Updated · Built and checked by the PlinzoTools team, formulas tested against published sources
Estimate heating load in BTU/hr from area, design temperature difference, and an overall envelope U-factor.
30000 BTU/hr
Unlike the BTU Calculator's flat per-square-foot rate, this tool estimates heating load using the actual physics driver behind heat loss: the temperature difference between inside and outside, scaled by an overall envelope U-factor representing how well-insulated the building's walls, windows, and roof are as a whole.
Enter your home's area, the design temperature delta (your desired indoor temperature minus your region's outdoor winter design temperature, which climate data or a local HVAC contractor can provide), and an overall envelope U-factor (0.3 is a reasonable starting point for an averagely-insulated home; a lower number means better insulation).
Heating Load (BTU/hr) = Area × Temperature Delta × U-Factor
This is a simplified, single-factor estimate for educational and rough-planning purposes -- a full Manual J heating load calculation breaks the envelope into individual components (walls, windows, roof, floor, infiltration) each with their own specific U-factor and area, which a real HVAC sizing decision should be based on.
The delta is the gap between the indoor temperature you want to hold and the outdoor temperature the system must handle. It is not the average winter temperature. Designers use a low temperature that is exceeded only a small share of the time, so the equipment can cope with cold snaps. If you enter a milder outdoor value, the result will be lower and the equipment may struggle on the coldest nights. Ask a local HVAC contractor or look up regional design data for your area rather than guessing.
Hypothetical example: you want 70 F inside and the local design low is 15 F. The delta is 55 F. For a 1,800 sq ft home with a U-factor of 0.25, the load is 1,800 x 55 x 0.25 = 24,750 BTU/hr. If you lower the thermostat target to 65 F, the delta becomes 50 F and the load drops to 22,500 BTU/hr, about 9 percent less.
A U-factor measures how readily heat passes through a building surface. Lower means better insulated. In this tool the value acts as one blended number applied to floor area, so it is a simplification: a real envelope has walls, windows, roof, and floor with different values and different areas. If your home has very large windows or a big vaulted ceiling, a single blended number may understate the load. Treat the U-factor as a way to show how insulation upgrades reduce heating need, rather than as a precise measurement.
Because the formula scales directly with the U-factor, you can test what better insulation might do. Run the tool with your current estimate and then with a lower value that represents added attic insulation or window replacement. The difference in BTU/hr shows the approximate reduction in peak heating demand, which can allow a smaller system or lower operating cost. This is only directional, and an energy auditor can quantify actual savings.
Use the number to gauge whether a proposed furnace or heat pump is in a sensible range. A contractor's proposal that is far above your estimate is worth questioning, and ask whether a Manual J calculation was done. For code compliance or equipment purchase, rely on a professional load calculation and local requirements.
The difference between your desired indoor temperature and your region's outdoor winter design temperature -- a standard climate-data figure (not the coldest day ever recorded, but a statistically representative low used for sizing) available from ASHRAE climate data or a local HVAC professional.
Lower U-factors mean better insulation -- a well-insulated modern home might use 0.15-0.2, while an older, poorly insulated home might be closer to 0.4-0.5. 0.3 is a reasonable rough middle estimate without a detailed envelope audit.
A local HVAC contractor can tell you, and design data for many locations is published in ASHRAE climate tables. Do not use the record low.
Only as a rough check. Furnaces are sized from a full load calculation, and efficiency and ducts also matter.
Because the load is proportional to the temperature difference, a smaller gap between inside and outside means less heat to replace.
No. Infiltration is not a separate input, so leaky homes may need more capacity than the formula suggests.