Use this free roof calculator to find your roof area in seconds. Enter the building size in metres or feet, choose the roof pitch, and the roof calculator adds the slope and extra for waste instantly — no sign-up, no account, and no personal information needed. Your numbers are calculated in your browser and are not sent to us.
Contents: Formula · How to use · Examples · Tips · FAQ · Related tools
No sign-up required. This roof calculator works in your browser, so the dimensions you enter are not stored or sent to us. Roof work is dangerous: get a professional quote before you buy or install material.
Roof Calculator Formula
This roof calculator starts with the flat footprint of the building, then multiplies it by a slope factor for the roof pitch, because a sloped roof has more surface than the flat area below it. It then adds your chosen percentage for cuts and waste, and rounds up to a whole unit.
Flat area = Building length × Building width
Roof area = Flat area × Slope factor
Material needed = Round up ( Roof area × (1 + Waste ÷ 100) )
For example, a building 10 m × 8 m has a flat area of 80 square metres. At a medium 6/12 pitch, the slope factor is 1.118, so the roof area is 80 × 1.118 = 89.44 square metres. With 10% extra: 89.44 × 1.10 = 98.38, which rounds up to 99 square metres of material.
How to Use the Roof Calculator
Measure the length and width of the building footprint, then type them into the first two boxes and choose metres or feet. Choose the roof pitch from the list: the pitch is written as rise over run, so 6/12 means the roof rises 6 units for every 12 units across. Set the extra for cuts and waste (10% is a good default, 15% is safer for complex roofs), and tap Calculate Roof Area.
For a roof with several sections, work out each section separately and add the areas together. This roof calculator gives the surface area only; it does not price shingles, tiles, or labour.
Roof Calculator Examples
Example 1 — 10 m × 8 m building, 6/12 pitch, 10% waste. Flat area 80 sq m; roof area 80 × 1.118 = 89.44 sq m; material 89.44 × 1.10 = 98.38, rounded up to 99 sq m.
Example 2 — 10 m × 8 m building, flat roof, 10% waste. Flat area 80 sq m; roof area 80 × 1 = 80 sq m; material 80 × 1.10 = 88 sq m.
Example 3 — 12 ft × 10 ft building, 6/12 pitch, 10% waste. Flat area 120 sq ft; roof area 120 × 1.118 = 134.16 sq ft; material 134.16 × 1.10 = 147.58, rounded up to 148 sq ft.
These roof calculator examples show why pitch matters: a steep roof needs much more material than the flat footprint suggests.
Roof Calculator Tips
Measure from the ground or from plans where you can, and do not climb a roof to measure it. Add extra waste for valleys, hips, dormers, and complex shapes. For background on how roof pitch is defined, see Wikipedia's article on roof pitch. Always compare the roof calculator result with a local roofer's measurement before you order.
Roof Calculator FAQ
What does a roof calculator do? A roof calculator finds the surface area of a roof from the building size and the roof pitch, and adds extra for cuts and waste.
Why is the roof area larger than the building area? Because the roof is sloped. A steeper pitch has a larger slope factor, so the same building needs more roofing material.
What does 6/12 pitch mean? It means the roof rises 6 units for every 12 units it runs across. A 12/12 pitch rises the same as it runs, at 45 degrees.
Can I use feet and metres? Yes. Enter the building in feet or metres, and the roof calculator shows the result in both square metres and square feet.
Does the roof calculator price the roof? No. It gives area only. Multiply the area by your local price per square metre or square foot for a rough material cost.
How do I work out a roof with several sections? Work out each section separately and add the areas together. This roof calculator gives the surface area only; it does not price shingles, tiles, or labour.
Related Tools
Try the Tile Calculator for floors, the Wallpaper Calculator for walls, the Percentage Increase Calculator to measure a change, and the Discount Calculator to find a sale price. Return to the PDGPT homepage to browse all free calculators by category.
Sources and Method
Method: this page uses its own Roof Calculator Formula. Flat area = Building length × Building width, Roof area = Flat area × Slope factor, and Material needed = Round up ( Roof area × (1 + Waste ÷ 100) ). A sloped roof has more surface than the flat area below it, so a steeper pitch uses a larger slope factor. No external source is used for this calculation.
Last updated: October 28, 2026