Dive Tables: How to Read Them (Even If You Have a Dive Computer)
I know what you're thinking. "It's 2026. I have a $400 dive computer on my wrist. Why do I need to learn about tables printed on a plastic card?"
Fair question. Here's my answer: your dive computer is a black box that tells you numbers. Dive tables are the transparent version of those numbers. Understanding tables means understanding decompression theory — and that understanding makes you a safer diver even if you never touch a table again.
Also, computers die. Batteries fail. Screens flood. I've seen it happen on a liveaboard in the Red Sea — diver's computer flooded on dive one of a seven-day trip. The diver who understood tables kept diving. The one who didn't sat on the boat for a week.
What Are Dive Tables?
Dive tables are pre-computed models that tell you how long you can stay at a given depth without incurring mandatory decompression stops. They're based on the same decompression algorithms your [dive computer](/blog/best-dive-computers-2026) uses — primarily Bühlmann ZHL-16C or variants — but simplified into a paper format.
The most widely taught table is the PADI Recreational Dive Planner (RDP), designed by Dr. Raymond Rogers and Dr. Bruce Bassett using DSAT (Diving Science and Technology) data. Here's how to read it.
No-Decompression Limits (NDLs): The Key Numbers
Before we get into the tables, here are the NDLs at common recreational depths on air. These are the maximum times you can spend at a given depth on your first dive without requiring decompression stops:
| Depth | NDL (No-Deco Limit) | |---|---| | 10m / 35ft | 219 minutes | | 12m / 40ft | 140 minutes | | 14m / 50ft | 80 minutes | | 18m / 60ft | 56 minutes | | 22m / 70ft | 40 minutes | | 25m / 80ft | 30 minutes | | 30m / 100ft | 20 minutes | | 35m / 115ft | 14 minutes | | 40m / 130ft | 9 minutes |
Look at that progression. At 18 meters you get almost an hour. At 40 meters you get nine minutes. This is [Henry's Law](/blog/the-bends-scuba-diving) in action — deeper means faster nitrogen absorption, which means shorter safe exposure times.
The PADI RDP: Three Tables in One
Table 1: No-Decompression Limits and Pressure Groups
This table takes your depth and bottom time and assigns you a pressure group — a letter from A to Z that represents your nitrogen loading. Lower letters (A, B, C) mean less nitrogen. Higher letters (W, X, Z) mean more.
How to use it: 1. Find your depth on the left column (round UP to the next listed depth if yours isn't exact) 2. Find your actual bottom time along that row 3. Your pressure group is the letter at the top of the column where your bottom time falls
Example: You dive to 18 meters for 40 minutes. Find 18m on the left, follow across to 40 minutes. Your pressure group is Q.
Table 2: Surface Interval Credit
After surfacing, you off-gas nitrogen over time. This table converts your surface interval into a new (lower) pressure group.
How to use it: 1. Find your current pressure group from Table 1 on the left 2. Follow across to find the time range that matches your surface interval 3. Your new pressure group is at the top of that column
Example: You surfaced as group Q and spent 1 hour 30 minutes on the surface. Find Q on the left, find the time range that includes 1:30. Your new group is approximately F.
Table 3: Repetitive Dive Planner
Now you're planning dive two. Your new pressure group tells you how much residual nitrogen you're carrying. This table adjusts your NDLs for the next dive.
How to use it: 1. Find your new pressure group (from Table 2) on the top 2. Find your planned depth for dive two on the left 3. The table gives you two numbers: Residual Nitrogen Time (RNT) and Adjusted NDL 4. Your allowed bottom time for dive two is the Adjusted NDL
Example: New group F, second dive planned to 15 meters. Your RNT might be 24 minutes (nitrogen you're "starting with") and your Adjusted NDL is 56 minutes (your reduced maximum).
A Complete Worked Example
Dive 1: 24 meters for 25 minutes
- Table 1: Pressure group = S
Table 2: S with 2-hour surface interval → new group = D
Dive 2: 15 meters planned
- Table 3: Group D at 15 meters → RNT = 17 minutes, Adjusted NDL = 55 minutes
- Maximum bottom time for dive 2: 55 minutes at 15 meters
Rules for Using Tables
1. Always round depth UP to the next table value. If you dove to 17 meters, use the 18-meter row. 2. Always round time UP to the next table value. If you dove 37 minutes, use 40. 3. Use the deepest depth reached, even briefly. A quick drop to 30 meters followed by 40 minutes at 15 meters? Your profile is planned at 30 meters. 4. Plan the deepest dive first. On multi-dive days, your first dive should be the deepest. This is because residual nitrogen from a deep dive has a larger impact on subsequent shallow dives than vice versa. 5. Minimum surface interval: 1 hour between repetitive dives (PADI recommendation). 6. Maximum 3 dives per day using tables. Tables lose accuracy for highly repetitive multi-day diving.
Tables vs. Dive Computers
Tables are conservative by design. They assume you spend the entire dive at your maximum depth (square profile). In reality, most dives are multilevel — you start deep and ascend gradually. A dive computer tracks your actual profile in real time and gives you credit for time spent at shallower depths. This means computers typically give you 20-40% more bottom time than tables for the same dive.
But tables have one advantage: they're deterministic and transparent. You can see exactly what the model is doing. You can check your buddy's math. You can plan dives on a boat with no electricity. And you understand what your computer is actually computing, which makes you a better diver.
My Recommendation
Learn tables during your [certification course](/blog/how-to-get-scuba-certified). Understand what they teach you about nitrogen loading, pressure groups, and surface intervals. Then dive with a computer. But keep a set of tables in your save-a-dive kit, because the one time your computer floods at the start of a liveaboard trip, you'll be the person who keeps diving while everyone else watches from the deck.
I'm Chad. I trust math more than electronics. And I always carry backup.