Dive safety verdict
Planning aid only — for use by certified nitrox divers only. Oxygen toxicity can cause convulsions and drowning: never exceed ppO2 1.4 (working) / 1.6 (contingency). Always verify MOD with your dive computer and analyze your gas before every dive. This tool is not a substitute for dive training.
Numbers here are planning estimates from published formulas and public-domain tables (NOAA oxygen exposure tables, US Navy Diving Manual Rev 7). Real dives add variables this page cannot see: exertion, cold, CO2 retention, dehydration, medication, and individual susceptibility. Your certification training, your analyzed gas, and your dive computer are the authorities on every dive — in that order.
Nitrox calculator FAQ
What is MOD in nitrox diving?
MOD (Maximum Operating Depth) is the deepest depth at which a given nitrox mix can be breathed without exceeding your chosen ppO2 limit. The formula is MOD = (ppO2 / FO2 - 1) x 10 for meters (x 33.8 for feet). Example: EAN32 at a 1.4 ppO2 limit gives (1.4 / 0.32 - 1) x 10 = 33.75 m, which conservative charts round down to 33 m. Diving deeper than the MOD pushes your oxygen exposure into the seizure-risk zone.
Why is the ppO2 limit 1.4 bar instead of 1.6?
1.4 bar is the recommended maximum ppO2 for the working portion of a dive; 1.6 bar is accepted only as a short contingency/decompression limit. The margin matters because oxygen toxicity can cause convulsions underwater without warning, which usually leads to drowning. Individual susceptibility varies with exertion, cold, CO2 retention and medication, so 1.4 is the conservative standard taught by all major training agencies.
How do I calculate Equivalent Air Depth (EAD)?
EAD = ((depth + 10) x (1 - FO2) / 0.79) - 10 in meters, or ((depth + 33.8) x (1 - FO2) / 0.79) - 33.8 in feet. Example: EAN32 at 30 m gives ((30 + 10) x 0.68 / 0.79) - 10 = 24.4 m EAD. EAD lets you plan nitrogen loading for a nitrox dive using standard air decompression tables, which is why our NDL estimator uses the EAD method.
What is the best nitrox mix for my planned depth?
The richest mix for a depth is FO2 = ppO2 limit / ambient pressure (ATA). Example: for 30 m (4 ATA) at a 1.4 limit, FO2 = 1.4 / 4 = 35%, i.e. EAN35. In practice you use a standard available mix at or below that (e.g. EAN32), never above. Note the reverse check too: EAN36 at 1.4 has an MOD of 28.9 m, so it cannot be used for a 30 m dive.
How do I calculate ppO2 at depth?
ppO2 = FO2 x ambient pressure in ATA, where ATA = depth/10 + 1 (meters) or depth/33.8 + 1 (feet). Example: EAN32 at 30 m = 0.32 x 4 = 1.28 bar ppO2 - within the 1.4 working limit but close to it, so any deeper excursion would cross it. Our ppO2-at-depth tab color-codes the result: green at or below 1.4, amber between 1.4 and 1.6, red above 1.6.
What is CNS oxygen toxicity and how is CNS% calculated?
CNS (central nervous system) oxygen toxicity is the acute danger of high ppO2: twitching, visual disturbances and convulsions. Divers track it as a percentage of the NOAA single-exposure limit: CNS% = (minutes at ppO2 / NOAA limit minutes for that ppO2) x 100. Example: 30 minutes at 1.4 ppO2 with a 150-minute NOAA limit = 20% CNS. 100% means you have reached the published limit - plan well short of it.
How does CNS% carry over between dives in one day?
Oxygen toxicity exposure clears with roughly a 90-minute half-life at the surface. Our multi-dive clock applies residual = previous CNS% x 0.5^(surface interval / 90) before adding the next dive's exposure, counting dives within a 24-hour window. Example: 20% CNS followed by a 60-minute surface interval leaves about 12.6% residual. This follows the NOAA procedure used in technical dive planning.
What are OTUs in diving?
OTU (Oxygen Tolerance Unit) measures pulmonary (whole-body) oxygen toxicity, the slower lung-irritation side of oxygen exposure. One minute at a given ppO2 accrues a published OTU rate (e.g. 1.56 OTU/min at 1.4 ppO2), so a 30-minute dive at 1.4 ppO2 = 46.8 OTU. The NOAA Repex guidance caps a single day at 850 OTU, with lower cumulative allowances across consecutive days (1400 for 2 days, 1700 for 3, 1900 for 4, 2000 for 5). Our tracker sums your dives against both limits.
How do you blend nitrox by partial pressure?
Partial-pressure blending adds pure oxygen to a cylinder, then tops up with air. The oxygen pressure to add is: ((target pressure x target FO2) - (current pressure x current FO2) - (added gas pressure x 0.21)) / 0.79. Example: a cylinder with 50 bar of EAN32 filled to 200 bar of EAN32 needs (200x0.32 - 50x0.32 - 150x0.21) / 0.79 = 20.9 bar of O2, then air to 200 bar. Only trained gas blenders should do this - oxygen above 40% in contact with non-O2-clean equipment is a fire/explosion hazard.
How do you top up a nitrox tank with air?
Topping up with air dilutes the mix: final FO2 = (current pressure x current FO2 + added air pressure x 0.21) / final pressure. Example: 100 bar of EAN32 topped to 200 bar with air = (100x0.32 + 100x0.21) / 200 = 26.5% (EAN26.5). The resulting mix must be re-analyzed and re-labeled before diving - never trust the math alone; analyze your gas before every dive.
Can I use this calculator without nitrox certification, or instead of my dive computer?
No - on both counts. Planning aid only - for use by certified nitrox divers only. Oxygen toxicity can cause convulsions and drowning: never exceed ppO2 1.4 (working) / 1.6 (contingency). Always verify MOD with your dive computer and analyze your gas before every dive. This tool is not a substitute for dive training. Get certified through a recognized agency (PADI, SSI, NAUI, SDI, BSAC or equivalent) before breathing any nitrox mix, and let your computer be the final authority on every dive.
EAN32 vs EAN36 - which nitrox mix should I choose?
EAN32 (MOD 33.75 m at 1.4) suits dives to about 30 m; EAN36 (MOD 28.9 m at 1.4) is for shallower dives, roughly 25 m and up, where it gives a lower EAD and therefore longer no-decompression limits. Deeper than 30 m, nitrox buys you little bottom time and ppO2 forces you back toward air - that is why recreational nitrox diving lives in the 18-30 m band. Choose the mix your dive shop can analyze and your certification covers.
How do I estimate no-decompression limits for a nitrox dive?
Compute the Equivalent Air Depth for your mix and depth, then look up the air no-decompression limit at that EAD - that is what our NDL estimator does, using the public-domain US Navy air tables (Rev 7). Example: EAN32 at 30 m has an EAD of 24.4 m (~80 fsw), rounding up to the 90 fsw Navy row = 33 minutes NDL. Treat this as a planning cross-check only: repetitive dives need residual nitrogen accounting, and your dive computer's algorithm is the authority on the day.
How much does a nitrox fill cost?
Fill prices vary widely by region, shop and method (membrane vs partial-pressure blending) - there is no global price list, so this tool does not price fills. As a rule of thumb, nitrox fills cost more than air fills because of the oxygen and the extra analysis/labor involved; liveaboards and resorts in dive destinations often price them per tank or per day. Call local shops for current rates before budgeting a trip.
How long should my surface interval be between nitrox dives?
A minimum of about one hour between recreational dives is the common guidance, with longer intervals giving more nitrogen and CNS% clearance; our multi-dive CNS clock models the 90-minute oxygen half-life across your actual intervals. For flying after diving, the widely cited DAN guidance is 18 hours after multiple dives or multiple days of diving (12 hours minimum after a single no-decompression dive). When in doubt, wait longer - and follow your training agency's tables, not a web tool.
What is the nitrox MOD chart and how do I read it?
A MOD chart lists the maximum operating depth for common mixes (EAN21 through EAN40) at the 1.4 and 1.6 ppO2 limits. Conservative charts round DOWN to the whole meter/foot - for example EAN32 at 1.4 is 33.75 m raw, charted as 33 m. Our printable chart follows that convention. Print it, laminate it, and keep it with your dive slate - but always confirm the numbers against your analyzed mix and computer before the dive.