Time elapsed for the volume above to pass through.
Default Natural Gas: ~1030-1040 BTU/ft³ (Imperial) or ~38-40 MJ/m³ (Metric).
Gas Flow Rate:–
Gross Heat Input (kW):–
Gross Heat Input (BTU/hr):–
How to Calculate Gas Rates
Calculating the gas rate (or "gas rating") of an appliance is a fundamental procedure for gas engineers and HVAC technicians. It ensures that a boiler, furnace, or water heater is burning the correct amount of fuel as specified by the manufacturer. An incorrect gas rate can lead to inefficiency, damage to the appliance, or dangerous situations like incomplete combustion.
Understanding the Formula
The calculation determines the energy input into the appliance based on the volume of gas consumed over a specific time. The formula varies slightly depending on whether you are using an Imperial (cubic feet) or Metric (cubic meters) gas meter.
Imperial Meters (ft³)
For meters measuring in cubic feet, the formula for heat input is:
Gas Rate (ft³/hr) = (Volume × 3600) / Time (seconds)
Heat Input (BTU/hr) = Gas Rate × Calorific Value (BTU/ft³)
Heat Input (kW) = BTU/hr / 3412.14
Metric Meters (m³)
For meters measuring in cubic meters, the calculation often uses the Calorific Value in Megajoules (MJ):
Gas Rate (m³/hr) = (Volume × 3600) / Time (seconds)
Volume Measured: This is usually determined by watching the test dial on the gas meter. On imperial meters, one revolution is often 1 ft³. On metric meters, it might be 0.01 m³ or similar.
Time Taken: The exact time in seconds it takes for the chosen volume to pass through the meter. Using a digital stopwatch is recommended for accuracy.
Calorific Value (CV): This represents the energy content of the gas. Standard Natural Gas is approximately 1030-1040 BTU/ft³ or 38-40 MJ/m³, but this varies by region and supplier. Always check your local data for precision.
Why Gas Rating Matters
After installation or service, an appliance must be "gas rated" to verify it is delivering the manufacturer's specified power output (kW). If the calculated rate is significantly lower than the badge rating, the appliance may be under-fired (poor performance). If it is higher, it is over-fired (danger of overheating and carbon monoxide production).
function updateGasPlaceholders() {
var type = document.getElementById("meterType").value;
var cvInput = document.getElementById("calorificValue");
var volInput = document.getElementById("gasVolume");
var cvHelp = document.getElementById("cvHelp");
var volHelp = document.getElementById("volumeHelp");
if (type === "imperial") {
cvInput.value = "1040";
cvInput.placeholder = "e.g., 1040";
volInput.placeholder = "e.g., 1 (1 revolution)";
cvHelp.innerHTML = "Standard Natural Gas: ~1030-1040 BTU/ft³";
volHelp.innerHTML = "Usually 1 ft³ per revolution on older meters.";
} else {
cvInput.value = "39.0";
cvInput.placeholder = "e.g., 39.0";
volInput.placeholder = "e.g., 0.01 (small dial)";
cvHelp.innerHTML = "Standard Natural Gas: ~38-40 MJ/m³";
volHelp.innerHTML = "Metric dials are often 0.01 m³ or 0.1 m³ per rev.";
}
}
// Initialize defaults on load
updateGasPlaceholders();
function calculateGasRates() {
var type = document.getElementById("meterType").value;
var volume = parseFloat(document.getElementById("gasVolume").value);
var time = parseFloat(document.getElementById("timeTaken").value);
var cv = parseFloat(document.getElementById("calorificValue").value);
var resultDiv = document.getElementById("resultSection");
var displayFlow = document.getElementById("resFlowRate");
var displayKW = document.getElementById("resKW");
var displayBTU = document.getElementById("resBTU");
if (isNaN(volume) || isNaN(time) || isNaN(cv) || time <= 0) {
alert("Please enter valid numeric values for Volume, Time, and Calorific Value.");
return;
}
var flowRatePerHour = 0;
var heatInputKW = 0;
var heatInputBTU = 0;
if (type === "imperial") {
// Imperial Calculations
// Flow in ft3/hr
flowRatePerHour = (volume * 3600) / time;
// Energy in BTU/hr
heatInputBTU = flowRatePerHour * cv;
// Energy in kW (1 kW = 3412.14 BTU/hr)
heatInputKW = heatInputBTU / 3412.14;
displayFlow.innerHTML = flowRatePerHour.toFixed(2) + " ft³/hr";
displayBTU.innerHTML = Math.round(heatInputBTU).toLocaleString() + " BTU/hr";
displayKW.innerHTML = heatInputKW.toFixed(2) + " kW";
} else {
// Metric Calculations
// Flow in m3/hr
flowRatePerHour = (volume * 3600) / time;
// Energy in kW. Formula: (m3/hr * MJ/m3) / 3.6
// Explanation: m3/hr * MJ/m3 = MJ/hr. 1 kWh = 3.6 MJ. So MJ/hr / 3.6 = kW.
heatInputKW = (flowRatePerHour * cv) / 3.6;
// Energy in BTU/hr (1 kW = 3412.14 BTU/hr)
heatInputBTU = heatInputKW * 3412.14;
displayFlow.innerHTML = flowRatePerHour.toFixed(3) + " m³/hr";
displayBTU.innerHTML = Math.round(heatInputBTU).toLocaleString() + " BTU/hr";
displayKW.innerHTML = heatInputKW.toFixed(2) + " kW";
}
resultDiv.style.display = "block";
}