What is equivalent air depth?

Equivalent air depth is the depth at which breathing air would give you the same nitrogen partial pressure as your Nitrox mix does at your actual depth. Because decompression models are driven by inert gas, an air table or planner can be used at the EAD rather than the real depth.

Does a shallower EAD mean I can dive deeper?

No. EAD only describes nitrogen loading. Your depth limit comes from oxygen, via maximum operating depth. Enriching the mix makes both numbers shallower: the shallower EAD relaxes your time limit, while the shallower MOD tightens your depth limit.

Do I still need EAD if I dive a computer?

A Nitrox-capable computer set to the analyzed mix tracks inert gas directly, so you do not convert depths by hand. EAD remains useful for understanding why Nitrox extends no-stop time, for cross-checking a plan, and for table-based planning. For example, EAN32 at 30 meters has an unrounded EAD of 24.4 meters; a table user rounds that up to the next deeper table depth, never down to 24 meters. (A Nitrox-capable computer set to the analyzed mix tracks inert gas directly, so you do not convert depths by hand. EAD remains useful for understanding why Nitrox extends no-stop time, for cross-checking a plan, and for table-based planning. For example, EAN32 at 100 feet has an unrounded EAD of 81.5 feet; on a table in 10-foot steps you use the 90-foot row, never the 80-foot row.)

Is EAD the same as equivalent narcotic depth?

They are different questions that can share arithmetic. EAD asks about decompression and counts only nitrogen. Equivalent narcotic depth asks about impairment. Some planning conventions also count oxygen as narcotic, and under that convention a Nitrox mix is treated as no less narcotic than air at the same depth.

Diving guide

Equivalent Air Depth (EAD) Explained

Work out the air depth a Nitrox dive is decompression-equivalent to, and see why EAD never changes your oxygen limit.

Nitrox extends no-stop time because you breathe less nitrogen at the same depth. Equivalent air depth puts a number on that: it is the shallower depth an air diver would have to be at to absorb nitrogen as fast as you are.

What EAD actually answers

Decompression models track inert gas, not oxygen. For a recreational Nitrox mix the inert gas is nitrogen, so two dives that produce the same nitrogen partial pressure produce comparable nitrogen loading. EAD converts your real depth on a rich mix into the air depth that matches it, which lets an air table or an air-based planner be used for a Nitrox dive.

The comparison is deliberately narrow. EAD says nothing about oxygen exposure, gas supply, narcosis, or thermal stress. It answers one question — how much nitrogen — and answers nothing else.

The EAD formula

Convert depth to absolute pressure, scale by the ratio of your nitrogen fraction to the nitrogen fraction of air, then convert back to depth:

\text{EAD} \approx \left(\text{depth} + 10\right) \times \frac{\text{FN}_2}{0.79} - 10

\text{EAD} \approx \left(\text{depth} + 33\right) \times \frac{\text{FN}_2}{0.79} - 33

Where $\text{FN}_2$ is the fraction of nitrogen in your mix — 0.68 for EAN32, because the balance of a 32% oxygen mix is 68% nitrogen. Depth and EAD are in metersfeet of seawater.

The 0.79 is the nitrogen fraction of air. Strictly, air is about 78% nitrogen plus roughly 1% argon and other trace gases; decompression practice lumps those inert gases together as 79%, which is why the same figure appears in agency tables.

The 10 and 33 come from the planning approximation that 10 meters or 33 feet of seawater adds one atmosphere, with one atmosphere at the surface at sea level. That is why the formula uses ≈ rather than =. Fresh water and altitude change the depth–pressure relationship, so use a table or computer setting made for those conditions rather than this sea-level form.

Try it: Equivalent Air Depth

Compare the nitrogen loading of a Nitrox dive with an air dive.

Actual depth

Oxygen percentage

24.4 m

EAD forEAN32at 30 m

33.8 m

MOD at 1.4 ATA

A worked example

Take EAN32 at 30 meters99 feet, so $\text{FN}_2 = 0.68$ . Under the seawater approximation the absolute pressure is 4 ATA. Substituting into the formula:

\text{EAD} \approx (30 + 10) \times \frac{0.68}{0.79} - 10 \approx 24.4\,\text{m}

\text{EAD} \approx (99 + 33) \times \frac{0.68}{0.79} - 33 \approx 80.6\,\text{ft}

Scaling 4 ATA by 0.68 ÷ 0.79 gives 3.44 ATA of air-equivalent pressure, and converting back to depth leaves an unrounded EAD of 24.4 meters80.6 feet.

One dive, two ways to describe it

EAN32 carries 68% nitrogen instead of 79%, so at 4 ATA it produces the nitrogen loading air would produce at about 3.4 ATA. The Nitrox dive and its shallower air equivalent are interchangeable to a decompression model — that equivalence is the entire meaning of EAD. Depths use the seawater, sea-level approximation of 10 m33 ft per atmosphere.

That is the whole benefit in one line: for decompression purposes the dive behaves like an air dive about 5.6 meters18 feet shallower, which is why the no-stop time is longer.

To use that result with a table, round EAD up to the next deeper table depth, never down. Here that means the next row deeper than 24.4 m, not the 24 m rowthe 90 ft row on a table in 10 ft steps, not the 80 ft row. Pretending the dive was shallower than the calculation says is the unsafe direction, and rounding up gives back some of the no-stop benefit.

Equivalent air depth reference

Raw EAD equation results for binary oxygen/nitrogen mixes in seawater at sea level (10 m or 33 ft per atmosphere), rounded to the nearest 0.1 m or 0.1 ft for display. Metric and imperial depths are calculated independently. Before using an air table, round up to the next deeper table depth.

Actual depth	EAN32EAD	EAN36EAD	EAN40EAD	EAN50EAD
6 m20 ft	3.8 m12.6 ft	3.0 m9.9 ft	2.2 m7.3 ft	0.1 m0.5 ft
12 m40 ft	8.9 m29.8 ft	7.8 m26.1 ft	6.7 m22.4 ft	3.9 m13.2 ft
18 m60 ft	14.1 m47.1 ft	12.7 m42.3 ft	11.3 m37.6 ft	7.7 m25.9 ft†
24 m80 ft	19.3 m64.3 ft	17.5 m58.5 ft	15.8 m52.8 ft	11.5 m†38.5 ft†
30 m100 ft	24.4 m81.5 ft	22.4 m†74.7 ft†	20.4 m†68.0 ft†	15.3 m†51.2 ft†
36 m120 ft	29.6 m†98.7 ft†	27.3 m†90.9 ft†	24.9 m†83.2 ft†	19.1 m†63.8 ft†

† The actual depth exceeds that mix's calculated MOD at 1.4 ATA. The EAD remains nitrogen-equivalence math, not permission to use the mix at that depth.

A richer mix makes both EAD and MOD shallower

This is the trap. Enriching a mix makes EAD shallower, which flatters the decompression picture. It also raises oxygen partial pressure, which makes maximum operating depth shallower too. The two numbers move the same way but mean opposite things: the shallower EAD relaxes your time limit, while the shallower MOD tightens your depth limit. EAN36 has a friendlier EAD than EAN32 at the same depth and a shallower hard ceiling.

So a shallower EAD never licenses a deeper dive. Depth is limited by oxygen; time is limited by inert gas. Confirm the ceiling with the MOD calculator before you think about no-stop time at all.

Where EAD still earns its place

A Nitrox-capable computer set to the analyzed mix tracks inert gas directly, so most divers never convert a depth by hand. EAD remains worth understanding because it explains why the computer gives you longer, it lets you sanity-check a plan before the dive, and it is still the mechanism behind table-based Nitrox planning.

It is also the honest way to see what you are spending. Using the extra no-stop time consumes the decompression margin the mix bought you. Keeping the air profile and breathing the richer mix banks that margin instead. Neither choice removes decompression risk. See no-decompression limits for what the limit itself represents.

EAD is an educational conversion for trained Nitrox divers, not a planning authority. It assumes an analyzed mix and says nothing about oxygen exposure, gas supply, or narcosis. Follow the tables, computer settings, and procedures taught by your training organization.

Sources

Diving Medical Technician Reference and Formulas · NOAA Diving Program — Gives the EAD formula (1 − FO2) × (D + 33) ÷ 0.79 − 33 and reproduces the U.S. Navy equivalent air depth table, whose EAD is for table selection only and rounded to the next greater depth.
NOAA Diving Manual: Diving for Science and Technology (6th ed.) · NOAA — Reference text for Nitrox physics, equivalent air depth, and the NOAA Nitrox tables.
What Is Nitrox? · PADI — Supports Nitrox extending no-decompression limits through lower nitrogen absorption, and the shallower maximum operating depth of richer mixes.
Breathing Gases · Divers Alert Network — Supports inert-gas content setting no-decompression time, and the 1.4 working PPO2 limit that defines MOD.
Rapture of the Tech: Depth, Narcosis and Training Agencies · DAN Europe (Alert Diver) — Survey of training-agency practice showing that agencies are split on whether oxygen counts as narcotic in END calculations.

Always use current training materials and operational procedures for dive planning.

Equivalent Air Depth (EAD) Explained

What EAD actually answers

The EAD formula

Try it: Equivalent Air Depth

A worked example

One dive, two ways to describe it

Equivalent air depth reference

A richer mix makes both EAD and MOD shallower

Where EAD still earns its place

Sources

Frequently Asked Questions

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