What Are Gradient Factors?
Gradient factors (GF) are a way to adjust how conservative or aggressive your Bühlmann dive computer calculates your decompression limits. They were introduced by Erik Baker in 1998 as a practical tool for customizing the Bühlmann ZHL-16C algorithm, and they have since become the standard way divers fine-tune their computers.
Gradient factors are expressed as two numbers: GF Low and GF High, written as a pair like 30/70 or 45/95. Each number is a percentage of the Bühlmann M-value — the theoretical maximum nitrogen pressure a tissue compartment can tolerate at a given depth.
GF Low: Your Deep Ceiling
GF Low controls the deepest point at which the algorithm will require you to stop during ascent. A lower GF Low means the computer will ask you to make your first stop deeper. This is primarily relevant for decompression diving — if you never exceed your NDL, GF Low rarely comes into play.
For example, with GF Low 30, the algorithm will trigger a stop when your leading tissue reaches 30% of its M-value. With GF Low 50, it waits until 50% — meaning you ascend higher before your first stop.
GF High: Your Surface Limit
GF High controls how much nitrogen the algorithm allows in your leading tissue when you reach the surface. A lower GF High means more time spent on shallow stops, resulting in a more gradual off-gassing. This is the setting that most directly affects total stop time and safety margins for all divers, including recreational.
GF High of 95 means you surface at 95% of the M-value — close to the theoretical maximum but still within limits. GF High of 70 means you surface at only 70% of the M-value, with a wider safety margin.
Common Gradient Factor Settings
| Setting | GF Low / GF High | Who Uses It | Effect |
|---|---|---|---|
| Factory Default | 40/85 – 45/95 | Recreational divers | Moderate conservatism, suitable for warm-water no-deco diving |
| Conservative Rec | 35/75 | Cautious rec divers | Longer safety stops, shorter NDLs |
| Tech Standard | 30/70 | Tech divers (deco) | Deep first stop, extended shallow stops |
| Aggressive | 50/80+ | Specific scenarios | Reduced margins — not recommended without experience |
When to Adjust (and When Not To)
Leave them alone if: You are a recreational diver, you dive within NDL limits, and you complete safety stops on every dive. The factory default GF settings are well-tested and appropriate for the vast majority of diving.
Consider adjusting if: You are starting decompression diving under qualified instruction, you dive in cold water or have personal risk factors for DCS, or your instructor recommends specific settings for your training level. Changes should be made with a clear understanding of the trade-offs, not based on forum advice.
Conservatism on RGBM Computers
Suunto, Cressi, and older Mares computers use RGBM and do not offer gradient factors. Instead, they provide conservatism presets — typically low, medium, and high (or P0, P1, P2 on Suunto). These presets adjust internal parameters of the RGBM model to be more or less conservative, but you do not see or control the specific numbers. For recreational diving, the default (medium) setting is appropriate for most divers.
Gradient factors are a powerful tool, but they are not a toy. Adjusting them without understanding the consequences can increase your DCS risk. If you do not understand what a setting change does, leave it at the factory default.