Why is altitude diving more dangerous than sea-level diving?

At altitude, atmospheric pressure is lower than at sea level. When you surface from a dive, dissolved gas in your tissues expands against a lower surrounding pressure — creating a steeper pressure gradient and a higher DCS risk than the same dive at sea level.

What is theoretical depth and why is it used in altitude diving?

Theoretical depth is the sea-level equivalent of your actual dive depth at altitude, found using the PADI altitude conversion table. Because the RDP was designed for sea-level pressure, you must convert your planned actual depth to theoretical depth before looking up no-decompression limits. Always round the theoretical depth up to the next RDP column — never down.

What is the rule if you have not acclimatised for 6 hours before an altitude dive?

If you dive within 6 hours of arriving at altitude, you must add a starting pressure group before planning the dive. Count 2 pressure groups for every 1,000 ft of altitude to find your starting group.

Which site should you dive first if diving at two different altitudes on the same day?

Always dive the higher altitude site first. Diving the lower altitude site first means you accumulate more nitrogen before reaching the higher, lower-pressure site, which significantly increases decompression risk on the second dive.

Altitude Diving — PADI IDC and Divemaster Theory

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Watch Will Welbourn explain why altitude diving increases DCS risk, how to convert actual depth to theoretical depth, and all the key rules tested in the PADI IDC and Divemaster exams.

Why altitude changes everything

At altitude, atmospheric pressure is lower than at sea level. That one fact drives every altitude diving rule.

Key concept — Pressure Gradient The pressure gradient is the difference between the gas pressure dissolved in your tissues and the pressure surrounding you at the surface. The greater the gradient, the greater the DCS risk. Think of it as a slope — the steeper the slope, the more dangerous it is.

When you surface from a dive, your body contains dissolved gas under pressure. That gas wants to come out. At sea level, the surrounding pressure is 1.0 atm — a gentle slope. At 2,000 m / 6,000 ft, the surrounding pressure is only 0.8 atm — the same tissue loading now produces a steeper slope, and a much higher DCS risk.

Scenario	Tissue pressure at surface	Surrounding pressure	Gradient
Same dive at sea level	1.4 atm	1.0 atm	0.4 atm — gentle slope
Same dive at 2,000 m / 6,000 ft	1.4 atm	0.8 atm	0.6 atm — steeper slope, higher risk

Exam trap The altitude conversion table uses altitude in feet, not barometric pressure. The two are closely related, but the tables are based on altitude. Don't confuse the two.

The altitude diving rules

Convert actual depth to theoretical depth — use the PADI altitude conversion table before planning any dive
Adjust safety stop depth — also found on the altitude conversion table
No 6-hour acclimatisation = treat as a repetitive dive — count 2 pressure groups per 1,000 ft of altitude
Halve your ascent rate — 30 ft / 9 m per minute instead of 60 ft / 18 m
Maximum 2 altitude dives per day
Dive the higher altitude first if diving at two different altitudes on the same day
Limit exertion — lower O₂ partial pressure at altitude means you become hypoxic more easily
Allow time to rewarm between dives — colder temperatures slow off-gassing
Don't drive to altitude immediately after sea-level diving — follow flying-after-diving logic
Nitrox at altitude: plan as if using air — use the standard air RDP with theoretical depths; don't use nitrox tables to extend NDL

Rule 1 & 2 — Theoretical depth and safety stop

The altitude conversion table converts your planned actual depth to a theoretical depth — the depth you must use when looking up NDLs on the RDP. It also gives you the adjusted safety stop depth for your altitude.

Worked Example — 80 ft dive at 10,000 ft

Look up 80 ft actual depth at 10,000 ft altitude on the conversion table → theoretical depth = 116 ft
Round up to next RDP column → use 120 ft
Look up 120 ft on the RDP → NDL = 13 minutes
Adjusted safety stop depth at 10,000 ft → 10 ft for 3 minutes

Compare: the same 80 ft dive at sea level has an NDL of 30 minutes. At 10,000 ft that drops to just 13 minutes.

Exam trap Always round the theoretical depth up to the next depth column on the RDP — never down. Rounding down would underestimate nitrogen loading.

Rule 3 — No 6-hour acclimatisation

If you arrive at altitude and dive within 6 hours, the lower surrounding pressure will have already caused your body to off-gas faster than normal — PADI treats this the same as nitrogen carried over from a previous dive. You must factor in a starting pressure group.

The rule Count 2 pressure groups for every 1,000 ft of altitude. The result is your starting pressure group for the dive.

Worked Example — Immediate dive at 6,000 ft to 60 ft actual depth

Calculate starting pressure group: 6,000 ft ÷ 1,000 = 6 × 2 = 12 pressure groups → count 12 letters from A → start in Pressure Group L
Look up theoretical depth: 60 ft at 6,000 ft → 75 ft → round up to 80 ft
Look up adjusted NDL on RDP: Pressure Group L at 80 ft → 8 minutes

NDL = 8 minutes. A full 6-hour wait before diving would return you to Pressure Group A, giving a much longer NDL.

Exam trap The 6-hour rule is mandatory above 8,000 ft. Below 8,000 ft it is strongly recommended — but if you don't wait, you still must apply the pressure group adjustment. Exams may test whether you know the distinction.

Rules 4 & 6 — Ascent rate and dive order

Rule	Sea level	At altitude	Why
Maximum ascent rate	60 ft / 18 m per min	30 ft / 9 m per min	Steeper pressure gradient at the surface — slower ascent reduces off-gassing rate
Dive order (two altitudes)	—	Higher altitude first	Conservative tables apply to the second dive; descending in altitude is the safe direction

Oxygen, Nitrox & cold at altitude

Reduced oxygen — limit your exertion

The partial pressure of oxygen is lower at altitude. You become hypoxic (short of breath) more easily. Swim slowly on the surface, rest before descending, and take it easy on the return swim.

Nitrox at altitude — use air tables

Nitrox at altitude rule Use the standard air RDP with theoretical depths. Do not use nitrox-specific tables to extend your NDL at altitude. Nitrox still helps with oxygen levels and nitrogen off-gassing, but the NDL extension benefit must not be applied.

Cold between dives

Altitude environments are typically colder. Cold reduces off-gassing efficiency. Allow adequate surface intervals and time to rewarm between dives.

Exam trap — nitrox at altitude A common exam question asks whether you can use nitrox tables to extend NDLs at altitude. The answer is no — always plan as if using air.

Quick-reference summary

Rule	The number / answer
Pressure groups per 1,000 ft (no 6-hr wait)	2
Maximum altitude dives per day	2
Ascent rate at altitude	30 ft / 9 m per min
Safety stop at 10,000 ft	10 ft for 3 min
Safety stop at sea level	15 ft for 3 min
Mandatory 6-hr acclimatisation above	8,000 ft
Depth table input	Theoretical depth (rounded up), not actual depth
Nitrox NDL tables at altitude	Do NOT use — plan as air
Two altitudes same day — which first?	Higher altitude first