Why must you never hold your breath while ascending?

Gas volume is inversely proportional to absolute pressure. Rising from 10 m to the surface halves the pressure, so the air in your lungs tries to double. Full lungs can barely stretch, so the excess becomes a pressure difference, and only about 0.1 bar — a little over a metre of water — can rupture alveoli and cause a pneumothorax or arterial gas embolism. Keep breathing normally all the way up.

How is SAC calculated, and what is the rule of thirds?

SAC is your breathing rate converted to 1 atm. At depth you use SAC × absolute pressure / 1 atm, so 20 L/min at 20 m (about 3 bar) means 60 L of surface gas per minute. The rule of thirds splits your gas into one third out, one third back and one third for emergencies, standard practice for cave and wreck diving.

How do you calculate the MOD for nitrox?

The maximum operating depth is where oxygen partial pressure reaches the limit: MOD = (ppO₂ limit / FO₂ − 1) × 10 m in sea water, roughly. EAN32 gives about 33 m at 1.4 bar and 40 m at 1.6 bar. Going deeper risks CNS oxygen toxicity and convulsions underwater. The tool uses your water type and altitude exactly.

Why check weighting at the end of the dive?

A full cylinder holds several kilograms of air — a 12 L steel tank at 200 bar carries about 2.9 kg. You breathe it away, so the cylinder gets lighter. If you are neutral at the start you will be light at the end and may struggle to hold a 5 m safety stop. Weight yourself to be neutral at 5 m with about 50 bar left and an empty BCD.

Can I use these no-deco times to plan a dive?

No. This is a teaching demonstration of Bühlmann ZHL-16C with a square profile, no ascent, no repetitive dives and no personal factors. For real dives use a certified dive computer or proper dive tables, stay conservative and always make a safety stop.

🤿

🤿Scuba Diving Physics Calculator - Pressure, Gas Planning, Nitrox MOD, Buoyancy

The physics from your dive course, made interactive. Every 10 m of sea water adds about one atmosphere, gas volume follows Boyle's law, and the biggest changes happen near the surface. The tool animates why holding your breath on ascent is dangerous and works out gas use, oxygen partial pressure, buoyancy and Bühlmann tissue loading. Everything here is educational — it does not replace a dive computer or proper training.

Free · No sign-up · Runs in your browserLast updated: 2026-10-29

Loading tool...

How to use Scuba Diving Physics Calculator

1Pick sea or fresh water and, for mountain lakes, the surface altitude
2In Pressure & Boyle set a depth and lung volume, then press Ascend to watch the lungs and a sealed balloon expand
3In Gas & rule of thirds choose a cylinder and SAC to see time to reserve and the turn pressure, or work out your SAC from a logged dive
4In Partial pressure & nitrox set the oxygen fraction and depth for ppO₂, MOD, EAD, END and the CNS clock
5Use Buoyancy & weighting and No-deco limits to adjust your kit and profile and watch each compartment's nitrogen load

FAQ

Why must you never hold your breath while ascending?: Gas volume is inversely proportional to absolute pressure. Rising from 10 m to the surface halves the pressure, so the air in your lungs tries to double. Full lungs can barely stretch, so the excess becomes a pressure difference, and only about 0.1 bar — a little over a metre of water — can rupture alveoli and cause a pneumothorax or arterial gas embolism. Keep breathing normally all the way up.
How is SAC calculated, and what is the rule of thirds?: SAC is your breathing rate converted to 1 atm. At depth you use SAC × absolute pressure / 1 atm, so 20 L/min at 20 m (about 3 bar) means 60 L of surface gas per minute. The rule of thirds splits your gas into one third out, one third back and one third for emergencies, standard practice for cave and wreck diving.
How do you calculate the MOD for nitrox?: The maximum operating depth is where oxygen partial pressure reaches the limit: MOD = (ppO₂ limit / FO₂ − 1) × 10 m in sea water, roughly. EAN32 gives about 33 m at 1.4 bar and 40 m at 1.6 bar. Going deeper risks CNS oxygen toxicity and convulsions underwater. The tool uses your water type and altitude exactly.
Why check weighting at the end of the dive?: A full cylinder holds several kilograms of air — a 12 L steel tank at 200 bar carries about 2.9 kg. You breathe it away, so the cylinder gets lighter. If you are neutral at the start you will be light at the end and may struggle to hold a 5 m safety stop. Weight yourself to be neutral at 5 m with about 50 bar left and an empty BCD.
Can I use these no-deco times to plan a dive?: No. This is a teaching demonstration of Bühlmann ZHL-16C with a square profile, no ascent, no repetitive dives and no personal factors. For real dives use a certified dive computer or proper dive tables, stay conservative and always make a safety stop.

Features

Absolute pressure, sea-level equivalent depth for altitude diving, and a Boyle's-law volume table
Breath-hold ascent animation that warns once transpulmonary pressure passes ~0.1 bar and shows where it happens
SAC-based gas use, time to reserve, rule-of-thirds turn pressure, and SAC from a logged dive
Nitrox: ppO₂, MOD at 1.4/1.6, best mix, EAD, END and NOAA CNS%
Buoyancy breakdown — body, wetsuit compression, full vs empty cylinder, lead — with BCD air needed and proper weighting
Bühlmann ZHL-16C 16-compartment model with gradient factors, compared with the PADI RDP

Related tools

View all 196 Physics Tools →