Roofing Calculator

How This Calculator Works

A sloped roof has more surface area than the flat footprint it covers, and how much more depends on how steep it is. Enter your roof’s footprint length and width plus its pitch, and this calculator returns the actual surface area to be shingled, how many roofing “squares” that is (the standard unit roofing material is sold in — 1 square = 100 square feet), and how many bundles of shingles to order.

Roof pitch is expressed as “rise per 12” — how many inches a roof rises for every 12 inches of horizontal run. A 6-in-12 pitch is a moderate, common residential slope; 0 is completely flat; 12-in-12 is a steep 45-degree slope.

The Formula

$$\vC{\text{Pitch Multiplier}} = \frac{\sqrt{\vA{\text{Rise}}^2 + 12^2}}{12}$$ $$\vD{\text{Roof Surface Area}} = \vB{\text{Footprint Area}} \times \vC{\text{Pitch Multiplier}}$$ $$\text{Squares Needed} = \frac{\vD{\text{Roof Surface Area (sq ft)}}}{100}$$

Rounded up.

A flat roof (0-in-12) has a multiplier of exactly 1 — no extra area. A 12-in-12 (45°) pitch has a multiplier of exactly √2 ≈ 1.414 — both easy to sanity-check against the underlying geometry.

Worked Example

A 12m x 10m footprint at a 6-in-12 pitch:

1. Pitch multiplier: $\dfrac{\sqrt{\vA{6}^2 + 12^2}}{12} = \dfrac{\sqrt{180}}{12} \approx \vC{1.118}$.
2. Footprint area: $12 \times 10 = \vB{120} \text{ sq m}$.
3. Roof surface area: $\vB{120} \times \vC{1.118} \approx \vD{134.16} \text{ sq m} \approx \vD{1{,}444} \text{ sq ft}$.
4. Squares needed: $\vD{1{,}444} \div 100 \approx 14.44$ → 15 squares (rounded up).
5. Recommended order with a waste margin (10% by default, adjustable in the calculator above): 16.5 squares, about 50 bundles of shingles.