Scuba Surface Interval Calculator
Calculate how your pressure group changes during a surface interval between dives.
Based on PADI dive tables for repetitive dive planning.
After a scuba dive, residual nitrogen remains in your body tissues. The surface interval is the time you spend at the surface between dives, allowing this nitrogen to slowly off-gas. Longer surface intervals reduce residual nitrogen and increase your NDL for the next dive.
Pressure groups: Dive tables assign a letter (A–Z) after each dive based on accumulated nitrogen. The longer your surface interval, the lower your pressure group becomes, and the longer your NDL on the next dive.
General guidelines:
- Minimum surface interval for a second dive: 10 minutes (not recommended)
- Typical surface interval: 45 min – 2 hours
- After 12 hours, you are considered a “clean slate” (no residual nitrogen)
Rule of 100: A simple heuristic — after a 40-minute dive at 20 m, allow at least 1 hour surface interval before repeating. Longer or deeper dives need proportionally longer surface intervals.
Flying after diving: Never fly within 12 hours of a single no-decompression dive, 18 hours after multiple dives, or 24 hours after dives requiring decompression stops. Altitude decreases ambient pressure, which can cause residual nitrogen to form bubbles.
Dive computers handle this automatically. Manual tables are useful for understanding the underlying principles and as a backup if your computer fails.