Sac Rate Calculator App

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Scuba SAC Rate Calculator

Calculate your Surface Air Consumption rate to plan safer dives.

Imperial (Feet/PSI) Metric (Meters/Bar)
Leave blank for just PSI/Bar per min

Your Results:

Your SAC Rate:

Your RMV (Respiratory Minute Volume):

What is SAC Rate in Scuba Diving?

SAC stands for Surface Air Consumption. It is a measurement used by scuba divers to determine how much air they would consume per minute if they were at the surface (1 atmosphere of pressure). Understanding your SAC rate is one of the most critical skills for gas management and dive planning.

Why Calculate Your SAC Rate?

  • Gas Planning: Know exactly how long your tank will last at a specific depth.
  • Safety: Predict when you will reach your "turn pressure" or "reserve."
  • Improvement: Track your SAC rate over time to see how your buoyancy and relaxation improve your air consumption.
  • Different Tanks: Calculate how much air you would use if you switched from an Aluminum 80 to a Steel 100 tank.

The Math Behind the Calculation

The SAC rate formula accounts for the fact that air is compressed as you go deeper. To find your surface rate, we must "normalize" your consumption based on the Ambient Pressure (ATA) at the depth you were diving.

Step 1: Calculate Pressure at Depth (ATA)
Imperial: ATA = (Depth / 33) + 1
Metric: ATA = (Depth / 10) + 1

Step 2: Calculate Air Consumed
Pressure Used = Starting Pressure - Ending Pressure

Step 3: Calculate SAC
SAC = (Pressure Used / Time) / ATA

SAC vs. RMV: What's the Difference?

While often used interchangeably, there is a technical difference:

Term Measurement Description
SAC Rate PSI/min or Bar/min The drop in tank pressure per minute at the surface. This is specific to a particular tank size.
RMV Cu Ft/min or Liters/min The actual volume of gas you breathe per minute. This remains constant regardless of the tank you use.

Practical Example

Imagine you dive to 66 feet for 30 minutes. You start with 3000 PSI and finish with 1000 PSI.

  1. ATA: (66 / 33) + 1 = 3 ATA.
  2. Pressure Used: 3000 – 1000 = 2000 PSI.
  3. Consumption per min: 2000 / 30 = 66.6 PSI/min.
  4. SAC Rate: 66.6 / 3 = 22.2 PSI/min.

Tips for Reducing Your SAC Rate

If you find your SAC rate is higher than your buddy's, try the following:

  • Master Buoyancy: Constant adjustments and "finning" to stay level waste energy and oxygen.
  • Slow Down: Move slowly. Water is 800 times denser than air; any fast movement increases resistance and effort.
  • Streamline: Ensure your gauges, octo, and hoses are tucked in to reduce drag.
  • Breathe Deeply: Slow, deep, rhythmic breaths are more efficient for gas exchange than short, shallow breaths.
function toggleUnits() { var units = document.getElementById('units').value; var depthUnit = document.getElementById('depthUnit'); var pressUnit1 = document.getElementById('pressUnit1'); var pressUnit2 = document.getElementById('pressUnit2'); var tankLabel = document.getElementById('tankLabel'); if (units === 'imperial') { depthUnit.innerText = 'ft'; pressUnit1.innerText = 'PSI'; pressUnit2.innerText = 'PSI'; tankLabel.innerText = 'e.g. 80 cu ft'; } else { depthUnit.innerText = 'm'; pressUnit1.innerText = 'Bar'; pressUnit2.innerText = 'Bar'; tankLabel.innerText = 'e.g. 12 L'; } } function calculateSAC() { var units = document.getElementById('units').value; var depth = parseFloat(document.getElementById('avgDepth').value); var startP = parseFloat(document.getElementById('startPressure').value); var endP = parseFloat(document.getElementById('endPressure').value); var time = parseFloat(document.getElementById('diveTime').value); var tankSize = parseFloat(document.getElementById('tankSize').value); if (isNaN(depth) || isNaN(startP) || isNaN(endP) || isNaN(time) || time = startP) { alert("Ending pressure must be lower than starting pressure."); return; } // Calculate ATA var ata = (units === 'imperial') ? (depth / 33) + 1 : (depth / 10) + 1; // Calculate Pressure Used var pressureUsed = startP – endP; // Calculate SAC (Pressure based) var sacRate = (pressureUsed / time) / ata; var sacUnit = (units === 'imperial') ? 'PSI/min' : 'Bar/min'; document.getElementById('sacRateValue').innerText = sacRate.toFixed(2); document.getElementById('sacRateUnit').innerText = sacUnit; // Calculate RMV if tank size is provided var rmvContainer = document.getElementById('rmvContainer'); if (!isNaN(tankSize) && tankSize > 0) { var rmv = 0; var rmvUnit = ""; if (units === 'imperial') { // Formula for Imperial RMV: (SAC * Tank Volume) / Working Pressure // Assuming standard 3000 PSI working pressure for common AL80 // More accurate RMV: SAC / (Working Pressure / Tank Capacity) // We'll assume the input 'tankSize' for Imperial is the total Cu Ft at a standard 3000 PSI rmv = (sacRate * tankSize) / 3000; rmvUnit = "Cu Ft/min"; } else { // Formula for Metric RMV: SAC * Tank Volume (Liters) rmv = sacRate * tankSize; rmvUnit = "Liters/min"; } document.getElementById('rmvValue').innerText = rmv.toFixed(2); document.getElementById('rmvUnit').innerText = rmvUnit; rmvContainer.style.display = 'block'; } else { rmvContainer.style.display = 'none'; } // Interpretation var interpretation = ""; if (units === 'imperial') { if (sacRate < 15) interpretation = "Excellent air consumption! You are very relaxed."; else if (sacRate < 25) interpretation = "Good average air consumption for a recreational diver."; else interpretation = "Your consumption is a bit high. Focus on relaxation and buoyancy."; } else { if (sacRate < 1.0) interpretation = "Excellent air consumption! You are very relaxed."; else if (sacRate < 1.7) interpretation = "Good average air consumption for a recreational diver."; else interpretation = "Your consumption is a bit high. Focus on relaxation and buoyancy."; } document.getElementById('interpretation').innerText = interpretation; document.getElementById('sacResult').style.display = 'block'; }

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