Repetitive Dive Interval Calculator
Estimate the surface interval needed before a repetitive scuba dive.
Uses a simplified residual nitrogen model based on your first dive depth and time.
This is an estimator, not a dive table
Read this first: actual repetitive-dive planning requires either certified printed tables (PADI RDP, NAUI, BSAC 88) or a dive computer running a recognised algorithm (Bühlmann ZH-L16, DSAT, RGBM). This page exists to help you understand the math behind surface intervals, not to replace those tools. Skipping this paragraph and using the calculator’s number for a real dive is how people get hurt.
No-decompression limit (NDL) basics
Nitrogen dissolves into your tissues at depth because the partial pressure of nitrogen in your lungs rises with ambient pressure. The deeper and longer you stay, the more dissolves. The NDL is the maximum bottom time at a given depth before you would need to do a mandatory decompression stop on the way up. The PADI RDP numbers most divers know by heart:
| Depth | NDL (PADI RDP) |
|---|---|
| 10 m / 33 ft | 219 min |
| 12 m / 40 ft | 147 min |
| 15 m / 50 ft | 72 min |
| 18 m / 60 ft | 56 min |
| 21 m / 70 ft | 40 min |
| 24 m / 80 ft | 30 min |
| 27 m / 90 ft | 25 min |
| 30 m / 100 ft | 20 min |
| 36 m / 120 ft | 13 min |
| 40 m / 130 ft | 8 min |
These are not safety margins; they are the limits. Most divers turn the dive well before NDL.
Why surface intervals matter
Nitrogen offgasses through the lungs at a roughly exponential rate. The standard “compartment half-time” for fast tissues is about 5 minutes; for medium tissues, 20-60 minutes; for slow tissues, several hours. After 6 hours of surface interval, almost all dissolved nitrogen from a recreational dive is gone. After 24 hours, even theoretical residuals are negligible.
For a same-day repetitive dive, a 60-minute exponential approximation works for the medium tissues that govern NDL planning:
residual_fraction ≈ initial_fraction × e^(−interval / 60)
So a dive that used 80% of NDL drops to about 30% after 60 minutes surface interval, and about 11% after 120 minutes.
The rules every recreational diver follows
- Plan the deepest dive first, the shallower dive second. Reverse profiles increase nitrogen load disproportionately.
- Minimum 60 minutes between dives. Some agencies require 1 hour; 2 hours is conservative and easy.
- No flying for 18 to 24 hours after the last dive of a series.
- Stay hydrated. Dehydration slows offgassing measurably.
- Don’t dive on consecutive days hung over or sick — both raise DCS risk.
A computer is honest about exactly where you are on each tissue compartment. A simplified estimator is a teaching tool, not a substitute.