Tool 03

Roof area calculator

Convert a footprint and a pitch into an estimated roof surface. The formula, the slope factor and the conversion are printed with the result, so you can check the arithmetic yourself.

Rectangular gable approximationGround measurements onlyFree, no signup

1. Units

FeetMetres

2. Footprint length (ft)

Measure on the ground or read it from your plans.

Footprint width (ft)

3. Roof pitch (rise per 12)

Enter the rise, not the angle: a 6/12 roof is 6. Use 0 for a flat roof. Never climb onto the roof to measure — use plans, or estimate the pitch from the ground.

Pitch is expressed as rise per 12 units of horizontal run. A 0/12 roof is flat in this approximation.

4. Do your measurements include the overhang?

Yes — measured to the roof edgeThe footprint you entered is the full roof footprint.No — wall-to-wall measurementWe will also show an adjusted figure assuming 1 ft of overhang on each side.

5. Material allowance (optional)

percent

Kept separate from the geometric roof area and shown on its own line. It is not added behind the scenes — set it to 0 if you only want the geometry.

Roof surfaceGeometry only

1,341.6 sq ft

Estimated roof surface before any material allowance.

Plan (footprint) area: 1,200 sq ft
Slope factor: 1.1180
Material allowance (10%): + 134.2 sq ft
With your allowance: 1,475.8 sq ft
Roofing squares (100 sq ft): 14.8
In square feet: 1,342 sq ft
In square metres: 124.6 sq m

Approximation for a rectangular gable or shed roof. Dormers, hips, valleys, garages and multiple roof planes need measuring separately.

The formula

plan area = length × width

slope factor = √(1 + (rise ÷ 12)²)

roof surface = plan area × slope factor

squares = sq ft ÷ 100

40 × 30 = 1200.0 sq ft

√(1 + (6 ÷ 12)²) = 1.1180

1200.0 × 1.1180 = 1341.6 sq ft

Worked example

A 40 ft × 30 ft footprint is 1,200 sq ft of plan area. At 0/12 the roof surface stays 1,200 sq ft; at 6/12 it is about 1,341.6 sq ft, because the slope factor is 1.1180. That is 13.4 squares before any material allowance.

What the result includes

Plan area, slope factor and calculated roof surface.
Roofing squares, where one square is 100 square feet of roof surface.
Both metric and imperial figures, converted at 1 sq ft = 0.09290304 m².
Your material allowance as a separate line, never folded into the geometry.

What it leaves out

Hips, valleys, dormers, multiple roof planes and outbuildings are not included. The calculator assumes one rectangular gable or shed plane. It is an approximation for planning, not an aerial measurement and not a materials order.

If you are pricing a repair rather than a covering, the repair cost calculator is the right tool — it prices the amount of repair, not the whole roof.