Altitude Diving Calculator

For dives in mountain lakes. See the air pressure at the lake, the depth to look up on a sea-level table and how shallow to hold your stops.

Results update as you type.

Quick examples

Load a full example, then change any value.

Lake altitude (m)

Height of the lake surface above sea level. Take it from a map or the dive centre.

Real depth in the lake (m)

The deepest point of the dive, measured in the lake.

Water

Most mountain lakes are fresh water.

Stop depth on your table (m)

The DECO 2000 table and this site use a 5-minute safety stop at 3 m.

Quick picks

Sea-level equivalent depth

Altitude dive

At 1500 m the air pressure is 845.6 hPa, so 18 m in the lake counts as 21.6 m on a sea-level table. Hold your 3 m stop at 2.5 m.

Above 700 m the DECO 2000 table no longer applies. Use a table made for this altitude, a sea-level table with the equivalent depth above, or a dive computer set for altitude.

Air pressure at the lake: 0.846 bar
Correction factor: 1.198
Your 3 m stop at the lake: 2.5 m
Ascent rate that matches 10 m/min: 8.3 m/min
Depth on a computer set to salt water: 17.6 m

No correction up to 300 m. DECO 2000 covers 0-700 m. Most dive computers stop at 3000 m.

Was this calculator useful?

These tools are educational planning aids; they are not a substitute for dive training, current printed tables or a dive computer. Verify results against your training and always plan within the limits of your certification.

About altitude diving

A mountain lake has less air above it. Sea-level tables assume 1013 hPa at the surface, so the same dive at altitude ends with a bigger pressure drop when you surface, and the tables do not allow for it.

The Cross correction in the US Navy Diving Manual converts the lake dive to a deeper sea-level dive with the same pressure ratio, so you can read an ordinary table. Stops move shallower by the same ratio.

With your numbers

Enter valid values to see the calculation worked through with your numbers.

Formula

Air pressure at the lake (ISA)

Palt = 1013.25 × (1 − 2.25577×10^−5 × h)^5.25588

Sea-level equivalent depth

dsea = d × (Psea) ÷ (Palt)

Stop depth at the lake

salt = s × (Palt) ÷ (Psea)

Where

h: lake altitude above sea level (m)
Psea: 1013.25 hPa, standard sea-level pressure
d: real depth in the lake (m)
s: stop depth from a sea-level table (m)

How it's calculated

Air pressure at the lake: P = 1013.25 × (1 − 2.25577 × 10^-5 × h)^5.25588 hPa, the ICAO/ISA standard atmosphere for 0-11 km (US Standard Atmosphere 1976 constants, en.wikipedia.org). At 1524 m (5000 ft) it gives 843.1 hPa, and the US Navy uses 843 mb.
Sea-level equivalent depth = real depth × 1013.25 ÷ P. This is the Cross correction of the US Navy Diving Manual, section 9-12.1.1 (navydiving.tpub.com).
Stop depth at the lake = table stop × P ÷ 1013.25 (section 9-12.1.2). The same ratio gives the ascent rate that matches 10 m/min. The Navy notes this third correction can safely be ignored, and a slower ascent does no harm.
No reduction for fresh water: the Navy assumes every dive is in sea water (section 9-12.3). The computer reading row uses 1000 kg/m³ for fresh and 1025 kg/m³ for sea water.
Bands: no correction up to 300 m (US Navy 1000 ft rule, PADI, Suunto's 0-300 m range). DECO 2000 and the Bühlmann 1986 tables cover 0-700 m (divetable.eu). Most computers stop at 3000 m (suunto.com).
Checked against US Navy Rev 7 Table 9-4 (omao.noaa.gov): 59 of 60 stop depths match to the foot and the last is 0.5 ft off. 206 of 207 equivalent depths for 2000-10,000 ft match after rounding up to the table row, and the other one is a row deeper in the Navy table.
Depths are shown to 0.1 m, pressure to 0.1 hPa and the factor to 3 decimals. Every step uses unrounded values.

Good to know

Wait 12 hours at the lake's altitude before the first dive if you can (US Navy). If you dive sooner, the nitrogen you brought up from lower ground counts like a previous dive. Suunto asks for at least 3 hours.
Switch your dive computer on at the lake. Many computers read the air pressure at switch-on and correct for altitude by themselves. Others need an altitude range set by hand, for example Suunto's 0-300, 300-1500 and 1500-3000 m.
Fresh water is about 2.5% lighter than sea water. The water setting on a computer changes only the depth on the screen, not its decompression maths. Like the US Navy, this calculator does not reduce the depth for fresh water, which keeps a small margin.
Driving over a mountain pass or flying after a dive is a separate limit with its own waiting times. This calculator does not replace altitude training or a dive computer set for altitude.

Altitude correction table

Standard-atmosphere air pressure for common lake altitudes, the correction factor, the sea-level equivalent of a 18 m dive and where a 3 m stop moves to.

Altitude	Air pressure	Factor	18 m dive on the table	3 m stop at the lake
0 m	1013.3 hPa	1.000	18.0 m	3.0 m
300 m	977.7 hPa	1.036	18.7 m	2.9 m
600 m	943.2 hPa	1.074	19.3 m	2.8 m
1000 m	898.7 hPa	1.127	20.3 m	2.7 m
1500 m	845.6 hPa	1.198	21.6 m	2.5 m
2000 m	795.0 hPa	1.275	22.9 m	2.4 m
2500 m	746.8 hPa	1.357	24.4 m	2.2 m
3000 m	701.1 hPa	1.445	26.0 m	2.1 m

Sources

How to use the altitude diving calculator

1
Enter the altitude of the lake from a map or the dive centre, and the deepest depth you plan to reach.
2
Choose the water type and the stop depth your sea-level table uses. DECO 2000 uses a 3 m safety stop.
3
Plan with the sea-level equivalent depth on your table, rounded up to the next row as usual, and hold the stops at the shallower depths shown.

What the results mean

Air pressure falls as you go up. At 1500 m it is about 846 hPa, 17% less than at sea level. Sea-level tables assume the full 1013 hPa at the surface, so they give too much time for a lake dive at altitude.

The sea-level equivalent depth is the depth to look up on a sea-level table. An 18 m dive in a lake at 1500 m is read as 21.6 m, then rounded up to the next depth row of your table. Stops move shallower by the same ratio: a 3 m safety stop becomes 2.5 m at 1500 m and about 2.1 m at 3000 m.

DECO 2000, the table used in CMAS training, is computed for 0-700 m altitude. Up to 700 m you read it with your real depth and no correction. Above 700 m use a table made for that altitude, a sea-level table with this correction, or a dive computer set for altitude.

Most modern dive computers measure the air pressure when they are switched on and correct for altitude by themselves. Some need an altitude range selected by hand. Old mechanical depth gauges with a sealed reference read too shallow at altitude, and the US Navy adds 1 fsw (0.3 m) for every 1000 ft (305 m).

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