Home Heat Loss & Heating Load Calculator is a free, in-browser tool that estimates how fast a room or home loses heat and how much that costs to replace over a season. It uses the real U · A · ΔT engineering model rather than a rule-of-thumb, and every figure is computed locally as you type.
Conductive loss through a surface is Q = U · area · ΔT, where U is the assembly U-value (or 1 / R if you enter an R-value), area is the surface size and ΔT is the indoor-minus-outdoor design temperature difference. Add optional air-infiltration loss from air changes per hour and room volume (0.018 · ACH · volume · ΔT in imperial, 0.33 · ACH · volume · ΔT in metric) to get the total design load in BTU/hr and kW. Both imperial and metric inputs are supported.
Enter heating degree days (HDD), a system efficiency and an energy price and it projects the annual heating energy (kWh) and cost, converting to per-kWh, per-therm or per-cubic-metre fuel. Use it to size a furnace or heat pump, compare insulation upgrades, or estimate a winter heating bill.
Frequently asked questions
What is ΔT and what value should I use?
ΔT is the difference between your indoor design temperature and the outdoor design (coldest expected) temperature. For example a 70°F indoor and 30°F outdoor gives ΔT = 40°F. In metric use the same difference in °C. Larger ΔT means proportionally more heat loss.
How do I find my U-value or R-value?
Use the R-Value / U-Value calculator to build up an assembly, read a manufacturer's rated U-value, or use typical values — an insulated wall is roughly U-0.06 to U-0.10 (R-10 to R-17). Switch the mode to enter whichever number you have; the tool converts R to U automatically.
What are heating degree days (HDD)?
HDD sum how far and how long outdoor temperature stays below a base (typically 65°F / 18°C) over a year, so they capture how cold your climate is. Annual heat energy is UA · 24 · HDD, where UA is your total loss divided by ΔT. Look up your location's HDD from a weather service and enter it to get an annual estimate.
Does this size my furnace exactly?
It gives a solid design heat loss you can match to equipment output, but a full Manual J load also accounts for windows, orientation, internal gains and duct losses individually. Treat the total as a well-grounded estimate and add a modest safety margin, not an oversized one.