Ten metres of water = one atmosphere

Every 10 m of seawater adds about 1 atmosphere of pressure.

Why it works

ρgh for seawater gives ~101 kPa per 10.06 m. At 30 m a diver is under 4 atm total — 1 of air plus 3 of water.

Do it exactly

Estimate with the rule, then check it against the calculator that models it properly.

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More in Environmental

• Lightning distanceCount the seconds from flash to thunder, divide by 3 for kilometres (by 5 for miles).
• Air cools as you climbTemperature drops about 6.5 °C for every 1,000 m of altitude.
• Pressure halves every ~5.5 kmAtmospheric pressure falls by half roughly every 5,500 m you climb.
• Boiling point falls with altitudeWater boils about 1 °C cooler for every 300 m of altitude.

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