Calculate the actual thermal efficiency of your wood stove. Compare older stoves with modern EPA-certified models to see fuel savings. Maximizing your stove's efficiency reduces firewood consumption and minimizes the environmental footprint of your home heating.
Enter the flue gas temperature measured near the stove collar, the room temperature, and the fuel you burn. The calculator applies the stack-loss method: every degree the exhaust carries above room temperature is heat that escaped up the chimney instead of warming the space. Loss is (stack − room) ÷ (theoretical flame temperature − room) × 100, and efficiency is whatever remains.
The reference temperature in the denominator depends on fuel chemistry: 1,900°C for cordwood, 1,800°C for pellets, and 2,000°C for coal. With a 250°C stack reading in a 20°C room on wood, the loss is 230 ÷ 1,880 ≈ 12.2% for a modeled efficiency of 87.8%; letting the stack climb to 400°C drops the figure to about 79.8%.
Results map into four bands: 80% and above is excellent, 65–79% good, 50–64% fair, and below 50% poor. Bear in mind this is a single-loss model — it tracks sensible heat leaving through the flue and does not subtract latent moisture losses or unburned fuel, so the number works best as a comparative indicator between burns and settings rather than a laboratory rating.
The heat in your exhaust had two possible destinations: the room or the sky. The stack-loss method treats the gap between flue temperature and room temperature as the share of combustion heat that took the chimney exit. On wood in a 20°C room, every 18.8°C of extra stack temperature costs about one percentage point of modeled efficiency, because the denominator is fixed at 1,900°C minus room temperature.
The denominator is the theoretical flame temperature of each fuel under ideal combustion: about 1,900°C for cordwood, 1,800°C for pellets — drier than cordwood but with a slightly cooler flame — and 2,000°C for coal. A hotter theoretical flame means the same stack reading represents a smaller fraction of the available heat, so identical temperatures score slightly differently across the three fuels.
Laboratory ratings come from full combustion analysis, accounting for moisture evaporation, unburned hydrocarbons, and standardized fuel loads. This calculator isolates one loss channel, sensible heat in the stack, so it generally reads higher than a certified HHV figure. Its value is in the trend: tracking how your own number moves between burns as you adjust draft, fuel dryness, and air settings.