Understanding Dive Tables

Before dive computers, every dive was planned with tables. Even today, understanding how tables work makes you a better-informed diver — and they remain a reliable backup when electronics fail.

What dive tables are

A dive table is a pre-calculated grid that tells you how long you can stay at a given depth without requiring decompression stops. It is the practical application of decompression theory — taking complex multi-compartment gas absorption models and boiling them down to simple lookup values.

The most common recreational tables are the PADI Recreational Dive Planner (RDP), the NAUI dive tables, and the DCIEM tables. Technical diving uses more complex tables like Buhlmann or VPM-based tables.

How they work

Dive tables are built from decompression algorithms that model how nitrogen dissolves into and out of different body tissues over time. The algorithm tracks multiple tissue “compartments” — theoretical groups of tissues with different gas absorption rates.

For each depth, the table lists the maximum time you can spend before the fastest-loading compartment reaches its M-value (the maximum tolerable dissolved gas level). This time is your no-decompression limit (NDL).

Reading a basic table

Table 1: No-decompression limits

The simplest table gives you maximum bottom time at each depth:

Depth (msw)	No-deco limit (min)
12	147
15	80
18	56
21	40
25	29
30	20
35	14
40	9

(Approximate values — actual limits vary by table.)

At 18 meters, you can stay for up to 56 minutes without a mandatory decompression stop. At 30 meters, only 20 minutes. The relationship is not linear — deeper dives have disproportionately shorter limits because nitrogen dissolves faster at higher partial pressures.

Table 2: Pressure groups

After each dive, you are assigned a pressure group (A through Z) based on your depth and time. Higher letters mean more residual nitrogen. During your surface interval, you move to lower letters as you off-gas.

Table 3: Repetitive dive planning

For your second dive, your current pressure group determines a residual nitrogen time (RNT) — an addition to your bottom time. Your adjusted no-deco limit at the new depth equals the normal limit minus the RNT.

Assumptions and limitations

Dive tables assume:

Square profile: You descend immediately to your maximum depth and stay there for the entire bottom time. Real dives are usually multi-level, spending time at various depths
Normal health: Tables assume a healthy diver with normal physiology
Sea level: Most tables are calculated for sea level. Altitude diving requires correction
Standard conditions: No excessive cold, exertion, or dehydration

These assumptions make tables conservative for typical multi-level dives (you usually spend less time at maximum depth than the table assumes) but potentially insufficient for unusual profiles.

Tables vs. dive computers

Advantages of tables

No batteries to fail: A printed table always works
Transparency: You can see exactly how the limits were calculated
Backup planning: Essential for planning dives when your computer is unavailable
Training tool: Understanding tables builds a deeper understanding of decompression theory

Advantages of computers

Multi-level credit: Computers recalculate continuously as your depth changes, giving credit for time spent shallower than your maximum depth. This typically yields 20-40% more no-deco time than tables for the same dive
Real-time tracking: No need to remember depth and time — the computer does it automatically
Repetitive dive precision: Computers track off-gassing in real time rather than using discrete pressure group steps
Safety features: Alarms, ascent rate warnings, ceiling depths, CNS tracking

The practical reality

Most recreational divers today use computers as their primary tool. Tables serve as:

A planning tool (estimating before the dive whether a plan is feasible)
A backup in case of computer failure
A training aid for understanding decompression theory

Altitude corrections

At altitude, atmospheric pressure is less than 1 bar. This means the pressure ratio between the surface and any given depth is greater than at sea level. A dive to 20 meters at altitude creates a greater decompression stress than the same depth at sea level.

Tables designed for sea level must be corrected for altitude. The standard approach is to use the “equivalent ocean depth” — converting your actual altitude depth to a deeper sea-level equivalent. Many modern dive computers have built-in altitude compensation.

Conservative practice

Even with tables, conservative practices improve safety:

Plan one depth group deeper: If your max depth is 22 meters, use the 25-meter row
Reduce bottom times: Stay 5-10 minutes under the limit, especially on deeper dives
Always do a safety stop: Tables may not require one, but 3-5 minutes at 5 meters is always worthwhile
Use nitrox with air tables: Breathing nitrox but planning with air tables gives you a built-in safety margin from reduced nitrogen loading

Safety considerations

Never exceed the table limits: The no-deco limit is a ceiling, not a target
Bottom time starts at descent: Your bottom time is measured from when you begin descending, not when you reach maximum depth
Round up depth, round up time: If your max depth was 23 meters, use the 25-meter row. If your bottom time was 17 minutes, use 20
Know which table you are using: Different tables give different limits. The PADI RDP, NAUI tables, and DCIEM tables are not interchangeable — pick one and stick with it
Carry a table: Even if you dive with a computer, having a waterproof table card in your pocket is good practice

Sources

PADI Recreational Dive Planner (RDP), Imperial and Metric versions
NOAA Diving Manual, 6th Edition
Buhlmann, A.A. (1984). “Decompression — Decompression Sickness.” Springer-Verlag
Hamilton, R.W. & Thalmann, E.D. (2003). “Decompression practice.” Bennett and Elliott’s Physiology and Medicine of Diving