NOAA Nitrox Tables vs. NOAA Air Tables: What Changes When You Plan on EANx
Short answer
NOAA publishes separate table sets for air and for specific nitrox mixes, and they are not interchangeable: you read the table that matches the gas in your cylinder. The practical difference is a trade — at a given depth a nitrox table allows a longer no-decompression time than the air table, because there is less nitrogen in the breathing gas, but the nitrox table stops at a shallower depth, because the extra oxygen imposes its own ceiling. In short: the air table goes deeper, the nitrox table lasts longer.
All the current sets live on NOAA's diving tables page, with the governing procedures in the NOAA Diving Standards and Safety Manual.
Why a different table at all
The nitrogen clock a table tracks depends on the partial pressure of nitrogen you are breathing, not on depth alone. Air is about 79% nitrogen; a 32% nitrox mix is 68%. At the same depth, the nitrox diver is loading nitrogen more slowly, so the no-decompression limit stretches. That is the entire mechanism — there is no additional magic, and the gas does nothing to nitrogen already dissolved in you. Our explainer on residual nitrogen covers that side of the accounting.
NOAA's nitrox tables are published for named mixes rather than as a general-purpose calculator. You pick the table for the mix you actually have — analyzed and labeled, not assumed — and read depth and time from it the same way you would read an air table.
Equivalent air depth: the bridge between the two
The older way of connecting nitrox to air tables is equivalent air depth (EAD): the shallower depth at which air would deliver the same nitrogen partial pressure as your mix at your real depth.
EAD (feet) = ((nitrogen fraction × (depth + 33)) ÷ 0.79) − 33
On a 32% mix (68% nitrogen) at 100 feet: 0.68 × 133 = 90.4; divided by 0.79 = 114.5; minus 33 gives about 81 feet. So your nitrogen clock at 100 feet is running at roughly the rate an air diver experiences at 81 feet — which is where the extra allowable time comes from. The arithmetic is worth understanding even if you never use it, because it makes the whole nitrox advantage concrete. In practice, read the published nitrox table for your mix rather than converting by hand; the table already incorporates the procedures NOAA intends you to use with it.
What stays the same
- The reading method. Depth and bottom time still land you in a pressure group, surface intervals still move you to a lower letter, and residual nitrogen still gets added to the next dive. The structure of the table is familiar; only the numbers inside it change.
- Repetitive-dive bookkeeping. Dive one still shapes dive two, and mixing gases across a day means being careful about which table governs which segment.
- Ascent discipline. Rate limits and stops are part of the table, not optional extras. The physiology behind why ascent rate matters is summarized in the StatPearls decompression sickness reference.
What is genuinely new with nitrox
Oxygen becomes a planning quantity in its own right. Each mix has a depth ceiling derived from its oxygen fraction — see nitrox maximum operating depth for the formula — and cumulative oxygen exposure across a day is tracked separately from nitrogen, with its own limits in the NOAA manual. That is two sets of books instead of one, which is why nitrox is taught as its own course rather than as a footnote to an air class.
How this differs from the agency-table comparison
If your question is about NOAA versus the recreational training agencies — different model assumptions, different conservatism, different intended user — that is a separate comparison, covered in NOAA dive tables vs. recreational tables. This page is about air versus nitrox within the NOAA family. And if you are deciding whether to learn tables at all when your computer handles mixes automatically, dive tables vs. dive computers lays out what each one actually does with the same inputs.