The total surface area of the fuel currently involved in the fire.
Typical values range from 0.01 for wood to 0.05 for plastics.
Energy released per unit mass burned (e.g., Wood ≈ 17,000 kJ/kg, Gasoline ≈ 44,000 kJ/kg).
Fraction of fuel completely oxidized (usually 0.6 to 0.9 for most fires).
Calculated HRR:
0 kW
0 MW
Understanding Heat Release Rate (HRR)
In fire protection engineering and fire dynamics, the Heat Release Rate (HRR) is considered the single most important variable in defining the fire hazard. It represents the rate at which heat energy is generated by the combustion process. Unlike temperature, which measures the "intensity" of heat, HRR measures the "power" of the fire.
The HRR Calculation Formula
The calculator uses the standard mass loss rate method to determine the fire's power:
Q̇ = A × ṁ" × ΔHc,eff × χ
Q̇: Total Heat Release Rate (kW)
A: Surface area of the fuel (m²)
ṁ": Mass loss rate per unit area (kg/m²·s)
ΔHc,eff: Effective heat of combustion (kJ/kg)
χ: Combustion efficiency (dimensionless fraction)
Why Does HRR Matter?
HRR determines the severity of the fire environment. Higher HRR values lead to:
Faster smoke production and reduced visibility.
Quicker transition to Flashover (where all combustible items in a room ignite).
Increased structural damage to the building.
Activation of fire suppression systems (sprinklers) and detection systems.
Practical Examples
Fuel Source
Typical Peak HRR
Small Wastebasket
50 – 150 kW
Upholstered Easy Chair
200 – 1,000 kW
Christmas Tree (Dry)
3,000 – 5,000 kW
Pool Fire (1m diameter Gasoline)
1,500 – 2,500 kW
How to Interpret Results
A fire of 1,000 kW (1 MW) is often considered the threshold for flashover in a typical domestic living room. For context, a standard 1,500W space heater produces only 1.5 kW of heat. A fire reaching 10 MW is extremely large, typically involving multiple pieces of furniture or large industrial fuel loads, and generally exceeds the control capabilities of standard residential sprinklers.
function calculateHRR() {
// Get input values
var area = parseFloat(document.getElementById('burningArea').value);
var massLoss = parseFloat(document.getElementById('massLossRate').value);
var heatComb = parseFloat(document.getElementById('heatCombustion').value);
var efficiency = parseFloat(document.getElementById('efficiency').value);
// Validation
if (isNaN(area) || isNaN(massLoss) || isNaN(heatComb) || isNaN(efficiency)) {
alert("Please enter valid numerical values for all fields.");
return;
}
if (area <= 0 || massLoss < 0 || heatComb < 0 || efficiency < 0) {
alert("Values must be positive numbers.");
return;
}
// Logic: HRR (kW) = Area * MassLossRate * HeatOfCombustion * Efficiency
// Units: m^2 * (kg/m^2*s) * (kJ/kg) * unitless = kJ/s = kW
var totalHRR_kW = area * massLoss * heatComb * efficiency;
var totalHRR_MW = totalHRR_kW / 1000;
// Display results
document.getElementById('hrrKW').innerText = totalHRR_kW.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('hrrMW').innerText = totalHRR_MW.toLocaleString(undefined, {minimumFractionDigits: 3, maximumFractionDigits: 3});
// Contextual Description
var desc = "";
if (totalHRR_kW < 100) {
desc = "Fire Class: Small (e.g., waste paper basket fire). Low risk of immediate spread.";
} else if (totalHRR_kW < 1000) {
desc = "Fire Class: Moderate (e.g., small furniture item). Potential for room-scale involvement.";
} else if (totalHRR_kW < 5000) {
desc = "Fire Class: Severe (e.g., large sofa or bed). High probability of flashover in standard rooms.";
} else {
desc = "Fire Class: Extreme (e.g., industrial fuel load or multiple large items). Immediate structural threat.";
}
document.getElementById('fireIntensityDesc').innerText = desc;
// Show container
document.getElementById('hrrResultContainer').style.display = 'block';
}