Calculate how long your oxygen supply will last based on flow rate and cylinder size.
D Cylinder (Factor 0.16)
E Cylinder (Factor 0.28)
M Cylinder (Factor 1.56)
G Cylinder (Factor 2.41)
H/K Cylinder (Factor 3.14)
Standard safety buffer is 200 psi
Estimated Time Remaining:
How to Calculate Oxygen Flow Rate and Duration
Understanding how to calculate oxygen flow rate duration is critical for healthcare providers, respiratory therapists, and patients relying on supplemental oxygen. The calculation ensures that a patient has an adequate supply of oxygen for transport or daily activities without running out unexpectedly.
While the "flow rate" is typically a setting chosen on the regulator (measured in Liters per Minute, or L/min), the most common calculation performed involves determining the duration of flow available in a specific cylinder given that rate.
The Golden Formula:
Duration (minutes) = [(Gauge Pressure – Safe Residual Pressure) × Cylinder Factor] / Flow Rate
Step-by-Step Calculation Guide
Determine the Cylinder Factor: Different tank sizes hold different volumes of compressed oxygen. Each size has a specific constant (conversion factor).
Check the Pressure Gauge: Read the current pressure in pounds per square inch (psi).
Subtract Residual Pressure: It is unsafe to drain a tank completely. A "safe residual pressure" of 200 psi is standard practice to prevent contaminants from entering the tank.
Divide by Flow Rate: Divide the result by the prescribed flow rate (L/min).
Common Oxygen Cylinder Factors
To use the formula manually, you need to know the factor for your specific tank size:
Cylinder Type
Factor (L/psi)
Typical Use
D Cylinder
0.16
Portable / Backpack
E Cylinder
0.28
Portable with Cart
M Cylinder
1.56
Large Home Medical
G Cylinder
2.41
Institutional
H / K Cylinder
3.14
Large Institutional Backup
Example Calculation
Let's calculate the duration for a patient using an E Cylinder at a flow rate of 2 L/min.
This converts to approximately 4 hours and 12 minutes of remaining oxygen supply.
Estimating FiO2 from Flow Rate
Sometimes "calculating flow rate" refers to estimating the fraction of inspired oxygen (FiO2) a patient receives via a standard nasal cannula. A general rule of thumb (Rule of 4) is:
FiO2 ≈ 20% + (4 × Flow Rate in L/min)
1 L/min ≈ 24% FiO2
2 L/min ≈ 28% FiO2
3 L/min ≈ 32% FiO2
4 L/min ≈ 36% FiO2
Note: This estimation becomes less accurate at rates above 6 L/min or if the patient is breathing through their mouth.
function calculateOxygenDuration() {
// Get Input Values
var cylinderFactor = parseFloat(document.getElementById("cylinderType").value);
var flowRate = parseFloat(document.getElementById("flowRate").value);
var currentPressure = parseFloat(document.getElementById("currentPressure").value);
var residualPressure = parseFloat(document.getElementById("residualPressure").value);
// UI Elements
var resultBox = document.getElementById("resultBox");
var errorDisplay = document.getElementById("errorDisplay");
var resultMinutes = document.getElementById("resultMinutes");
var resultHours = document.getElementById("resultHours");
// Reset Display
resultBox.style.display = "none";
errorDisplay.style.display = "none";
// Validation
if (isNaN(flowRate) || flowRate <= 0) {
errorDisplay.innerText = "Please enter a valid Flow Rate greater than 0.";
errorDisplay.style.display = "block";
return;
}
if (isNaN(currentPressure) || currentPressure < 0) {
errorDisplay.innerText = "Please enter a valid Current Pressure.";
errorDisplay.style.display = "block";
return;
}
if (isNaN(residualPressure) || residualPressure < 0) {
errorDisplay.innerText = "Please enter a valid Safe Residual Pressure.";
errorDisplay.style.display = "block";
return;
}
// Logic Check: Available Pressure
var availablePressure = currentPressure – residualPressure;
if (availablePressure 0) {
timeString += hours + " Hour" + (hours !== 1 ? "s" : "");
}
if (mins > 0) {
if (hours > 0) timeString += " and ";
timeString += mins + " Minute" + (mins !== 1 ? "s" : "");
}
if (timeString === "") timeString = "Less than 1 minute";
resultHours.innerText = "(" + timeString + ")";
resultBox.style.display = "block";
}