Heat Loss Calculator

How the Heat Loss Calculator Works

The calculator uses the same heat-transfer relationship in either unit system: heat flow equals surface area times thermal conductance times temperature difference. If you enter R-value, the calculator converts it to U-value by taking the inverse. If you enter U-value directly, it uses that conductance as entered.

How to Use This Calculator

1. Select US/Imperial or Metric/SI units, then choose whether each surface will use R-value or U-value.
2. Enter the indoor design temperature and outdoor design temperature so the calculator can find the temperature difference.
3. Add the area and R-value or U-value for walls, windows, doors, and the roof or ceiling.
4. Click Calculate Heat Loss to see total BTU/hr, watts, per-degree load, effective envelope value, and the share from each surface.

R-Value vs U-Value

R-value measures resistance to heat flow. A higher R-value means less heat passes through the surface. U-value measures conductance. A lower U-value means less heat passes through the surface. They are inverses of one another when the same unit system is used: U = 1 ÷ R.

What the Results Mean

Total BTU/hr is commonly used for heating load and HVAC discussions in the US. Total watts is the same heat-flow rate in SI units. Per-degree load is the envelope UA value, showing how much heat flow changes for each degree of temperature difference. The surface share chart points to the part of the envelope that contributes most to the conductive load.

Limitations

This calculator is a steady-state conductive heat loss model. It does not include air infiltration, mechanical ventilation, duct losses, solar heat gain, internal heat gains, thermal mass, slab or basement ground coupling, orientation, wind exposure, moisture, or room-by-room distribution. Use it for screening and comparison, then confirm HVAC sizing with a complete load calculation when equipment decisions are involved.

Frequently Asked Questions

What formula does the heat loss calculator use?

For U-value inputs, heat loss equals area × U-value × temperature difference. For R-value inputs, U-value is 1 ÷ R-value, so heat loss equals area × temperature difference ÷ R-value.

Should I enter R-value or U-value?

Use whichever value you have. Insulation products often list R-value, while windows and doors often list U-value. R-value measures resistance to heat flow; U-value measures how easily heat passes through the assembly.

Why are windows often the largest heat loss path?

Windows usually have a much higher U-value than insulated walls or roofs. Even when window area is smaller, the lower thermal resistance can make glass and frames a major share of design heat loss.

Is air leakage included in this calculator?

No. This calculator estimates steady-state conductive heat transfer through entered surfaces. Air infiltration, ventilation, duct losses, solar gains, internal gains, thermal mass, and ground losses are separate loads.

Can this replace a Manual J or professional load calculation?

No. It is a fast planning estimate for surface heat loss. HVAC sizing should also consider local design weather, infiltration, ventilation, room-by-room loads, ducts, equipment performance, and applicable building standards.