The Simple Definition
Your no-decompression limit (NDL) is the maximum time you can spend at your current depth before the decompression algorithm requires mandatory stops during your ascent. As long as your NDL shows time remaining, you can ascend directly to the surface (following a normal ascent rate and completing a safety stop) without mandatory decompression obligations.
How NDL Is Calculated
The algorithm tracks nitrogen absorption in multiple theoretical tissue compartments. Each compartment has a maximum tolerated nitrogen level (the M-value in Bühlmann, or equivalent limits in RGBM). Your NDL is determined by whichever tissue compartment is closest to its maximum — this is the "leading tissue." As that tissue approaches its limit, NDL counts down. If it reaches the limit, NDL hits zero and you are in mandatory decompression.
NDL is dynamic: it changes with every depth change. Ascend a few meters and your NDL typically increases because the leading tissue is now below its depth-specific limit and may even begin off-gassing. Descend deeper and NDL decreases because nitrogen absorption accelerates.
Factors That Affect Your NDL
Depth: The deeper you are, the faster nitrogen absorbs into tissues, and the shorter your NDL. At 12 meters, NDL on air can exceed 120 minutes. At 30 meters, it may be 15–20 minutes.
Gas mix: Nitrox reduces the nitrogen fraction, which extends NDL. EAN32 at 30 meters gives a longer NDL than air at 30 meters because you are absorbing less nitrogen.
Residual nitrogen: If you have residual nitrogen from a previous dive, your NDL on the next dive will be shorter because you start with a higher nitrogen load.
Algorithm and conservatism: More conservative settings (lower gradient factors on Bühlmann, higher conservatism on RGBM) produce shorter NDLs at the same depth.
Altitude: Diving at altitude reduces NDL because the ambient surface pressure is lower, meaning the pressure difference during ascent is proportionally larger.
What Happens When NDL Hits Zero?
You enter mandatory decompression. Your computer switches from showing NDL to showing required stop depths and times. You now must complete these stops before surfacing. Skipping mandatory deco stops significantly increases your DCS risk.
For recreational divers, entering deco is not ideal but it is not an emergency — as long as you have enough air to complete the stops and ascend calmly. Your computer will guide you through the required stops. The best approach is to monitor your NDL throughout the dive and begin your ascent with comfortable NDL remaining.
Building in a Safety Margin
Experienced recreational divers typically plan to begin their ascent with at least 5–10 minutes of NDL remaining, rather than pushing to zero. This provides a buffer for unexpected delays during ascent (helping a buddy, clearing gear, slow ascent past currents). Combined with a three-minute safety stop, this practice keeps you well within safe limits on every dive.
Watch your NDL like you watch your air gauge: plan to start ascending while you have plenty remaining, not when the numbers are critical.