🌡️ Heat Loss Calculator
Enter a surface's area, its U-value, and the inside-to-outside temperature difference to see how much heat it loses in BTU/hr — the first step in spotting where your home leaks warmth.
🧮 Calculate Fabric Heat Loss
What is a Heat Loss Calculator?
It quantifies the heat escaping through the fabric of your home. Give it the area of a wall, roof, floor, or window, its U-value, and the temperature gap you want to maintain, and it returns the steady heat flow in BTU/hr — the warmth your heating has to keep replacing.
Use it to compare insulation options, prioritise the upgrades that save the most, and build up a whole-room or whole-house heating load one element at a time. Lower the U-value or the temperature gap and watch the loss fall.
❓ Frequently Asked Questions
How does the heat loss calculator work?
It multiplies the surface area of a building element by its U-value (how readily heat passes through it) and by the temperature difference between inside and outside. The result is the steady-state fabric heat loss through that element in BTU per hour.
What is a U-value?
A U-value measures thermal transmittance — how much heat flows through a square foot of a material for each degree of temperature difference. Lower is better: a modern insulated wall might be around 0.05, single glazing closer to 1.0. Adding insulation lowers the U-value and cuts heat loss proportionally.
How do I find my whole-room heat loss?
Calculate the loss for each element — each wall, the roof or ceiling, the floor, and every window and door — then add them together, plus an allowance for air changes. The total is the heating load that element-by-element fabric loss puts on your system on a cold day.
Where does insulation make the biggest difference?
Wherever the area is large and the U-value is high. Uninsulated roofs and single-glazed windows usually lose the most per square foot, so improving those first typically gives the fastest payback. Run the numbers before and after to compare.