Heat Load Calculator (U-value method)

Heat loss = U × area × ΔT for each surface, summed.

Inputs

Insulated wall: 0.06. Single-pane wall: 1.0.

Double-pane low-e: 0.30. Single pane: 1.0.

Result

Loading calculator…
—

General calculation reads

Amazon affiliate
How Not to Be Wrong
Jordan Ellenberg - math thinking
Shop on Amazon
The Joy of X
Steven Strogatz - accessible math
Shop on Amazon
Scientific calculator (Casio fx-991EX)
Shop on Amazon

As an Amazon Associate we may earn from qualifying purchases. This does not add cost for you.

How to use this calculator

  • Enter wall area and U-value (or R-value's inverse).
  • Enter window area and U-value.
  • Enter ΔT (indoor − outdoor).

About this calculator

Heat loss through walls and windows: each surface loses heat at U × area × ΔT. Lower U-value = better insulation. R-value is the inverse (R = 1/U). Modern walls: R-19 (U≈0.05). Modern double-pane low-e windows: R-3 (U≈0.30). Single-pane windows: R-1 (U≈1.0) — they leak heat ~3× faster.

Frequently asked

U-value vs R-value?+
U = heat-flow rate per ft² per °F. R = 1/U. R-30 wall has U-value ≈ 0.033.
How much loss through windows?+
Often 25-35% of total heat loss in older homes despite being only 10-15% of envelope.
Should I include ceiling and floor?+
Yes for full Manual J. This calculator simplifies to walls + windows; add ceiling/floor for completeness.
How to find U-value?+
Window manufacturer label (NFRC). Wall: from R-value of insulation + framing.
Air leakage?+
Significant in old homes. Add 30-50% to envelope-only calculation. Modern tight homes: <10%.

Related calculators

BTU for Room Heating Calculator
Heating BTU/hr = ft² × insulation factor × ΔT.
BTU for Room Cooling Calculator
BTU/hr needed = ft² × climate factor (20-30 BTU/ft² for typical US climates).
Furnace Size Calculator
Furnace BTU = ft² × climate-zone factor (30-60 BTU/ft²).
AC Tonnage Calculator (sqft OR BTU input)
1 ton = 12,000 BTU/hr cooling. Convert sqft → tons (rule-of-thumb sizing) OR BTU → tons (direct).
Ductwork CFM Calculator
CFM (cubic feet per minute) = BTU/hr × 0.07. Used for sizing ducts.
Wall Area for Paint Calculator
Perimeter × ceiling height − doors and windows = paintable area.

More tools you might like

Hand-picked tools that pair well with this one — same audience, same intent.

Wall Area for Paint Calculator
Calc

Perimeter × ceiling height − doors and windows = paintable area.

BTU for Room Cooling Calculator
Calc

BTU/hr needed = ft² × climate factor (20-30 BTU/ft² for typical US climates).

BTU for Room Heating Calculator
Calc

Heating BTU/hr = ft² × insulation factor × ΔT.

AC Tonnage Calculator (sqft OR BTU input)
Calc

1 ton = 12,000 BTU/hr cooling. Convert sqft → tons (rule-of-thumb sizing) OR BTU → tons (direct).

Furnace Size Calculator
Calc

Furnace BTU = ft² × climate-zone factor (30-60 BTU/ft²).

Body Surface Area (BSA) Calculator
Health

Estimate body surface area in m² using Mosteller and Du Bois formulas — for clinical reference, anesthesia dosing, and physiologic indexing.