What's the difference between a dive table and a dive computer?
Short version: the same math, fed different information. A table assumes you spent your entire dive at your deepest depth. A computer uses the dive you actually did, second by second. On a real dive, that difference is worth a lot of bottom time — and it isn't free.
The square profile
A table is built around one picture: you drop straight to a depth, stay there, and come straight back up. Divers call it a square profile. The dive I'll use on this page — one of mine, in Grenada, breathing EAN32 (nitrox, 32% oxygen) — reached 76 ft at its deepest, so the table puts me in its 80-ft column, and on PADI's EANx32 table the no-stop limit at 80 ft is 45 minutes.1 The table counts bottom time from the moment you start down until the moment you start your final ascent — all of it charged at that deepest depth.1
One column only, redrawn from PADI's EANx32 Recreational Dive Planner to show how a table is read — for illustration, not for planning a dive. Left: the pressure group; right: the longest bottom time for that group at 80 ft. A table always rounds up: to the next deeper column, and to the next longer time.
The table's picture of a dive: straight down to 80 ft, 45 minutes there, straight back up. Every dive the table plans is drawn this way.
The equation gives the same answer
That 45 isn't magic. It's the tissue-loading equation from What does my dive computer actually measure?, turned around to ask a different question: not “how much nitrogen after t minutes?” but “how many minutes until this compartment hits its limit?”
t = T × log2[ (Pi − P0) ÷ (Pi − Plimit) ]
Worked example — one compartment at 80 ft on EAN32
Every number that goes into the equation comes from something simple:
| Number | What it is | Where it comes from | Value |
|---|---|---|---|
| Psurface | Air pressure at the surface | Sea level | 1.013 bar (1.00 ATA) |
| Pvapor | Water vapor in your lungs | Your lungs are saturated with water vapor at body temperature, 37 °C (99 °F): 47 mmHg, the same at any depth. It takes up a slice of every breath, so the other gases share what's left | 0.063 bar (0.062 ATA) |
| P0 | Nitrogen in the compartment before the dive | Fresh from the surface, your tissues hold the nitrogen of surface air, 79% of what's left after water vapor: (1.013 − 0.063) × 0.79 | 0.75 bar (0.74 ATA) |
| Pambient | Total pressure at 80 ft | Seawater adds one atmosphere every 33 ft: 1.013 × (1 + 80 ÷ 33) | 3.47 bar (3.42 ATA) |
| Pi | Nitrogen you breathe at 80 ft | EAN32 is 68% nitrogen, again after water vapor: (3.47 − 0.063) × 0.68 | 2.32 bar (2.29 ATA) |
| T | The compartment's half-time | Bühlmann's compartment 4 (the sixteen are listed on the computer page) | 18.5 min |
| Plimit | The most nitrogen this compartment may carry back to the surface | Bühlmann's limit for compartment 4 at surface pressure, from his two published coefficients for it2,3 | 2.05 bar (2.02 ATA) |
- Known: T = 18.5 min, P0 = 0.75 bar, Pi = 2.32 bar, Plimit = 2.05 bar (Table 1).
- Unknown: minutes at 80 ft until this compartment reaches its limit.
- Why: the compartment closes half its gap to Pi every 18.5 minutes; we want the moment it crosses Plimit.
- Equation: t = T × log2[ (Pi − P0) ÷ (Pi − Plimit) ]
- Substitute: t = 18.5 × log2[ (2.32 − 0.75) ÷ (2.32 − 2.05) ] = 47 minutes (in bar; ATA gives the same answer).
Do the same for all sixteen compartments and the smallest answer is your no-stop limit:
| Compartment | Half-time (min) | Minutes to its limit |
|---|---|---|
| 1–2 | 5–8 | never — they fill up before they reach it |
| 3 | 12.5 | 61 |
| 4 | 18.5 | 47 ← first to the line |
| 5 | 27 | 49 |
| 6 | 38.3 | 52 |
| 7 | 54.3 | 62 |
| 8 | 77 | 78 |
| 9–16 | 109–635 | 100 and up |
Forty-seven minutes from the equation, forty-five from the table. The two minutes are the difference between models: PADI's tables aren't built on Bühlmann's numbers but on their own model with its own limits, tested on recreational divers and rounded on the cautious side.4 Same idea, same arithmetic, nearly the same answer. (That 2.05 bar is the one limit printed on this site. It's here to show the arithmetic — not to plan a dive with.)
Nobody dives a square: a real dive in Grenada
Here's the catch. You don't magically appear at depth and magically reappear on the surface, and you don't spend the whole dive pinned to the deepest point of the reef. So here's the dive itself: one of mine, Grenada, 30 July 2026, on EAN32, straight from my Garmin Descent X50i's log. (What nitrox itself buys you, compared with air, is its own question — that's the next FAQ, using this same dive.)
From my Garmin Descent X50i's log, one reading a second. The dashed box is how the table counts the dive; the dot is the computer's tightest moment.
| Date and time | 30 July 2026, 9:28 a.m. local time |
|---|---|
| Where | Off Grenada's southwest coast; entry at 12.02119° N, 61.79391° W (map) |
| Dive of the day | First — 45 hours since my previous dive |
| Water | Salt; 29 °C (84 °F) at depth, 31 °C (88 °F) at the surface |
| Deepest point | 23.1 m (76 ft) |
| Average depth | 12.5 m (41 ft) |
| Time underwater | 40 minutes |
| Bottom time, as a table counts it | 29 minutes (start of descent to leaving the bottom) |
| Safety stop | About 9 minutes at 3–6 m (11–19 ft) |
| Gas | EAN32 |
| Cylinder pressure | 195 bar (2,830 psi) at the start, 53 bar (770 psi) at the end |
| Computer | Garmin Descent X50i: Bühlmann ZH-L16C, gradient factors 35/75 (“High”) |
Down to the high 60s in five minutes, more than twenty minutes between 60 and 69 ft along the reef, straight up at minute 27, and a long, lazy safety stop. Nothing like a square.
| What it sees | Room left at the tightest moment | |
|---|---|---|
| EANx32 table | 30 minutes at 80 ft (limit 45): group M | 15 min |
| Computer, no added conservatism | the dive, second by second | 51 min |
| Computer, my settings (gradient factors 35/75) | the dive, second by second | 20 min |
On the table
The deepest point was 76 ft, so the table uses its 80-ft column: limit 45 minutes. Bottom time, from the start of the descent until I left the bottom: 29 minutes, which the table counts as 30. That's group M (Figure 1), 15 minutes short of the limit — with every one of those 30 minutes charged as if I'd spent it at 80 ft.
On the computer
The computer runs the same kind of equation every second, at the depth I was actually at. With no conservatism added, its tightest moment came at minute 24, at 68 ft, with 51 minutes of no-stop time still left. That's the square tax made visible: the table's 15 minutes against the computer's 51, for the same 29 minutes underwater.
But I don't dive with the conservatism switched off. With my settings — gradient factors 35/75, which Garmin calls “High” — the room left drops to 20 minutes, close to the table's 15. The computer's extra time was real; I chose to give most of it back.
And the model can be checked against the real thing. On the day, my Descent X50i was set to the gas I actually had, 30%, with the same 35/75. Run that way, the model's lowest no-stop time is 15.1 minutes, at minute 25. My X50i logged 15.0 minutes, at minute 25. The arithmetic on this page is the arithmetic on my wrist.
Where the extra time comes from — and what it costs
The table's square isn't wrong; it's deliberately pessimistic. Every minute you spent shallower than your maximum, it charged at your maximum, and that overcharge was built-in margin. A computer hands that margin back to you as bottom time.
Four things follow:
- Spend it knowingly. A diver who rides every dive to the last minute of no-stop time is diving closer to the line all day, and how close you push the line is the strongest predictor of getting bent that we have.5 (More on that here.)
- Deep first, then shallower. The credit comes from working up: every minute spent shallower than your deepest point loads you less than the table assumes. Going deeper later in a dive spends that margin fast.
- Your settings decide. On my Grenada dive the computer found 51 minutes of room; my conservatism setting took it back to 20, close to the table's 15. Conservatism is a choice to give time back. That's the point of it.
- The computer still doesn't know you. It tracks the dive precisely and your body not at all — no age, no fitness, no hydration, no idea how hard you just finned. (Here's why.)
So use the computer for what it's good at — following the dive you really did — and treat the extra time it gives you as a margin you're allowed to keep, not one you're required to spend.
How the numbers were worked out: Bühlmann ZH-L16C limits, no gradient factors unless stated, 1 atmosphere per 33 ft of seawater, the dive stepped through in one-second intervals; the Grenada dive uses my computer's own log, one reading a second, nitrox run as EAN32 (the cylinder analyzed at 30% that morning; the page uses 32% so PADI's EANx32 table applies), “my settings” are gradient factors 35/75; set as my computer was, the model matched its logged no-stop time to within a few seconds. Your computer's model and settings will give different numbers; the pattern will be the same.
For education only. This page explains the physics and the reasoning behind dive tables and computers. It is not for dive or gas planning. Plan every dive with your training, your dive computer and your gauges, and follow your instructor or dive professional.
References
- PADI. Recreational Dive Planner and EANx32 Recreational Dive Planner (tables), with instructions for use. Rancho Santa Margarita (CA): PADI. ↩ab
- Bühlmann AA. Decompression–Decompression Sickness. Berlin: Springer; 1984. ↩
- Baker EC. Understanding M-values. Immersed. 1998;3(3):23–27. ↩
- Hamilton RW, Rogers RE, Powell MR, Vann RD. Development and validation of no-stop decompression procedures for recreational diving: the DSAT Recreational Dive Planner. Diving Science and Technology Corp.; 1994. ↩
- Marroni A, Kot J, Pieri M, Pelliccia R, Balestra C. Identification of DCS risk factors in recreational diving: a multifactorial model based on the DAN DSL Database 2024. International Maritime Health. 2026;77(1):1–12. doi:10.5603/imh.108038 ↩