Gas Planning Calculators

SAC (Surface Air Consumption) rate measures how much breathing gas you consume per minute, normalized to surface pressure. It is also called RMV (Respiratory Minute Volume) when expressed in volume terms. When calculated from a dive log using tank pressure drop and tank volume, SAC and RMV produce the same number in cubic feet per minute.

Knowing your SAC rate lets you predict how long a given tank will last at any depth. It is one of the most important numbers in dive planning, especially for technical diving, overhead environments, and any situation where running out of gas is not an option.

Your SAC rate varies with fitness, stress, water temperature, current, workload, and experience. Track it over many dives to find your personal baseline. A typical recreational diver falls between 0.5 and 0.8 cu ft/min.

Closed-circuit rebreather (CCR) divers must carry enough open-circuit bailout gas to reach the surface if their unit fails at the worst possible moment. The amount of bailout gas required depends directly on your SAC rate, the depth of failure, and the ascent profile including any decompression stops.

A common approach is to assume a stressed SAC rate (typically 2x your resting SAC) and calculate the gas consumed during an ascent at 1 minute per 10 feet, plus time at decompression stops. For example, with a resting SAC of 0.5 cu ft/min and a stressed factor of 2x, your planning SAC becomes 1.0 cu ft/min. From 100 feet, a 10-minute ascent at an average depth of 50 feet (2.5 ATA) requires 1.0 * 10 * 2.5 = 25 cu ft of gas, not counting deco stops.

The bailout calculator above gives a simplified estimate for a direct ascent. Real-world bailout planning must also account for decompression obligations, gas switches, and a stress multiplier. Always consult with your instructor and use dedicated deco planning software.

Partial pressure blending is the most common method for making nitrox and trimix in dive shops. The idea is straightforward: you add each pure gas component (helium first, then oxygen) to the tank individually, then top off with air. Each gas is added to a specific pressure, and the resulting mix is determined by the ratio of each gas's partial pressure to the total final pressure.

The fill order matters. Helium goes in first because it is the lightest gas and mixes most easily. Oxygen is added second. Air is added last as a top-off. When starting with a partially filled tank, the existing contents must be accounted for in the calculations.

Important safety notes: any mix containing more than 40% oxygen requires O2-clean equipment and special handling procedures. Hypoxic mixes (less than 18% O2) are not safe to breathe at the surface and require careful labeling and handling. Always analyze your final mix with a calibrated analyzer before diving.