Note: This calculator uses the "Pump-Up" method. You must time how long it takes your compressor to fill the tank from a specific start pressure to an end pressure.
True Flow Rate: 0.00 CFM
Metric Flow Rate: 0.00 LPM
Calculated based on standard atmospheric pressure (14.7 PSI) and ideal gas laws.
function calculateCFM() {
// Retrieve inputs by ID
var tankGal = parseFloat(document.getElementById("tankVolume").value);
var startP = parseFloat(document.getElementById("startPsi").value);
var endP = parseFloat(document.getElementById("endPsi").value);
var timeSec = parseFloat(document.getElementById("fillTime").value);
// Validation
if (isNaN(tankGal) || isNaN(startP) || isNaN(endP) || isNaN(timeSec)) {
alert("Please enter valid numbers for all fields.");
return;
}
if (timeSec <= 0) {
alert("Time must be greater than zero.");
return;
}
if (endP <= startP) {
alert("Ending pressure must be higher than starting pressure.");
return;
}
// Calculation Logic
// Formula: CFM = (Tank_Vol_CuFt * (End_PSI – Start_PSI)) / (Atmospheric_Pressure * Fill_Time_Minutes)
// Constants
var gallonsPerCuFt = 7.48;
var atmosphericPsi = 14.7; // Standard sea level pressure
// Conversions
var tankCuFt = tankGal / gallonsPerCuFt;
var pressureDiff = endP – startP;
var timeMinutes = timeSec / 60;
// Core Calculation
var cfm = (tankCuFt * pressureDiff) / (atmosphericPsi * timeMinutes);
// Convert CFM to LPM (Liters Per Minute)
// 1 CFM = 28.3168 LPM
var lpm = cfm * 28.3168;
// Display Results
document.getElementById("cfmResult").innerHTML = cfm.toFixed(2);
document.getElementById("lpmResult").innerHTML = lpm.toFixed(0);
// Show the result box
document.getElementById("result").style.display = "block";
}
Understanding Air Compressor Flow Rate (CFM)
When selecting or troubleshooting an air compressor, the most critical metric is CFM (Cubic Feet per Minute). While manufacturers often advertise Horsepower (HP) or Tank Size (Gallons) in large letters, CFM determines whether the compressor can actually run your tools.
CFM measures the volume of air the compressor pump can deliver into the tank or to a tool at a specific pressure. If your tool consumes air faster than the compressor can deliver it (higher CFM demand than supply), the pressure will drop, and you will be forced to stop working to let the tank recharge.
Why Use the Pump-Up Test?
Compressor ratings on stickers can sometimes be misleading. "Displacement CFM" is a theoretical number calculated by cylinder bore and stroke, assuming 100% efficiency. However, real-world "Delivered CFM" is always lower due to heat, friction, and valve inefficiencies.
The Pump-Up Test used by the calculator above provides the most accurate, real-world measurement of your specific machine's performance. By timing how long it takes to fill a known volume (your tank) between two pressures, we can use physics to determine exactly how much air is being moved.
How to Perform the Test
Drain the Tank: Ensure there is no water in the tank, as water reduces the available air volume and will skew the calculation.
Check Tank Size: Find the tank capacity in gallons (usually stamped on the data plate).
Prepare the Timer: Have a stopwatch ready on your phone.
Start the Compressor:
For a 0-Full test: Open the drain valve until the gauge reads 0 PSI, close it, start the compressor, and start the timer immediately. Stop the timer when the compressor shuts off.
For a Recovery test: Run the compressor until it cuts out. Bleed air until it kicks back on (note the start PSI). Start the timer. Stop the timer when it kicks off (note the end PSI).
Input Data: Enter your numbers into the calculator above to get your true CFM.
Common Air Tool CFM Requirements
To ensure your compressor is large enough, compare your calculated CFM against the requirements of your tools. A safe rule of thumb is to have a compressor that provides 1.25 to 1.5 times the CFM of your hungriest tool.
Air Tool
Average CFM Required @ 90 PSI
Brad Nailer / Stapler
0.3 – 1.5 CFM
Impact Wrench (1/2″)
3.5 – 5.0 CFM
Air Ratchet
4.0 – 6.0 CFM
Paint Spray Gun (HVLP)
9.0 – 12.0 CFM
DA Sander
10.0 – 14.0 CFM
Die Grinder
12.0 – 16.0 CFM
Sandblaster
15.0 – 25.0+ CFM
The Formula Behind the Calculation
For those interested in the physics, the calculator uses the Ideal Gas Law derived for volumetric flow. The simplified formula for industrial estimation is:
CFM = (V × ΔP) / (14.7 × t)
Where:
V = Tank volume in Cubic Feet (Gallons / 7.48)
ΔP = The difference in pressure (End PSI – Start PSI)
14.7 = Atmospheric pressure at sea level (PSI)
t = Time in minutes
Note: This calculation provides Free Air Delivery (FAD) approximations. Altitude and extreme temperatures can affect these results slightly.