygen partial pressure (commonly 1.4 ATA working, 1.6 contingency) and FO2 is the fraction of oxygen in the mix (0.32 for EAN32).

What is the MOD of EAN32?

For EAN32 at a 1.4 PO2 limit, MOD = ((1.4 / 0.32) - 1) x 10 = 33.75 metres, which you round down to 33 metres. At a 1.6 contingency limit it is 40 metres.

Why do you round MOD down and not up?

The PO2 limit is a safety ceiling. Rounding up would put you past it. Always round MOD down to the nearest metre to stay inside the limit you calculated.

Dive theory, decoded

How to Calculate MOD (Maximum Operating Depth) for Nitrox

MOD is one of the most testable, and most misunderstood, calculations on the dive theory exam. Here is the formula, worked examples, the rule that quietly costs people marks, and a quick-reference table you can memorize.

What MOD actually means

Maximum operating depth is the deepest you can take a given breathing gas before the partial pressure of oxygen (PO2) climbs past your safe limit. Breathe a gas deeper than its MOD and you raise the risk of central nervous system oxygen toxicity, which underwater can be sudden and serious. MOD is how you put a hard number on that ceiling before you ever get in the water.

The MOD formula

In metric, the formula is:

MOD (m) = ( ( PO2 limit / FO2 ) − 1 ) × 10

PO2 limit is the maximum oxygen partial pressure you will accept. The standard working limit is 1.4 ATA; 1.6 ATA is treated as a contingency ceiling, not a planning target. FO2 is the fraction of oxygen in your mix, so EAN32 is 0.32 and EAN36 is 0.36.

The rule that saves marks: always round down. A MOD of 33.75 m becomes 33 m, never 34 m. The limit is a safety ceiling, and rounding up spends the exact margin you just calculated. Examiners (and the water) reward the conservative answer.

Worked examples

EAN32 at a 1.4 PO2 limit

( ( 1.4 / 0.32 ) − 1 ) × 10 = ( 4.375 − 1 ) × 10 = 33.75 m → 33 m

EAN36 at a 1.4 PO2 limit

( ( 1.4 / 0.36 ) − 1 ) × 10 = ( 3.89 − 1 ) × 10 = 28.9 m → 28 m

Notice the pattern: richer mixes (more oxygen) give you a shallower MOD. More O2 is more bottom time and less narcosis, but it caps how deep you can safely go.

Quick-reference table (1.4 ATA working limit)

Mix	FO2	MOD at 1.4	MOD at 1.6
Air	0.21	56 m	66 m
EAN32	0.32	33 m	40 m
EAN36	0.36	28 m	34 m
EAN40	0.40	25 m	30 m

All figures rounded down to the nearest metre.

The flip side: best mix

MOD answers "how deep can this gas go." The related question is "what mix should I breathe for a target depth," which uses the same relationship rearranged:

Best mix FO2 = PO2 limit / ( depth / 10 + 1 )

For a 30 m dive at a 1.4 limit: 1.4 / (30/10 + 1) = 1.4 / 4 = 0.35, so EAN35 is your richest safe mix. Round the oxygen fraction down here too.

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Frequently asked questions

What PO2 limit should I plan to?

1.4 ATA is the standard working limit for recreational and most technical bottom phases. 1.6 ATA is a contingency ceiling, often reserved for decompression stops under controlled conditions, not a number you plan a dive around.

Does MOD account for nitrogen narcosis?

No. MOD is an oxygen limit only. Narcosis is handled separately through equivalent narcotic depth (END). A gas can be within its MOD and still leave you noticeably narked.

Is the formula different in feet?

The logic is identical; you swap the 10 for 33 (feet of seawater per atmosphere): MOD (ft) = ((PO2 limit / FO2) - 1) x 33. Most theory exams outside the US use metric.

Independent study aid for exam preparation only. Not affiliated with or endorsed by any certification agency, and not a substitute for certified training or proper dive planning. Always dive within your training and follow your computer or tables.