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Water Pressure at Depth Calculator

Example: 33 ft of seawater: 29.4 psi, twice surface air pressure

How hard does the water push at the bottom of a pool, on a dive or beside the Titanic? Enter a depth to see absolute and gauge pressure, the load on a porthole, how much a pocket of air shrinks, and where your depth sits among famous dives.
Last reviewed by SparkCalc editorial team · September 2026
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Calculation results updated

Your depth beside famous dives

Pressure from the surface down

Absolute Pressure

Compared With Surface Air

Gauge Pressure

Porthole Load

Air Squeeze

Your Depth Among Famous Dives

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How to use this calculator

  1. Choose feet and psi or metres and bar.
  2. Enter the depth and pick seawater or fresh water.
  3. For a lake in the mountains, enter the height of its surface above sea level.
  4. Read the absolute pressure, the load on a porthole and how much a pocket of air shrinks.
  5. Compare your depth with dive limits, record dives and wrecks in the chart and the Famous Depths table.

How We Calculate This

Gauge pressure = ρ × g × h, where ρ is water density (1,025 kg/m³ for surface seawater, 1,000 kg/m³ for fresh water), g is standard gravity (9.80665 m/s²) and h is depth. Absolute pressure adds the air pressure at the surface, from the NASA Glenn troposphere model: T = 15.04 − 0.00649 × altitude in °C and p = 101.29 × ((T + 273.1) / 288.08)^5.256 kPa, which gives about 101.3 kPa at sea level. Atmospheres divide absolute pressure by 101,325 Pa. The porthole load is gauge pressure on a round window 30 cm (about 12 in) across, since air pressure acts on both sides. The air squeeze uses Boyle's law at constant temperature. Seawater density rises slightly with depth and cold, so in the deep ocean the true pressure can differ from this model by a few percent.

Methodology last reviewed: September 2026. How SparkCalc works

Sources: NOAA Ocean Service: How does pressure change with ocean depth? · NASA Glenn Research Center: Earth Atmosphere Model (metric units) · Greenaway, Sabine, and others (2021). Revised depth of the Challenger Deep from submersible transects. Deep Sea Research Part I (NOAA Institutional Repository) · NOAA Office of General Counsel: R.M.S. Titanic, History and Significance · PADI: How Deep Can a Scuba Diver Go? Open Water vs. Advanced Open Water Limits · DeeperBlue: Molchanov Sets New Absolute Freediving World Record in Constant Weight · NOAA Fisheries: Sperm Whale · Schorr, G. S. and others (2014). First Long-Term Behavioral Records from Cuvier's Beaked Whales Reveal Record-Breaking Dives. PLOS ONE 9(3): e92633

Depth, not the size of the water, sets the pressure

Pressure depends only on how much water is stacked above a point, not on how wide the lake or ocean is. The bottom of a narrow 10 metre tank and a spot 10 metres down in the open sea feel almost the same pressure. What changes the answer is density: salt water is about 2.5 percent heavier than fresh water, so it builds pressure a little faster.

Why the first few metres matter most to divers

Going from the surface to 10 metres of seawater doubles the absolute pressure and halves the volume of any trapped air. Going from 30 to 40 metres adds the same pressure but changes air volume far less in proportion. That is why ear and mask squeeze are felt most near the surface, and why the last few metres of an ascent need the most care.

The deep ocean in numbers

NOAA Fisheries notes that sperm whales routinely hunt around 2,000 feet down, and tagged Cuvier's beaked whales have reached 2,992 metres, the deepest recorded mammal dive. The Titanic rests near 3,800 metres. The Challenger Deep in the Mariana Trench, measured at 10,935 metres in 2021, carries more than a tonne of water load on every square centimetre.

Key terms

Absolute pressure
The total pressure at depth: the weight of the water plus the air pressure at the surface.
Gauge pressure
The pressure from the water alone, measured relative to the surface. It reads zero at the surface.
Atmosphere (atm)
A unit equal to average air pressure at sea level: 101,325 pascals, 1.013 bar or 14.7 psi.
Boyle's law
At a fixed temperature, the volume of a gas falls in proportion as its pressure rises.

Frequently Asked Questions

How do you calculate water pressure at depth?

Multiply the water density by gravity and by the depth. That gives gauge pressure, the part caused by the water. Add the air pressure at the surface, about 14.7 psi or 1.013 bar at sea level, to get absolute pressure, which is what your body and any sealed container feel.

What is the difference between gauge and absolute pressure?

Gauge pressure counts only the weight of the water above you, so it reads zero at the surface. Absolute pressure adds the atmosphere on top. Divers and physics problems usually quote absolute pressure; many depth sensors report gauge pressure.

How much does pressure increase every 10 metres or 33 feet?

NOAA puts it at one atmosphere for every 33 feet (10.06 metres) of seawater. Fresh water is slightly less dense, so it takes about 10.3 metres to add the same amount. At 10 metres of seawater you are under roughly twice the pressure you feel on land.

What is the water pressure at the Titanic and the Challenger Deep?

The Titanic lies about 3,800 metres (12,500 feet) down, where seawater presses with roughly 380 times surface air pressure, over 5,500 psi. The Challenger Deep, measured at 10,935 metres, is close to 1,100 times surface pressure, about 16,000 psi.

Why does air shrink as you go deeper?

A sealed, flexible pocket of air obeys Boyle's law: its volume falls in proportion to the absolute pressure. At 33 feet of seawater the pressure has doubled, so a balloon filled at the surface is half its size. This is why divers equalise their ears and never hold their breath while ascending.

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This calculator is for education and curiosity. It is not a dive planner or dive computer and does not account for gas mixtures, decompression, temperature or equipment ratings. Plan real dives with training, certified equipment and established dive tables.

References