EAD Calculator: Equivalent Air Depth for Nitrox Divers in Feet
Calculate your Equivalent Air Depth for any nitrox blend and depth in US feet. Unique features include the exact nitrogen savings you gain versus air (in bar), NDL extension in plain minutes, a reverse EAD mode to find what blend gives a target EAD, and an all-blend comparison table. The only US EAD calculator that shows you the actual nitrogen partial pressure reduction driving your extended bottom time.
Two modes: Find EAD for your blend and depth, or use Reverse EAD to find what blend achieves a target EAD at your planned depth.
| Blend | EAD | ppN2 | NDL | NDL Gain |
|---|---|---|---|---|
| Enter depth above to see all blends compared | ||||
Red dashed line shows nitrogen partial pressure (ppN2) breathing air at each depth. Blue solid line shows ppN2 on your nitrox blend. The gap between lines is the nitrogen you save at every depth on nitrox , this gap is what extends your NDL. Tooltip shows exact N2 savings at each depth. Source: NOAA Diving Standards and Safety Manual (2023).
What Equivalent Air Depth Measures That Your Depth Gauge Never Shows
Your depth gauge tells you how deep you are. It says nothing about how much nitrogen your body is actually absorbing at that depth. Two divers at exactly the same depth on different gas mixes are absorbing nitrogen at completely different rates, and that difference in nitrogen absorption is what separates their respective no-decompression limits. EAD is the single number that makes this difference visible and plannable.
When you breathe standard air at 80 feet, your lungs are drawing gas at 3.424 atmospheres of absolute pressure. Air is 79 percent nitrogen, so the partial pressure of nitrogen you are breathing, the ppN2, is 0.79 times 3.424, which equals 2.70 bar. That 2.70 bar of dissolved nitrogen is what drives your decompression obligation. Your body does not care about your actual depth directly. It responds to the partial pressure of inert gas, meaning nitrogen, in the gas you breathe.
On EAN32 at the same 80 feet, the nitrogen fraction drops to 0.68. The ppN2 becomes 0.68 times 3.424, which equals 2.33 bar. You are breathing 0.38 bar less nitrogen at the same depth. Your body is behaving as if you were at a shallower depth breathing air at that same 2.33 bar ppN2. The Equivalent Air Depth calculation finds exactly how shallow that would be on air, and that shallower figure is what you take to your dive table or confirm with your dive computer to find your NDL.
NOAA EAD Formula (2023): EAD in feet equals ((FN2 divided by 0.79) times (depth plus 33)) minus 33. This formula appears in NOAA Diving Standards and Safety Manual Appendix 2, and is consistent with TDI Imperial Formula 8 and the PADI Enriched Air Diver Specialty materials. Source: NOAA Office of Marine and Aviation Operations.
Why Nitrogen Partial Pressure Is What Actually Drives Your NDL
Decompression tables and dive computer algorithms track nitrogen loading in body tissues. The rate at which nitrogen dissolves into those tissues is proportional to the partial pressure of nitrogen in the breathing gas at depth. This is Henry’s Law applied to human physiology: the amount of gas that dissolves in a liquid is proportional to the partial pressure of that gas above the liquid. Your blood and tissues are the liquid; your breathing gas under pressure is the gas above it.
No-decompression limits exist because beyond a certain total nitrogen load in specific tissue groups, ascending directly to the surface risks forming nitrogen bubbles that can cause decompression sickness. The NDL is the maximum time at a given depth, breathing a given gas, before that critical nitrogen load is reached. When you breathe nitrox, you load nitrogen more slowly at every depth compared to air, so the NDL extends. The EAD converts this slower loading rate into a depth equivalent that maps directly onto the air tables you already know.
Three Key Numbers Every Nitrox Diver Needs to Know
EAD calculation produces three numbers that together complete your nitrox dive plan. First is the EAD itself in feet: the depth number you look up in air tables to find your NDL. Second is your actual nitrogen partial pressure at the planned depth on your blend: this is what this calculator calls ppN2, and seeing it numerically is uniquely useful because it makes the nitrogen savings from nitrox concrete rather than abstract. Third is the ppO2 at your planned depth: you need this to confirm you are below the MOD for your blend and within the 1.4 bar working ppO2 limit. All three appear together in this calculator because dive planning requires all three, not just the EAD in isolation.
Reverse EAD: Planning the Other Direction
Most EAD discussions start with the blend and find the EAD. But experienced nitrox divers often plan in reverse: they know what NDL they need for a particular dive site and work backward to the required blend. If you want to match the nitrogen profile of a 50-foot air dive at a planned depth of 80 feet, what blend do you need? The Reverse EAD formula answers this directly. You divide 0.79 by the ratio of your target EAD plus 33 to your planned depth plus 33, which gives you the nitrogen fraction needed, and subtracting from 1 gives the O2 fraction. This tool is unique among US EAD calculators in providing this reverse calculation in feet rather than meters.
The NOAA EAD Formula: Every Step Calculated and Explained
The mathematics of EAD follows directly from the definition of nitrogen partial pressure and a simple algebra rearrangement. Understanding the derivation helps you trust the output and lets you verify calculations manually against your PADI slate on a dive boat without a smartphone.
Step-by-Step EAD Calculation
// 80 ft: (80/33) + 1 = 3.424 ATA
ppN2 on nitrox = FN2 x ATA
// EAN32 at 80 ft: 0.68 x 3.424 = 2.328 bar
ppN2 on air = 0.79 x ATA
// Air at 80 ft: 0.79 x 3.424 = 2.705 bar
N2 savings = Air ppN2 – Nitrox ppN2
// 2.705 – 2.328 = 0.377 bar less nitrogen
EAD (ft) = ((FN2 / 0.79) x (depth + 33)) – 33
// EAN32 at 80 ft: ((0.68/0.79) x 113) – 33 = 97.3 – 33 = 64.3 ft
Reverse EAD (find blend for target EAD):
FN2_needed = 0.79 x (target_EAD + 33) / (depth + 33)
// Target EAD 50 ft at 80 ft: 0.79 x 83/113 = 0.580; O2 needed = 42%
O2_needed% = (1 – FN2_needed) x 100
How to Use EAD with PADI Tables
PADI’s approach to using EAD with air tables involves two standard steps. First, calculate the EAD using the formula above. Second, round up to the next deeper table entry. For an EAD of 64.3 feet, round up to 70 feet (the next deeper PADI table depth). Look up the NDL for 70 feet in PADI Table 1, which gives you 40 minutes. This 40 minutes is your NDL for the nitrox dive, compared to 30 minutes on air at 80 feet. The rounding-up step is conservative: it treats you as if you had slightly more nitrogen exposure than the exact EAD calculation produces, adding a small safety margin.
How Dive Computers Handle EAD
Most modern dive computers with a nitrox mode do not require you to calculate EAD manually. When you set the oxygen percentage from your O2 analyzer reading, the computer’s algorithm tracks nitrogen loading using the actual ppN2 for your specific blend at each moment during the dive. This is more accurate than a manual EAD calculation because it responds to your actual depth profile in real time rather than using a single planned depth. The computer’s displayed NDL effectively incorporates the EAD advantage without showing you a specific EAD number. Understanding EAD as the underlying concept helps you interpret what your computer is doing and why the NDL counter on nitrox runs down more slowly than on air at the same depth.
The Reverse EAD: Finding a Blend for a Target NDL
The reverse calculation answers the question: “I need at least 60 minutes of NDL at 80 feet. What blend do I need?” Working backward: 60 minutes of NDL corresponds to a PADI table entry of 60 minutes, which is the 50-foot row (80 minutes actual). So you want EAD to be approximately 50 feet. The reverse EAD formula gives you: FN2 equals 0.79 times (50 plus 33) divided by (80 plus 33), equals 0.79 times 0.735, equals 0.580. O2 fraction equals 1 minus 0.580 equals 0.420, so 42 percent oxygen. Since EAN40 is the recreational limit, you cannot quite reach this EAD with recreational nitrox. EAN40 at 80 feet gives an EAD of about 53 feet, providing the 70-foot table NDL of 40 minutes, short of the 60-minute goal. The reverse calculation tells you this limitation before you walk into the dive shop, not after you have already planned the dive assuming a target that is not achievable.
PADI NDL and Nitrogen Pressure Reference Tables for US Nitrox Divers
These tables contain the verified reference data behind this calculator. NDL values are from the PADI Recreational Dive Planner Table 1 (first-dive, Group A). Nitrogen partial pressure values are calculated from standard EAD and ppN2 formulas using NOAA’s seawater constants (33 feet per ATA).
EAD and ppN2 for Common Blends at Key US Dive Depths
| Depth | Blend | EAD (ft) | ppN2 (bar) | Air ppN2 (bar) | N2 Savings | NDL (PADI) |
|---|---|---|---|---|---|---|
| 60 ft | EAN32 | 44 ft | 1.91 | 2.23 | 0.32 bar | 140 min |
| 60 ft | EAN36 | 32 ft | 1.71 | 2.23 | 0.52 bar | 205 min |
| 70 ft | EAN32 | 56 ft | 2.22 | 2.58 | 0.36 bar | 80 min |
| 70 ft | EAN36 | 43 ft | 1.98 | 2.58 | 0.60 bar | 140 min |
| 80 ft | EAN32 | 64 ft | 2.33 | 2.71 | 0.38 bar | 40 min |
| 80 ft | EAN36 | 52 ft | 2.08 | 2.71 | 0.63 bar | 80 min |
| 100 ft | EAN32 | 80 ft | 2.76 | 3.19 | 0.43 bar | 30 min |
| 100 ft | EAN28 | 88 ft | 2.89 | 3.19 | 0.30 bar | 25 min |
| 110 ft | EAN28 | 95 ft | 3.16 | 3.46 | 0.30 bar | 25 min |
| 130 ft | EAN24 | 107 ft | 3.69 | 3.94 | 0.25 bar | 16 min |
PADI RDP Table 1 Air NDL Reference (US Feet)
| Depth (ft) | Air NDL (min) | Applies to EAD Range | Use This Row When |
|---|---|---|---|
| 35 | 205 | 0 to 35 ft | EAD is 35 ft or shallower |
| 40 | 140 | 36 to 40 ft | EAD rounds up to 40 ft |
| 50 | 80 | 41 to 50 ft | EAD rounds up to 50 ft |
| 60 | 55 | 51 to 60 ft | EAD rounds up to 60 ft |
| 70 | 40 | 61 to 70 ft | EAD rounds up to 70 ft |
| 80 | 30 | 71 to 80 ft | EAD rounds up to 80 ft |
| 90 | 25 | 81 to 90 ft | EAD rounds up to 90 ft |
| 100 | 20 | 91 to 100 ft | EAD rounds up to 100 ft |
| 110 | 16 | 101 to 110 ft | EAD rounds up to 110 ft |
| 120 | 13 | 111 to 120 ft | EAD rounds up to 120 ft |
| 130 | 10 | 121 to 130 ft | EAD rounds up to 130 ft |
Source: PADI Recreational Dive Planner Table 1, first-dive no-decompression limits. Always round EAD up to the next deeper table entry for a conservative result. Dive computers using Buhlmann ZH-L16C or RGBM algorithms typically allow longer NDLs than these table values.
Three Real US EAD Nitrox Dive Planning Scenarios
These examples use actual US dive sites to show how EAD calculation feeds directly into a complete, plannable dive. Follow each one to understand how the formula outputs translate into real decisions made at the dock.
Taylor is spending a full day at Alexander Springs, a freshwater spring system in the Ocala National Forest where maximum depth is about 20 feet. She is planning three dives with a focus on macro photography and minimal surface intervals. At 20 feet on EAN36, the ATA is only 1.606. Her EAD is ((0.64/0.79) times 53) minus 33, which equals 42.9 minus 33, equaling 9.9 feet. The EAD rounds to 10 feet, effectively surface-level for decompression purposes.
The practical meaning: her nitrogen absorption on this dive is negligible, and she can plan three back-to-back dives with minimal surface interval without any meaningful nitrogen accumulation. The EAD here is less about NDL extension and more about the multi-dive day advantage: each dive ends with essentially no nitrogen loading, so cumulative pressure groups never become a limiting factor. Her ppN2 at 20 feet on EAN36 is only 1.03 bar, compared to 1.26 bar on air.
Jordan is diving the deep reefs at Point Lobos, one of California’s premier cold-water dive sites, to 90 feet on EAN32 that he picked up at a Monterey dive shop. On air at 90 feet, his PADI NDL is 25 minutes. He runs the EAD calculator: at 90 feet (3.727 ATA), his EAD is ((0.68/0.79) times 123) minus 33, which equals 105.9 minus 33, equaling 72.9 feet. Rounding to 80 feet gives a PADI NDL of 30 minutes, only a modest gain.
The ppN2 breakdown reveals why: at 90 feet, the ATA multiplier is high, and even with EAN32’s nitrogen reduction, his ppN2 is 2.53 bar compared to 2.94 on air. The savings are 0.41 bar, meaningful but not transformative at this depth. Had he used EAN36 instead, his EAD would be 60 feet and NDL would be 55 minutes, doubling his bottom time. This demonstrates why best mix nitrox at the site’s maximum depth is more valuable than a standard EAN32 fill for deeper California diving.
Dana wants 55 minutes of NDL at 110 feet on the USS Vandenberg wreck. PADI Table 1 gives 55 minutes at 60 feet on air, so she sets a target EAD of 60 feet at her planned 110-foot max depth. Using the Reverse EAD: FN2 needed equals 0.79 times (60 plus 33) divided by (110 plus 33), equals 0.79 times 0.650, equals 0.514. O2 needed equals (1 minus 0.514) times 100, equals 48.6 percent.
Since 48.6 percent exceeds the EAN40 recreational cap, she cannot achieve a 60-foot EAD at 110 feet with recreational nitrox. The calculator shows EAN40 at 110 feet gives EAD of 79 feet and NDL of 30 minutes. The reverse calculation answered her question before she even got to the dive shop: at 110 feet, recreational nitrox cannot deliver a 55-minute NDL, and she should either plan a shorter dive or choose a shallower section of the wreck. That is exactly the planning insight that saves a dive day.
Six Expert Tips for Using EAD Correctly in Your Nitrox Dive Plan
Always Round Your EAD Up to the Next Deeper Table Entry
If your exact EAD calculation produces 64.3 feet, you do not use the 60-foot table entry. You round up to the next deeper row, which is 70 feet, and use that NDL of 40 minutes. This rounding-up approach is conservative and is the standard PADI methodology. Rounding down (using the 60-foot NDL of 55 minutes for an EAD of 64 feet) would give you an optimistic NDL that is not supported by the conservative nitrogen loading model the PADI table is built on. This calculator rounds up automatically when displaying the PADI NDL for your EAD.
Set Your Dive Computer to the Analyzed O2 Percentage, Not the Label
Your dive computer tracks nitrogen loading using the O2 percentage you program into it, not the label on the tank. If your tank is labeled EAN32 but the analyzer reads 33.2%, program 33.2% into your computer. The computer will then track your actual nitrogen exposure rather than a rounded estimate. With nitrox, a 1 to 2 percent difference in O2 setting changes your effective EAD by several feet and can affect your NDL by 5 to 15 minutes on dives deeper than 80 feet. This is worth the 20 extra seconds to set correctly before every dive.
EAD Applies to NDL Planning, Not to Your Maximum Operating Depth
These are two entirely separate calculations that serve two different safety functions. EAD and the resulting NDL protect you from decompression sickness by tracking nitrogen absorption. MOD and the ppO2 limit protect you from oxygen toxicity by capping depth based on oxygen partial pressure. A blend with EAD of 40 feet can still have a MOD of only 95 feet if it is EAN36. You must calculate both independently and respect both limits simultaneously. Never substitute EAD for MOD or treat them as related constraints on the same issue.
Use the ppN2 Output to Explain EAD to Your Buddy
The abstract concept of EAD is much harder to explain to a non-technical diving buddy than the concrete ppN2 savings. Instead of saying “my EAD is 64 feet so I get 40 minutes instead of 30,” try: “On EAN32 I breathe 2.33 bar of nitrogen at 80 feet instead of 2.71 bar on air. My body thinks I am at a shallower depth, so I get more time.” The nitrogen partial pressure framing connects the chemistry to the physiology in a way that makes intuitive sense. This calculator is unique in displaying the ppN2 numerically for exactly this reason.
Use Reverse EAD When Specifying Fills at the Dive Shop
Instead of asking for a standard blend and then calculating whether it meets your NDL needs, work backward. Decide what NDL you need, find the EAD that provides it from the PADI table, and use the Reverse EAD formula to calculate the required blend. Then tell the shop: “I need a blend as close as possible to 38 percent oxygen.” This approach ensures your fill specification starts from your actual diving requirement rather than from what is easiest to fill. Not all shops can hit custom blends precisely, but specifying a target is always better than accepting whatever standard fill happens to be available.
EAD Gives Conservative Results Compared to Modern Dive Computer Algorithms
The PADI RDP table and the EAD method it uses are deliberately conservative: they assume a worst-case tissue loading model. Modern dive computers based on the Buhlmann ZH-L16C algorithm or RGBM typically display longer NDLs than the PADI RDP table for the same dive, including on nitrox dives. The EAD figures this calculator produces give you a conservative planning baseline. Your actual computer will likely give you more time, which is fine. Plan conservatively with EAD and tables, then follow your specific computer’s live NDL display during the dive for real-time guidance. Never override your computer’s NDL display based on table planning.
EAD Quick Reference: Formulas and NOAA Standards for US Divers
Print or screenshot this reference for your dive bag. All formulas follow NOAA Diving Standards (2023) using US feet and seawater depth constants.
| Calculation | Formula (US feet) | Authority / Note |
|---|---|---|
| EAD (forward) | ((FN2 / 0.79) x (depth + 33)) – 33 | NOAA Diving Standards (2023), PADI EA standard |
| ppN2 at depth | FN2 x ((depth / 33) + 1) | Henry’s Law applied at depth pressure |
| ppO2 at depth | FO2 x ((depth / 33) + 1) | Check against 1.4 bar MOD limit |
| Air ppN2 at depth | 0.79 x ((depth / 33) + 1) | 0.79 = nitrogen fraction in standard air |
| N2 savings (bar) | Air ppN2 – Nitrox ppN2 | Reason your NDL extends on nitrox |
| Reverse EAD (O2% for target EAD) | O2% = (1 – (0.79 x (EAD+33)/(depth+33))) x 100 | Find required blend for desired EAD |
| Table rounding rule | Round EAD UP to next deeper entry | PADI conservative standard for table use |
| EAN32 EAD at 80 ft | 64 ft (NDL: 40 min) | Most common US blend, standard reef depth |
| EAN36 EAD at 60 ft | 32 ft (NDL: 205 min) | Common shallow dive with extended NDL |
| Max recreational nitrox | EAN40 (40% O2) | PADI Enriched Air Diver cert. upper limit |
Frequently Asked Questions About Equivalent Air Depth and Nitrox Planning
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Editorial Standards and Legal Disclaimer
The EAD Calculator on this page is a free educational and planning tool. All EAD calculations use the NOAA Imperial Formula: EAD equals ((FN2 divided by 0.79) times (depth plus 33)) minus 33, consistent with the NOAA Diving Standards and Safety Manual (2023), TDI Imperial Formula 8, and the PADI Enriched Air Diver Specialty course materials. NDL values are from the PADI Recreational Dive Planner Table 1, first-dive no-decompression limits. NOAA diving safety information is available at the NOAA Office of Marine and Aviation Operations. DAN nitrox research references are available at DAN Alert Diver health resources.
EAD is an approximation method. Individual dive computers using Buhlmann ZH-L16C, RGBM, or other decompression algorithms track nitrogen loading more precisely than EAD with air tables and typically produce longer NDLs than PADI RDP table values. During actual dives, always follow your dive computer’s displayed NDL. Analyze your gas blend with a calibrated O2 analyzer before every nitrox dive and program the analyzed percentage into your computer.
Nitrox diving requires current Enriched Air Nitrox certification from a recognized agency. USCalculators.com is an independent educational resource not affiliated with PADI, NAUI, SSI, TDI, DAN, or NOAA. Calculator outputs are mathematical results based on standard formulas and user inputs. Always verify your dive plan with a qualified dive professional and dive within the limits of your current certification level.