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Roofing Calculator

Turn a roof’s footprint and pitch into the sloped area, squares, angle and rafter length.

By Nitin Raturi · Last reviewed

Your roof

How to use the Roofing Calculator

Turn a roof’s footprint and pitch into the sloped area to cover.

  1. Choose metres or feet.
  2. Enter the plan length and width, including overhangs.
  3. Enter the pitch as rise per 12 of run.
  4. Add a waste allowance, then read the area to order.

Good to know

  • A roofing square is 100 sq ft.
  • Hip roofs with one pitch on every side have the same area as a gable on the same footprint.
Unit

Unit

Whether your measurements are in metres or feet.

How to fill it in

  • Switching converts the length and width you entered.

Example

A 12 × 9 m footprint at 6 in 12 needs 132.82 m² (1,430 sq ft) with 10% waste.

Length

The roof’s plan length, measured on the ground or a drawing.

How to fill it in

  • Include the overhang at both ends.
  • Enter 0.1 to 1,000.

Width

The roof’s plan width across the slope, eave to eave.

How to fill it in

  • Include the overhang on both sides.
  • Enter 0.1 to 1,000.

Tip: For a gable roof the rafter spans half this width.

Pitch (rise per 12)

How many units the roof rises for every 12 units across.

How to fill it in

  • Enter 0 for flat, 6 for a common 6 in 12 slope, up to 24.
  • The angle in degrees is shown with the result.

Example

A 40 × 30 ft footprint is 1,200.00 sq ft flat, 1,341.64 sq ft at 6 in 12 and 1,697.06 sq ft at 12 in 12, before waste.

Keep in mind: Every roof plane is assumed to have the same pitch.

Waste allowance

Extra material for cutting, overlaps, ridges and mistakes.

How to fill it in

  • Enter 0% to 30%. Simple gables need less than roofs with hips and valleys.

Keep in mind: Follow the sheet or tile maker’s overlap guidance; it can need more.

Roof area to cover

1,475.80 sq ft

The sloped roof is 1,341.64 sq ft; with 10% waste, order for 1,475.80 sq ft, or 14.76 roofing squares.

Plan area
1,200.00 sq ft
Roof angle
26.6°
In square metres
137.11 m²
Gable rafter
16.77 ft
Ridge height
7.50 ft

Pitch factor 1.1180 (√(12² + rise²) ÷ 12) applied to the plan area. Every plane has the same pitch; rafter and ridge figures assume a simple gable across the width.

ChangelogHow we test this calculatorEmbed this calculator

How this calculator works

  1. Step 1Plan arealength × widthMeasured flat, including overhangs.
  2. Step 2Pitch factor√(12² + rise²) ÷ 12How much longer the slope is than the run.
  3. Step 3Order area+ wasteExtra for cuts and overlaps.

A sloped roof covers more area than the ground it sits over. The pitch factor is the slope length for every unit of horizontal run, from Pythagoras.

Multiply the plan area by the pitch factor to get the sloped area. This works for gable and hip roofs, as long as every plane has the same pitch.

Worked example: 40 × 30 ft at 6 in 12

Roof area for a 40 × 30 ft footprint
Factor
√(12² + 6²) ÷ 12
Area
plan × factor × (1 + waste)
  1. Plan area40 × 301,200 sq ft
  2. Pitch factor√180 ÷ 121.1180
  3. Sloped area1,200 × 1.11801,341.64 sq ft
  4. With 10% waste1,341.64 × 1.11,475.80 sq ft

That is 14.76 roofing squares of 100 sq ft. The roof angle is 26.6°, and a gable rafter from eave to ridge is 16.77 ft.

Read the result

  • Flat1,320.00 sq ft
  • 6 in 121,475.80 sq ft
  • 12 in 121,866.76 sq ft
Order area for the same 40 × 30 ft footprint, with 10% waste

Steeper roofs need more material: at 12 in 12 (45°) the same house needs 1,866.76 sq ft, against 1,320.00 sq ft flat.

In metres, a 12 × 9 m footprint at 6 in 12 needs 132.82 m², or 1,430 sq ft, with 10% waste.

Assumptions and limits

  • One pitchEvery roof plane has the same slope.
  • Overhangs inMeasure the plan including eaves and verges.
  • Area onlySheet counts depend on the product’s cover width and laps.
  • No openingsSkylights and chimneys are not subtracted.

Roof pitch is a slope. Angles, triangles and areas all feed the estimate.

For the angle of any rise over run, use the slope calculator. The triangle calculator solves gable shapes, and the area calculator covers flat sections.

Common mistakes

  • Mistake: Using the floor area as the roof area.

    Instead: Multiply by the pitch factor.

  • Mistake: Leaving out the overhang.

    Instead: Measure from the outer edge of the eaves.

  • Mistake: Ordering the exact area.

    Instead: Add a waste allowance; hips and valleys need more.

  • Mistake: Mixing degrees with rise per 12.

    Instead: 6 in 12 is about 26.6°, not 6°.

Checking the method

A separate script re-works every golden value with trigonometry, dividing each plan area by the cosine of the roof angle.

Read how we test our calculators, or browse more everyday tools.

Frequently asked questions

How much roof does a 40 × 30 ft house at 6 in 12 have?

1,341.64 sq ft of sloped roof, or 1,475.80 sq ft with 10% waste (14.76 squares).

What is a pitch factor?

The sloped length per unit of run: √(12² + rise²) ÷ 12. At 6 in 12 it is 1.1180.

How much does pitch change the area?

For a 40 × 30 ft footprint with 10% waste: 1,320.00 sq ft flat, 1,475.80 sq ft at 6 in 12 and 1,866.76 sq ft at 12 in 12.

Does it work in metres?

Yes. A 12 × 9 m footprint at 6 in 12 needs 132.82 m² with 10% waste, or 1,430 sq ft.

Does a hip roof need more material than a gable?

Not in area, if every side has the same pitch; both are plan area × pitch factor. Hips do need more cutting waste.

Sources and methodology

The formulas are standard time-value-of-money results. We check every figure against golden values worked out independently, by closed form and by month-by-month simulation; see the methodology.

Written and maintained by Nitin Raturi, a software engineer (about the author). Last reviewed 4 Nov 2026; next review due by . Found an error? Tell us.

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