Insulation Calculator

How to Calculate Insulation Needs

This calculator totals the square footage of insulation needed across walls, ceiling, and floor, with a waste factor for cuts and compression.

The formula

Total = (Wall Area + Ceiling Area + Floor Area) × (1 + Waste% ÷ 100)

Step-by-step

1. Measure wall surface area (exterior wall length × height, minus doors/windows for precision).
2. Measure ceiling area (floor footprint for flat ceilings, plus 10% for vaulted).
3. Measure floor area if insulating under the floor (e.g., over a crawlspace).
4. Enter a waste percentage (10% is standard — insulation compresses and needs cutting around outlets and joists).

Worked example

400 sq ft walls + 200 sq ft ceiling + 200 sq ft floor = 800 sq ft. With 10% waste: 800 × 1.10 = 880 sq ft of insulation.

R-value matters

Insulation is sold by R-value (thermal resistance), not just area. Check your local building code for required R-values: typically R-13 for walls, R-30 to R-49 for ceilings, R-19 for floors in most US climate zones.