MOD / EAD / END Calculator

Gas Calculators — Key Features

• MOD calculator — find the maximum safe depth for any oxygen percentage
• EAD calculator — convert nitrox dives to equivalent air depth
• END calculator — determine narcotic depth for trimix gases
• Best Mix finder — optimal oxygen percentage for your target depth
• Support for nitrox and trimix gas mixes
• Configurable PPO₂ limits for different diving scenarios

Gas Calculators — How It Works

Enter your gas mix (O₂% and optionally He%) and the calculator instantly returns four planning numbers. MOD (Maximum Operating Depth) is the depth at which your mix reaches the chosen ppO₂ limit — typically 1.4 ata for working bottom gas (the CNS oxygen-toxicity threshold for active dives) and 1.6 ata for deco gas breathed at rest on a stop. EAD (Equivalent Air Depth) is the depth at which air would have the same nitrogen partial pressure as your nitrox — useful when consulting air-based tables on a nitrox dive. END (Equivalent Narcotic Depth) is the depth at which air would produce the same narcotic load as your trimix; the standard convention treats helium as non-narcotic and includes oxygen in the narcotic fraction. Best Mix solves the inverse: given a target depth and ppO₂, what nitrox blend stays within limits. All four values update live as you adjust parameters, so you can iterate on a complete gas plan in seconds.

Why Use Our Gas Calculators?

• Instant results — calculations update as you type
• All essential gas calculations in one place
• Supports both nitrox and trimix gas mixes
• Clear visual feedback on safety limits

Gas Calculators — Frequently Asked Questions

What is MOD and why does it matter?

MOD (Maximum Operating Depth) is the deepest depth at which a gas mix can be breathed without exceeding a chosen oxygen partial-pressure limit, typically 1.4 ata for working (bottom) gas and 1.6 ata for decompression gas breathed at rest. Exceeding the MOD risks acute central-nervous-system oxygen toxicity, which can trigger seizures underwater — a near-certain fatality in scuba. Every nitrox and trimix dive requires an MOD calculation before descent; the figure determines which gas is safe to breathe at which depth in the plan.

What's the difference between EAD and END?

EAD (Equivalent Air Depth) is the depth at which air would have the same nitrogen partial pressure as your nitrox mix; useful when consulting air-only tables on a nitrox dive (a shallower EAD means less decompression obligation). END (Equivalent Narcotic Depth) is the depth at which air would produce the same narcotic load as your trimix; the standard convention treats helium as non-narcotic and includes oxygen in the narcotic fraction, so adding helium reduces END dramatically. EAD applies to deco planning, END applies to gas selection for deep dives.

Is this calculator free?

Yes — all gas calculators on DiveToolbox (MOD, EAD, END, Best Mix) are completely free with no usage limits. The freemium model only paywalls advanced tools like the full Dive Planner, gas blending, successive-dive planning, and saved profiles. Calculators that solve a single equation in real time — MOD, EAD, END, unit conversions, gradient factors, gas density, ICD — remain free because they should be instantly available to any diver checking a number before splashing.

What ppO2 limit should I use?

Standard recreational practice uses 1.4 ata as the MOD ceiling for active (working) bottom gas, where elevated CO₂ from exertion increases CNS toxicity risk. 1.6 ata is the accepted ceiling for decompression gas breathed at rest on a stop, where the body is not working and CNS tolerance is higher. Some training agencies are more conservative (1.3 ata bottom, 1.5 ata deco); some technical programs allow 1.6 ata on shorter exposures. Always use the limit specified by your training agency and adjust downward for cold, exertion, or stress.

Can I calculate for trimix gases?

Yes — every gas calculator accepts both O₂ and helium percentages, computing MOD, EAD, END, and density for any nitrox or trimix blend up to 100% O₂ and 100% He. END calculations follow the standard convention: helium is non-narcotic, oxygen is counted in the narcotic fraction. For pure helium or heliox mixes the narcotic depth drops dramatically — END can be 10-15 metres shallower than actual depth, which is why trimix is the standard tool for deep diving beyond ~40 metres.

How does nitrox affect my no-decompression limits?

Nitrox increases your no-decompression limit at any given depth because there is less nitrogen to absorb. The trade-off is a shallower MOD: EAN32 (32% O₂) has an MOD of 33 m at 1.4 ppO₂ versus 56 m for air. The NDL extension is meaningful in the 18-30 m range typical of recreational diving — EAN32 roughly doubles NDL at 24 m compared to air. Use the [Gradient Factors](/en/gradient-factors) tool to see your specific NDL on any nitrox mix and depth.

→ How to Plan a Trimix Dive — A 6-step workflow connecting every calculator in the right order

→ How to Plan a Nitrox Dive — A 5-step recreational workflow — pick the blend, know your gas, stay inside NDL

→ Diving Glossary — Essential acronyms and concepts every diver should know

→ Nitrox Blends Compared: EAN32 vs EAN36 vs EAN40 — How three common nitrox mixes change MOD, NDL extension and depth ceiling for the same dive