Equivalent Air Depth Calculator for Nitrox
Equivalent air depth in feet equals ((1 - oxygen fraction) x (depth + 33) / 0.79) - 33. EAN32 at 100 feet gives an equivalent air depth of 81.5 feet, or 24.8 metres, which is why nitrox extends bottom time. It is purely a nitrogen calculation and says nothing about oxygen, so always pair it with the maximum operating depth for the same mix.
Equivalent air depth answers one narrow question: if I breathe this nitrox mix at this depth, what air dive would load my tissues with the same amount of nitrogen? Because every decompression table and every algorithm was built around nitrogen, converting a nitrox dive into its air equivalent is how a table can be used at all with a mix it was never written for. It is also the clearest way to see, before you pay for a fill, whether a richer mix will actually buy you time on the dive you are planning.
Worth stating up front: the calculator below is a planning and teaching tool. In the water you follow your computer, because a unit like the Shearwater Peregrine models tissue loading continuously across the whole profile rather than at one nominal depth, and no table can match that. What the equivalent air depth calculation gives you is understanding, and the ability to compare two mixes at the surface before you commit.
- Nitrogen fraction
- 68.0%
- Oxygen partial pressure
- 1.29 ata
- Depth saved
- 18.5 ft
Equivalent air depth across the common mixes
| Mix | Equivalent air depth | MOD at 1.4 | ppO2 at your depth | Usable here |
|---|
What is equivalent air depth?
Equivalent air depth is the depth at which an air dive would produce the same nitrogen partial pressure as your nitrox dive produces at its actual depth. It is always shallower than the real depth, because every nitrox mix contains less nitrogen than air does.
The reason it exists is historical and practical. Decompression tables and algorithms are built on models of how nitrogen dissolves into and comes out of tissue. Air is 79 percent nitrogen, and every classical table assumes that. Give a diver EAN32, which is 68 percent nitrogen, and the table no longer describes the dive. Rather than write a new table for every mix, you convert the dive into the air dive that would produce the same nitrogen exposure, and read the existing table at that depth.
What is the equivalent air depth formula?
EAD (fsw) = ((1 - FO2) x (depth + 33) / 0.79) - 33
EAD (m) = ((1 - FO2) x (depth + 10) / 0.79) - 10
Every term earns its place. (1 - FO2) is the nitrogen fraction of your mix. (depth + 33) converts depth to absolute pressure expressed in feet of sea water, since the atmosphere above the water is worth another 33 feet. Dividing by 0.79 asks what absolute pressure air would need to be at to deliver the same nitrogen. Subtracting 33 converts that absolute pressure back into a depth.
Work EAN32 at 100 feet. The nitrogen fraction is 0.68. Absolute pressure is 133 feet of sea water. Multiply: 90.44. Divide by 0.79: 114.48. Subtract 33: 81.5 feet. In metres the same dive is 30.5 m actual and 24.8 m equivalent.
You will occasionally see 0.79 replaced with 0.7902, or the nitrogen fraction of air taken as 0.78 with 1 percent argon treated separately. The difference at recreational depths is under a foot and it is not worth arguing about. What is worth noticing is that the correction is a multiplicative one on absolute pressure, which is why the depth saved grows as you go deeper: at 60 feet EAN32 saves you about 12 feet, and at 130 feet it saves nearly 23.
How much bottom time does that actually buy?
This is the question the calculation is really for. Convert your dive to its equivalent air depth, then read the no-decompression limit at that depth instead of at your real one.
| Actual depth | Air no-stop limit | EAN32 equivalent air depth | EAN32 no-stop limit | EAN36 equivalent air depth | EAN36 no-stop limit |
|---|---|---|---|---|---|
| 50 ft | 80 min | 38 ft | 140 min | 34 ft | 205 min |
| 60 ft | 55 min | 47 ft | 80 min | 42 ft | 80 min |
| 70 ft | 40 min | 56 ft | 55 min | 50 ft | 55 min |
| 80 ft | 30 min | 64 ft | 40 min | 59 ft | 55 min |
| 90 ft | 25 min | 73 ft | 30 min | 67 ft | 40 min |
| 100 ft | 20 min | 82 ft | 25 min | 75 ft | 30 min |
| 110 ft | 16 min | 90 ft | 20 min | 83 ft | 25 min |
| 120 ft | 13 min | 99 ft | 20 min | past MOD | not usable |
| 130 ft | 10 min | 107 ft | 16 min | past MOD | not usable |
Two things fall out of that table immediately. The first is that the gain is real and largest in the 50 to 90 foot band: EAN32 at 60 feet moves the nominal limit from 55 minutes to 80, and EAN36 at 80 feet moves it from 30 minutes to 55, which is nearly double. The second is that the gain shrinks and then evaporates at the deep end, not because the nitrogen maths stops working but because the oxygen ceiling arrives. EAN36 has a maximum operating depth of 95 feet, so its entry at 120 feet is not a shorter dive. It is not a dive at all.
Why must you always pair equivalent air depth with MOD?
Because they move in opposite directions and only one of them is a hard limit.
As you enrich a mix, nitrogen falls, so the equivalent air depth gets shallower and the no-stop time gets longer. That is the whole appeal. At the same time oxygen rises, so the maximum operating depth gets shallower too, and that ceiling is not negotiable: exceeding it risks a central nervous system oxygen toxicity event, and the failure mode is a convulsion underwater. Run any mix through the nitrox MOD calculator at the same time as this one, and take the shallower of the two answers as the ceiling for the dive.
The practical version of that rule is short. Pick the depth first. Then pick the richest mix whose maximum operating depth comfortably covers that depth, with margin for going a little deeper than planned. Then, and only then, look at what equivalent air depth says you have gained. Choosing a mix by the bottom time you want and hoping the depth works out is the sequence that gets people hurt.
Does a longer no-stop limit mean a longer dive?
Often not, and this surprises people who have just paid extra for nitrox. Extending the nitrogen limit only helps if nitrogen was the binding constraint, and at depth it very often is not.
At 100 feet you consume gas four times as fast as at the surface. A diver with a respiratory minute volume of 0.5 cubic feet per minute burns 2 cubic feet per minute at that depth, so an aluminium 80 with a sensible reserve gives roughly 22 minutes of bottom time. The nitrogen limit on EAN32 is 25 minutes. The gas runs out first, so the nitrox bought nothing on that dive except a slightly less loaded surface interval. Work out which limit binds on your own profile with the SAC rate calculator before you pay for the fill.
Where nitrox does earn its money is repetitive diving. Three dives a day on a liveaboard, or a week of two-tank mornings, accumulate residual nitrogen across surface intervals, and a leaner nitrogen load on every dive compounds. That, plus feeling less wrung out at the end of a diving week, is what regular nitrox divers actually report, and it is a more honest reason to buy it than a single deep dive that your gas supply was going to end anyway.
What are the real limits of this calculation?
- It assumes a square profile. The formula takes one depth. A real dive that drops to 100 feet, works at 70 and finishes at 30 does not have one depth, and treating the maximum as if it were the whole dive is conservative but crude. Computers do not have this problem, which is a large part of why they replaced tables.
- It says nothing about oxygen. Covered above, and worth repeating because it is the failure that actually hurts people.
- It says nothing about narcosis. Nitrogen narcosis at 100 feet on EAN32 feels like nitrogen narcosis at 100 feet on air. The small reduction in nitrogen partial pressure is not something a diver can perceive.
- It says nothing about oxygen exposure over time. The CNS clock and the pulmonary oxygen toxicity units both accumulate across a day, and both are tracked separately by your computer.
- It cannot make a dive shallower than it is. Ascent rate, safety stop and minimum gas reserve are all set by the real depth, not the equivalent one. Plan the reserve with the gas planning calculator at the actual depth.
A slate carrying the mix, the maximum operating depth, the planned maximum depth and the turn pressure removes most of the ways a nitrox dive goes wrong on the surface rather than at depth. A folding wrist slate costs less than one boat dive and gives every one of those numbers a place to live where you can read them at depth.
Related tools and reading
- Nitrox MOD calculator, the oxygen half of the same decision
- SAC rate calculator, to see whether gas or nitrogen limits your dive
- Gas planning calculator, for reserve and turn pressure
- Best dive computers, all of which track this continuously
- Dive computer specifications A to Z, including nitrox ceilings by model
- Intermediate scuba setup, the kit that supports nitrox diving
Frequently asked questions
What is the equivalent air depth formula?
Equivalent air depth in feet equals the nitrogen fraction of your mix, multiplied by depth plus 33, divided by 0.79, minus 33. For EAN32 at 100 feet that is 0.68 times 133, divided by 0.79, minus 33, which gives 81.5 feet. In metres, substitute 10 for 33 in both places and the same dive gives an equivalent air depth of 24.8 metres.
What is the equivalent air depth of EAN32 at 100 feet?
About 81.5 feet, or 24.8 metres. That means the nitrogen loading you take on at 100 feet breathing EAN32 matches the loading of an air dive to roughly 82 feet. Rounded up to the next line of the PADI Recreational Dive Planner, the 90 foot line, that is a no-decompression limit of 25 minutes against 20 minutes for a 100 foot air dive, which is where the extra bottom time comes from.
Does equivalent air depth account for oxygen toxicity?
No, and this is the single most important limitation of the calculation. Equivalent air depth is purely a nitrogen calculation. It tells you nothing about oxygen partial pressure, which is the limit that makes a rich mix dangerous at depth. Always pair an equivalent air depth figure with a maximum operating depth figure for the same mix, and treat the shallower of the two as your real ceiling.
Why is equivalent air depth always shallower than actual depth?
Because a nitrox mix contains less nitrogen than air does. Air is 79 percent nitrogen; EAN32 is 68 percent. At the same depth and the same ambient pressure, the nitrogen partial pressure in the nitrox mix is lower, so your tissues load more slowly. Equivalent air depth expresses that lower loading as the shallower air dive that would produce the same nitrogen partial pressure.
Should I use equivalent air depth if my computer already handles nitrox?
For planning and for understanding, yes. For the dive itself, follow the computer. A modern nitrox capable computer tracks tissue loading continuously and updates through the whole profile, which no table can do. The value of the equivalent air depth calculation is that it lets you see before the dive whether a richer mix will actually buy you useful time at your planned depth.
Does equivalent air depth reduce narcosis?
No. Narcosis at recreational depths is driven mainly by nitrogen partial pressure, and swapping a few percent of nitrogen for oxygen changes it very little. A diver at 100 feet on EAN32 should expect to feel much the same as at 100 feet on air. Claims that nitrox produces a clearer head at depth usually describe better hydration, warmth or carbon dioxide management rather than the mix.
How we choose: we compare published manufacturer specifications, published training agency standards, and verified owner reviews across retailers. We do not test gear in person, and every depth rating, cylinder capacity and algorithm name quoted here is the manufacturer's published figure unless we say otherwise, so confirm it on the current spec sheet before you buy. Nothing here is dive instruction, and no calculator output on this site is a dive plan. Scuba diving carries a real risk of decompression sickness, oxygen toxicity, barotrauma and drowning. Dive within the limits of your certification, verify every plan with your own computer, and buy the training before the gear that assumes it.
Logging your own SAC rate and gas plans? The Dive Kit & Air Planning Workbook is the paid version of these pages: 8 printable worksheets you fill in with your own numbers, plus the full PDF, $29.