Dive planning calculators
Six calculators covering the arithmetic every trained diver is expected to be able to check: surface air consumption, nitrox oxygen limits, nitrogen loading, gas reserves, lead and exposure protection. Each one shows its working rather than handing you a number.
Dive planning math is unusual among the things a hobby site can publish, because getting it wrong has physical consequences rather than merely embarrassing ones. That shapes how these tools are built. Every one of them prints the formula it used, states the assumptions it made, and warns when an input takes you outside recreational limits, so a trained diver can check the arithmetic rather than trust it.
- Equivalent Air Depth Calculator for NitroxFree equivalent air depth calculator. Enter your nitrox mix and planned depth to see the equivalent air depth for nitrogen... Open →
- Gas Planning Calculator: Rock Bottom and Turn PressureFree scuba gas planning calculator. Work out your rock bottom reserve and turn pressure from depth, cylinder size and respiratory... Open →
- Nitrox MOD Calculator: Maximum Operating DepthFree nitrox MOD calculator. Enter your oxygen fraction and partial pressure limit to get the maximum operating depth in feet and... Open →
- SAC Rate Calculator: Surface Air ConsumptionFree SAC rate calculator for scuba diving. Enter start and end pressure, dive time and average depth to get your surface air... Open →
- Scuba Weight Calculator: How Much Lead Do You Need?Scuba weighting calculator by body weight, exposure suit, water type and cylinder. Get a starting lead figure in pounds and... Open →
- Wetsuit Thickness Calculator for Scuba DivingWetsuit thickness calculator by water temperature, dive length and repetitive dives. Get a millimetre recommendation plus hood... Open →
Which calculator should you use first?
The SAC rate calculator, and it is not close. Your surface air consumption is the one personal number that makes every other gas calculation real: without it, a gas plan is a guess dressed up in arithmetic. It takes three figures off a single logged dive, and once you have it, the gas planning calculator can produce a rock bottom reserve and a turn pressure for the depth you are actually diving.
The formulas these tools use
| Quantity | Formula | Worked example |
|---|---|---|
| Ambient pressure | ata = depth in fsw / 33 + 1 | 99 ft = 4.00 ata |
| Oxygen partial pressure | ppO2 = FO2 x ambient pressure in ata | EAN32 at 99 ft = 1.28 |
| Maximum operating depth | MOD (fsw) = 33 x (ppO2max / FO2 - 1) | EAN32 at 1.4 = 111 ft |
| Equivalent air depth | EAD (fsw) = ((1 - FO2) x (depth + 33) / 0.79) - 33 | EAN32 at 100 ft = 81.5 ft |
| Surface air consumption | SAC = (psi used / min) / ((avg depth fsw / 33) + 1) | 2,000 psi, 40 min, 50 ft = 19.9 psi/min |
| Respiratory minute volume | RMV = SAC x (cylinder cu ft / service psi) | 19.9 in an AL80 = 0.51 cu ft/min |
Two of those deserve a warning label. MOD is an oxygen limit and EAD is a nitrogen calculation, and they answer different questions: EAD tells you nothing at all about oxygen exposure, so a nitrox dive is only planned when you have both. And RMV travels between cylinders while SAC does not: a psi per minute figure is only true for the cylinder it was measured in, which is why the SAC tool converts to volume.
What these calculators deliberately do not do
They do not calculate decompression schedules, gas switches, or anything for a dive with a planned decompression obligation. Those belong to technical training, and publishing them as a web form would imply a competence a page cannot confer. The technical diving page explains what those topics are, why each needs a course, and why this site stops where it does.
They also do not replace your computer. The no-decompression limits table exists to show how far apart published figures can be between agencies and algorithms, which is exactly the reason a table never overrides the instrument on your wrist.
How do these six calculators connect to each other?
They are not six separate tools, they are one chain with two branches. Understanding the shape of it is most of what turns arithmetic into planning.
The gas branch starts at the SAC rate calculator, which turns one logged dive into a personal consumption figure. That figure feeds the gas planning calculator, which converts it into a rock bottom reserve for a specific depth and cylinder, and from there into a turn pressure. Nothing downstream of the SAC figure is meaningful without it, which is why we say it is the number to work out first.
The gas branch has a hidden input, and it is the weighting calculator. Overweighting is the largest single cause of a high consumption figure, because extra lead is offset by extra air in the wing, which increases drag and forces a head-up swimming angle. A diver who fixes weighting will produce a different SAC figure within a handful of dives, so the two tools are genuinely coupled rather than merely adjacent.
The nitrox branch has two halves that answer different questions and must both be used. The MOD calculator answers the oxygen question, which is a hard ceiling: a depth you do not go past. The EAD calculator answers the nitrogen question, which is what buys you the bottom time that made nitrox attractive in the first place. EAD says nothing at all about oxygen, and a plan built on EAD alone is missing the half that has the ceiling in it.
The wetsuit calculator sits upstream of everything. Being cold raises consumption, reduces dexterity and clouds judgement, and is a recognised risk factor for decompression sickness. It also changes your weighting by several pounds. A diver who is underdressed will see it in their gas figures before they feel it.
A worked example that uses four of them
A diver plans a 60 foot reef dive on EAN32 in an aluminium 80, in 68 F water, doing two dives that day. Here is what each tool contributes.
| Step | Tool | Output | What it decides |
|---|---|---|---|
| 1 | Wetsuit thickness | 5 mm plus hood | 68 F with two dives pushes one step warmer than the plain band |
| 2 | Weighting | 14 to 18 lb | A 180 lb diver in a 5 mm suit in salt water, confirmed by a weight check |
| 3 | Nitrox MOD | 111 ft at 1.4 ata | 60 ft is comfortably inside the ceiling, so the mix is legal for the dive |
| 4 | Equivalent air depth | about 47 ft | Nitrogen loading matches a 47 ft air dive, which is where the bottom time comes from |
| 5 | SAC rate | 0.51 cu ft/min | The diver's own figure from five logged dives, taken as a median |
| 6 | Gas planning | turn at about 1,925 psi | Rock bottom around 850 psi, so half of the usable 2,150 psi is spent outbound |
Read step 6 against step 4 and you can see the point of doing both. The nitrox mix has given this diver far more no-decompression time than air would have, and their gas supply now runs out first. Once that happens, a bigger cylinder does more for bottom time than a richer mix does, and there is no way to see which limit is binding without your own consumption figure.
What these tools assume, so you can check them
- 33 feet of sea water per atmosphere, 34 feet in fresh water, and 10 metres in sea water. Every tool with a water selector applies the correct one.
- Ambient pressure in ata is depth divided by 33, plus 1. At 99 feet that is 4 ata, which is why consumption there is four times the surface rate.
- Average depth, not maximum depth, in every consumption calculation. Using maximum depth understates your SAC substantially, which is the dangerous direction.
- 1.4 ata as the working oxygen maximum and 1.6 ata as a contingency ceiling rather than a target.
- An elevated consumption rate for both divers in a reserve calculation, commonly 1.0 cubic feet per minute each rather than a relaxed 0.5, because a diver who has just gone out of gas is not relaxed.
- Cylinder buoyancy and capacity figures that vary by manufacturer, quoted as approximate and labelled as such wherever they appear.