Plan your NDL, gas supply, SAC rate, and nitrox limits
Planning a safe and enjoyable scuba dive requires more than just strapping on a tank and jumping in. Understanding your no-decompression limit (NDL), how long your gas supply will last, and whether your nitrox mix is safe for your planned depth are all critical steps in responsible dive planning. This free scuba dive time calculator brings together four essential dive planning tools in one place, designed for recreational divers of all experience levels. The physics of diving revolve around two fundamental gas laws that every diver should understand. Boyle's Law states that as pressure increases with depth, gas volume decreases proportionally — at 10 metres of seawater, ambient pressure doubles to 2 ATA (atmospheres absolute), meaning you consume gas from your tank twice as fast as at the surface. At 30 metres, consumption is four times the surface rate. Dalton's Law of Partial Pressures governs the oxygen and nitrogen components of your breathing gas — the partial pressure of each gas equals its fraction multiplied by the ambient pressure, which is why oxygen becomes toxic at depth and why nitrogen absorption drives decompression limits. The NDL calculator uses the PADI-aligned recreational dive table to estimate how long you can stay at a given depth without incurring mandatory decompression stops. If you go beyond the NDL, surfacing directly becomes dangerous — ascending without decompression stops at excessive depth exposure risks decompression sickness (DCS), a serious condition caused by dissolved nitrogen forming bubbles in your tissues and bloodstream. Our calculator applies a conservatism multiplier (Normal, Conservative, or Very Conservative) so you can plan with a margin of safety. The NDL table ranges from a generous 219 minutes at 10 metres down to just 9 minutes at 40 metres, illustrating the dramatic effect of depth on nitrogen absorption. For nitrox divers, the tool automatically computes the Equivalent Air Depth (EAD) and uses it for the NDL lookup, reflecting the extended bottom times that enriched air offers due to reduced nitrogen loading. The SAC/RMV calculator helps you determine your Surface Air Consumption (SAC) rate and Respiratory Minute Volume (RMV) from a dive you have completed. SAC tells you how many bar (or psi) of pressure you consume per minute at the surface, adjusted for depth. RMV expresses the same rate in litres per minute, making it cylinder-size independent — useful when you dive different tanks. Knowing your SAC and RMV is the foundation of all gas planning; typical recreational values range from 20–25 L/min for beginners down to under 10 L/min for very experienced technical divers. The calculator includes a breathing rate classification scale so you can see how your consumption compares to divers at different experience levels. The Gas Planning calculator takes your tank size, starting pressure, and RMV and tells you exactly how long your gas will last at a planned depth. It applies the Rule of Thirds — a standard dive planning framework where one-third of your gas is reserved as an emergency reserve, one-third is used for the return journey and ascent, and one-third is available for your bottom time exploration. This is the conservative standard taught by PADI and most recreational training agencies. A visual stacked bar chart shows the three allocation segments at a glance. If you enter a planned bottom time, the tool also calculates exactly how much gas you will need and shows what percentage of your total supply that represents, with a colour-coded bar indicating whether your plan fits safely within the usable gas allocation. The Nitrox Tools section covers everything you need for enriched-air diving: Maximum Operating Depth (MOD), Equivalent Air Depth (EAD), the partial pressure of oxygen (PO2) at your planned depth, and the best nitrox mix for your planned dive. MOD is the deepest you can safely dive on a given nitrox blend without exceeding your chosen PO2 limit (1.2, 1.4, or 1.6 ATA). Exceeding your MOD on nitrox risks oxygen toxicity — a potentially fatal condition that can cause seizures with no warning. The PO2 status indicator immediately shows whether your planned depth and nitrox mix combination is safe, at the limit, or dangerous, and a colour-coded PO2 gauge bar provides an intuitive visual reference. This calculator supports both metric (metres, bar, litres) and imperial (feet, psi, cubic feet) unit systems, salt water and fresh water (which have slightly different pressure gradients due to density differences of 1.025 vs 1.000 g/cm³), and includes common tank size presets and depth quick-buttons for fast entry. Results can be exported to CSV or printed as a dive plan summary. All calculations are performed locally in your browser — no data is sent to any server. IMPORTANT: This tool is for educational dive planning only. It is not a replacement for a calibrated dive computer, physical dive tables, or professional diver training. Always dive with a certified dive computer and never exceed your training limits. Conservative planning is always recommended — NDL values are maximum limits, not targets. Individual factors such as fitness, hydration, cold exposure, and previous dive history all affect actual nitrogen absorption.
Understanding Scuba Dive Planning
What Are NDL, SAC, and MOD?
The No-Decompression Limit (NDL) is the maximum time you can spend at a given depth on a single dive without needing to make mandatory decompression stops before surfacing. Recreational diving is conducted within NDL limits using standard dive tables or a dive computer. The PADI Recreational Dive Planner table underpins our NDL calculations. Surface Air Consumption (SAC) rate measures how fast you breathe through your tank, expressed as pressure loss (bar or psi) per minute at the surface. Respiratory Minute Volume (RMV) is the same concept in volume terms (litres per minute), making it independent of which cylinder you're using. Maximum Operating Depth (MOD) is the deepest safe depth for a given nitrox oxygen percentage and your chosen partial pressure of oxygen (PO2) limit — diving deeper risks acute oxygen toxicity.
How Are the Calculations Done?
NDL is computed by looking up your depth in the PADI-aligned recreational air table (10 m → 219 min, through 40 m → 9 min) with linear interpolation between table entries, then multiplying by a conservatism factor (1.0 normal, 0.85 conservative, 0.70 very conservative). For nitrox, the Equivalent Air Depth formula EAD = ((FN2 / 0.79) × (depth + 10)) − 10 is used to find the effective air depth before the table lookup. SAC rate is computed as SAC = (pressure_used / time) / ATA, where ATA = 1 + depth/10 for salt water (1 + depth/10.3 for fresh water). RMV = SAC × cylinder_volume. MOD = ((PO2_max / FO2) − 1) × 10 in metres, where FO2 is the oxygen fraction of your nitrox mix. Gas planning uses the Rule of Thirds: one third of start pressure is reserved, the remaining two-thirds divided between travel gas and bottom gas.
Why Dive Planning Matters
Decompression sickness (DCS) — sometimes called 'the bends' — occurs when dissolved nitrogen in your blood and tissues comes out of solution too quickly during ascent, forming bubbles. Even within NDL limits, too-fast ascents or skipping safety stops can cause DCS. Oxygen toxicity from nitrox misuse can cause underwater seizures with no warning, which are fatal for a submerged diver. Running out of gas underwater is one of the leading causes of diving fatalities. Understanding your SAC rate and applying the Rule of Thirds ensures you always have enough gas for emergencies. Conservative dive planning — treating NDL limits as maximums, not targets, and always keeping a reserve — is the single most important habit a recreational diver can develop.
Limitations of This Calculator
This calculator uses a simplified PADI-aligned recreational table for NDL and assumes a single-level dive at a constant depth. Real dives involve varying depths and multi-level profiles, which a dive computer handles dynamically using Bühlmann or RGBM algorithms — always more accurate than table-based estimates. The RMV calculation assumes a constant breathing rate throughout the dive; in reality, rates increase with exertion, cold, stress, and poor buoyancy. Salt vs fresh water calculations use standard density constants (1.025 vs 1.000 g/cm³) — altitude diving at lakes above sea level requires additional corrections not implemented here. NDL values do not account for repetitive dives; if you are planning a second dive within a surface interval, your residual nitrogen group must be factored in. Always verify your plan against your actual dive computer before entering the water.
Scuba Diving Formulas
Absolute pressure in atmospheres absolute. In salt water, pressure increases by 1 ATM every 10 metres (33 feet). In fresh water, it takes 10.3 metres (34 feet) per atmosphere due to lower density.
SAC rate measures air consumption normalized to surface pressure. Divide the pressure drop by the product of bottom time and absolute pressure at depth to get consumption in PSI per minute at the surface.
RMV converts SAC rate to actual litres per minute of gas consumed at the surface. Multiply SAC by the tank's internal volume and divide by the tank's working (rated) pressure. Typical recreational RMV ranges from 12–20 L/min.
The maximum depth at which a given nitrox mix stays below the oxygen toxicity threshold. PO2max is typically 1.4 ATA for recreational diving. FO2 is the fraction of oxygen in the mix (e.g., 0.32 for EANx32).
EAD converts a nitrox dive to the equivalent depth on air for decompression table purposes. Because nitrox has less nitrogen, the EAD is shallower than actual depth, giving longer no-decompression limits.
Maximum dive time based on gas supply. Usable gas is total gas minus reserve (Rule of Thirds: 1/3 out, 1/3 back, 1/3 reserve). Divide by the product of RMV and absolute pressure at depth.
Scuba Diving Reference Tables
PADI Recreational No-Decompression Limits (Air)
| Depth (m) | Depth (ft) | NDL (min) |
|---|---|---|
| 10 | 33 | 219 |
| 12 | 40 | 147 |
| 14 | 46 | 98 |
| 16 | 52 | 72 |
| 18 | 59 | 56 |
| 20 | 66 | 45 |
| 22 | 72 | 37 |
| 25 | 82 | 29 |
| 30 | 98 | 20 |
| 35 | 115 | 14 |
| 40 | 131 | 9 |
Common Nitrox Mixes
| Mix | O2 % | MOD at 1.4 ATA (m) | MOD at 1.6 ATA (m) | Best Use |
|---|---|---|---|---|
| Air | 21% | 56.7 | 66.2 | General recreational diving |
| EANx28 | 28% | 40.0 | 47.1 | Moderate depth, extended bottom time |
| EANx32 | 32% | 33.8 | 40.0 | Most popular recreational nitrox |
| EANx36 | 36% | 28.9 | 34.4 | Shallow reef diving |
| EANx40 | 40% | 25.0 | 30.0 | Very shallow or deco stops |
Typical SAC / RMV Values by Experience Level
| Experience Level | SAC (bar/min, 12L tank) | RMV (L/min) |
|---|---|---|
| Beginner | 1.7–2.1 | 20–25 |
| Intermediate | 1.25–1.7 | 15–20 |
| Experienced | 0.83–1.25 | 10–15 |
| Technical / Very Calm | <0.83 | <10 |
PO₂ Limits for Dive Planning
| Level | PO₂ (ATA) | Use Case |
|---|---|---|
| Conservative | 1.2 | Extended exposure, cold water, strenuous dives |
| Standard Recreational | 1.4 | Normal recreational diving |
| Contingency Maximum | 1.6 | Short exposures only (technical diving) |
Scuba Dive Planning Examples
NDL Calculation — 22m Salt Water Dive
Look up 22m in PADI NDL table: base NDL = 37 minutes
Conservatism factor (normal): 1.0 — no reduction
Adjusted NDL = 37 × 1.0 = 37 minutes
ATA = 1 + (22 / 10) = 3.2 ATA
Safety stop: 3 minutes at 5 metres recommended
SAC Rate from a Completed Dive
Calculate ATA: 1 + 18/10 = 2.8 ATA
Pressure consumption rate: 150 bar / 45 min = 3.33 bar/min at depth
Normalise to surface: SAC = 3.33 / 2.8 = 1.19 bar/min
Convert to RMV: 1.19 × 12L = 14.3 L/min
Gas Planning — Rule of Thirds at 20m
Total gas: 11.1L × 207 bar = 2,297.7 litres
Reserve (1/3): 2,297.7 / 3 = 765.9 litres
Usable gas (2/3): 2,297.7 × 2/3 = 1,531.8 litres
ATA at 20m: 1 + (20/10) = 3.0
Consumption at depth: 18 L/min × 3.0 = 54 L/min
Max dive time: 1,531.8 / 54 = 28.4 minutes
Nitrox MOD and EAD — EAN32 at 28m
MOD: ((1.4 / 0.32) - 1) × 10 = (4.375 - 1) × 10 = 33.75 metres
FN₂ = 1 - 0.32 = 0.68
EAD at 28m: ((0.68 / 0.79) × (28 + 10)) - 10 = (0.8608 × 38) - 10 = 22.7 metres
PO₂ at 28m: 0.32 × (1 + 28/10) = 0.32 × 3.8 = 1.216 ATA (safe)
How to Use This Calculator
Choose Your Calculator Mode
Select one of the four tabs: NDL (no-decompression limit), SAC/RMV (breathing rate from a completed dive), Gas Planning (how long your tank will last), or Nitrox Tools (MOD, EAD, best mix). Set your unit system (metric or imperial) and water type at the top of the input card.
Enter Your Dive Parameters
Use the depth quick-buttons (10/15/20/30 m or 33/60/66/100 ft) for common dive depths, or type any value. For SAC/RMV, enter your actual dive data: depth, bottom time, start and end tank pressure, and tank volume. For gas planning, enter your RMV (from a previous SAC calculation) and tank details.
Review Your Results
Results calculate instantly as you type. The NDL tab shows your maximum no-decompression bottom time and — if you entered an actual dive time — a progress ring showing how much of your NDL you used. The gas planning tab shows max dive time, Rule of Thirds breakdown, and whether your planned time requires more gas than your tank can safely supply.
Export or Print Your Dive Plan
Click 'Export CSV' to download your results as a spreadsheet, or 'Print Plan' to get a print-friendly summary you can carry to the dive site. Always verify your plan against your actual dive computer before entering the water — this tool is for planning, not in-water decision making.
Frequently Asked Questions
What is the NDL and why does it matter?
The No-Decompression Limit (NDL) is the maximum time you can spend at a given depth on a single dive without incurring mandatory decompression stops during your ascent. Exceeding the NDL means nitrogen has saturated your tissues enough that you must make stops at specific depths to allow safe off-gassing before reaching the surface. Surfacing directly after exceeding NDL risks decompression sickness (DCS), also called 'the bends,' which can cause joint pain, paralysis, or death. NDL values decrease rapidly with depth — at 10 m you have over 3 hours, but at 40 m you have only 9 minutes. Conservative planning treats NDL as an absolute maximum, not a target.
What is the difference between SAC and RMV?
Surface Air Consumption (SAC) rate expresses your breathing rate in pressure units per minute (bar/min or psi/min), normalized to surface pressure. It is useful for planning dives on the same cylinder you normally use. Respiratory Minute Volume (RMV) expresses the same rate as an actual volume of gas per minute (litres/min), making it cylinder-size independent. RMV is the preferred planning metric because it works the same whether you're diving an AL80, a 12L steel, or a 15L tank. Typical beginner RMV is 20–25 L/min; experienced divers often achieve 10–15 L/min. Knowing your RMV lets you instantly calculate how long any size tank will last at any depth.
What is the Rule of Thirds for gas planning?
The Rule of Thirds is a gas management framework taught by most recreational diving agencies. It divides your starting tank pressure into three equal portions: one third is held in reserve for emergencies and should never be used during a planned dive; one third is used for the outbound portion of the dive, including ascent and safety stop; and one third is available for actual bottom time. For example, starting at 200 bar, you reserve 67 bar (never use), use up to 67 bar for bottom time, and keep 67 bar for the return and ascent. This conservative approach ensures you always have gas for an emergency even if your buddy needs to share your tank.
Is nitrox actually safer than air for recreational diving?
Nitrox (enriched air with oxygen above 21%) reduces nitrogen loading at a given depth, effectively extending your NDL compared to air. For example, at 30 m on air your NDL is 20 minutes, but on EAN32 your equivalent air depth is about 26 m, giving a longer NDL. However, nitrox is not unconditionally safer — higher oxygen content means your Maximum Operating Depth (MOD) is shallower than for air. Diving below your MOD on nitrox risks acute oxygen toxicity, which can cause an underwater seizure without warning, which is almost always fatal for a diver. Nitrox requires specific training and a valid nitrox certification before use.
Why does the calculator show different NDL for salt vs fresh water?
Salt water is denser than fresh water (approximately 1.025 g/cm³ vs 1.000 g/cm³), so pressure increases slightly faster with depth in salt water than in fresh water. In salt water, you reach 1 additional atmosphere of pressure every 10 metres; in fresh water, it takes about 10.3 metres per atmosphere. For shallow recreational dives (under 30 m) the difference is small — less than 3% — but it affects the ATA calculation used in SAC, RMV, gas planning, and PO2 calculations. Most recreational tables and dive computers are calibrated for salt water. If you dive in a freshwater lake or quarry, use the fresh water setting for the most accurate results.
Do I still need a dive computer if I use this calculator?
Yes — absolutely. This calculator is a planning tool, not a replacement for a calibrated dive computer. A real dive computer tracks your actual depth profile second-by-second using Bühlmann or RGBM decompression algorithms that account for multi-level dives, ascent rates, and repetitive dive nitrogen groups. This calculator assumes a constant depth at your planned level, which is conservative but less accurate than a computer. In the water, your dive computer is your primary safety device. Never make in-water decisions based on pre-dive table calculations — always monitor your computer and ascend when it tells you to, regardless of what your planning figures showed.