Every 10 m of seawater adds about 1 atmosphere of pressure.
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.
In seawater. Fresh water needs about 10.3 m for the same atmosphere, and altitude changes the surface pressure you are counting from — a lake at 2,000 m starts near 0.8 atm, so a dive computer set for sea level reads the whole profile wrong.
At 10 m the rule says 101.3 kPa and the exact answer is 97.87 kPa — 3.5% high. It stays inside 5% across the whole range shown.
Fresh water at 10 m is 97.9 kPa, not 101.3 - the rule quietly rounds seawater and standard atmosphere into the same number.
The rule against the exact answer, computed across the range. Inside the shaded band the shortcut is close enough to use; outside it, reach for the calculator.
Estimate with the rule, then check it against the calculator that models it properly.
Open Altitude & Depth →Every 10 m of seawater adds about 1 atmosphere of pressure. ρgh for seawater gives ~101 kPa per 10.06 m.