Dive Decompression Calculator
Estimate no-decompression limits, safety stop requirements, and dive risk levels. For educational planning only — always use certified dive tables or a dive computer.
About this calculator
This calculator walks through a simplified, table-style version of the no-decompression limit (NDL) logic found in recreational dive tables like the PADI/DSAT tables — it is explicitly built as an educational approximation, not a substitute for a certified dive computer or the actual tables. Based on your planned depth, it looks up a fixed NDL in minutes from a step function (219 minutes at 10m or shallower, dropping in stages down to just 5 minutes past 40m), reflecting how quickly the body accumulates dissolved nitrogen at greater ambient pressure. If you mark the dive as repetitive, the calculator reduces that NDL to account for residual nitrogen still in your tissues from an earlier dive: a longer surface interval (capped in its benefit at 360 minutes) recovers more of the original limit, while a short interval keeps the reduction closer to half the fresh-dive NDL.
From there it flags whether a safety stop is warranted — required whenever your planned depth exceeds 10 meters or your bottom time passes half the NDL — always at the standard 5-meter stop depth, for 3 minutes normally or 5 minutes if you're using more than 80% of your no-decompression time. A risk-level score (low, moderate, high, or critical) simply reports what percentage of your NDL your planned bottom time consumes. Because the underlying model is a rough approximation and doesn't account for altitude, water temperature, exertion, or multiple repetitive dives in a day, use it only for rough planning intuition — always dive within the limits of a real dive computer or table and your training.
Inputs
Results
No-Deco Limit
56 min
How to Use This Calculator
- Enter your planned dive depth in meters.
- Set your intended bottom time in minutes.
- Indicate whether this is a repetitive dive and enter your surface interval in minutes if so.
- Read No-Decompression Limit (NDL) in minutes — do not exceed this without decompression training and equipment.
- Check whether a safety stop is recommended and at what depth (usually 5 m for 3 minutes).
- Review the Dive Risk Level (Low/Moderate/High/Critical) to assess how conservatively you are diving.
How the result changes with Planned Depth
| Planned Depth | No-Deco Limit |
|---|---|
| 9 | 219 min |
| 14 | 98 min |
| 27 | 20 min |
| 45 | 5 min |
What each input means
- Planned Depth
- Maximum planned depth in meters. Recreational diving limits are typically 18m (Open Water) to 40m (Deep Diver).
- Bottom Time
- Total time from descent to beginning of final ascent, in minutes.
- Repetitive Dive?
- Whether you have dived within the last 12 hours.
- Surface Interval
- Time spent at the surface between dives, in minutes. Only relevant for repetitive dives. Longer intervals reduce residual nitrogen.
What each result means
- No-Deco Limit
- Maximum bottom time before mandatory decompression stops are required. Stay within this limit for recreational diving.
- Safety Stop Required
- 1 = Yes (safety stop strongly recommended), 0 = No (still a good practice). Always do a safety stop on deeper dives.
- Safety Stop Depth
- Standard safety stop depth is 5 meters (15 feet).
- Safety Stop Duration
- Recommended safety stop duration. 3 minutes is standard; 5 minutes when approaching NDL limits.
- Risk Level
- 1 = Low (<50% NDL), 2 = Moderate (50-75%), 3 = High (75-100%), 4 = Critical (exceeds NDL).
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersPlanned Depth = 18, Bottom Time = 35, Repetitive Dive? = 0, Surface Interval = 60 = 4 input(s) provided
- Calculate No-Deco Limit56 = 56
- Calculate Safety Stop RequiredSafety Stop Required1 = 1
- Calculate Safety Stop DepthSafety Stop Depth5 = 5
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does the No-Deco Limit drop so sharply as depth increases?
The NDL step table goes from 219 minutes at 10m or shallower down to just 5 minutes past 40m, reflecting how ambient pressure — and therefore the rate of nitrogen absorption into body tissues — rises steeply with depth. Deeper water compresses the same time-at-depth into a much smaller dissolved-nitrogen budget, which is why even a few extra meters can cut your allowed bottom time dramatically.
How much does surface interval actually restore my repetitive-dive NDL?
The calculator computes a recovery factor as your surface interval divided by 360 minutes, capped at 0.9, and uses that to soften what would otherwise be a flat 50% NDL reduction for a repetitive dive. A very short interval barely moves off the 50% cut, while an interval near or past 360 minutes recovers nearly all of the original fresh-dive NDL, matching the real physiology of nitrogen off-gassing over time.
When does the calculator require a 5-minute safety stop instead of the standard 3 minutes?
A safety stop is flagged whenever your planned depth exceeds 10m or your bottom time passes 50% of the NDL, but the duration only bumps to 5 minutes once your bottom time exceeds 80% of the NDL. Below that 80% threshold but above the 50% or 10m trigger, the stop stays at the standard 3 minutes.
What does the Risk Level score actually measure?
It's simply your planned bottom time expressed as a percentage of your calculated NDL: under 50% is Low, 50-75% is Moderate, 75-100% is High, and anything exceeding 100% is Critical because you'd be planning to dive past the no-decompression limit entirely. It doesn't account for anything beyond that single ratio — no adjustment for depth, repetitive dives beyond the first, or actual physiological variation between divers.
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