Roofing Calculator
Calculate roof-plane area or horizontal projections with slope, roofing squares and whole bundles.
This tool runs entirely in your browser. Your data is never uploaded, never stored, and never leaves your device.
Estimate roof area from measured planes or horizontal projections with individual slopes, then calculate squares and optional bundles using actual coverage.
How to use it
- 1Choose measured roof-plane dimensions or horizontal projections before entering rectangles.
- 2Include overhangs and correct-basis exclusions; projections also need rise per 12 run.
- 3Enter your allowance and, if wanted, actual bundle coverage and price.
- 4Review surface area, roofing squares, whole bundles and the local JSON breakdown.
Example
- Input
- 600 ft² horizontal projection, 5:12 slope, 10% allowance, 32.5 ft²/bundle at USD 35.
- Output
- 650 ft² roof surface; 715 ft² required; 7.15 squares; 22 bundles; USD 770.00.
Main-product coverage only; accessories and design suitability require a separate takeoff.
What happens to your data
Roof measurements are calculated locally without uploading or saving them. The optional JSON download records your measurement basis and active product settings.
Last updated October 2026
A home's floor area is not its roof area. Overhangs, separate roof planes and slope change the surface to be covered. This roofing calculator makes the measurement basis explicit: enter lengths measured on the roof plane, or enter horizontal projected rectangles and each plane's rise per 12 horizontal run. It combines the surfaces, applies your chosen allowance and reports area and roofing squares.
If you know the actual coverage printed on a shingle bundle, it can also round to whole bundles and show material-only cost. It does not assume every product has three bundles per square, and it does not derive a complete roof system from footprint alone.
How it works
Measured-surface mode uses length × width − excluded area directly because the entered dimensions already lie on the sloped roof plane. Do not apply a second slope multiplier. Horizontal-projection mode uses that same rectangle formula for the plan projection, then multiplies it by sqrt(1 + (rise/12)²). For a 5:12 slope, the multiplier is 13/12. Rise and run use the same units; the ratio is dimensionless even in metric mode.
Enter each plane's appropriate horizontal projection, including overhangs. Exclusions in projection mode are horizontal projected exclusions too; do not subtract an actual sloped skylight area from a horizontal rectangle. In measured-surface mode, dimensions and exclusions belong to the actual plane. Switching measurement basis clears the rectangles so old values cannot silently acquire a new meaning. Irregular polygons need suitable non-overlapping rectangular subdivisions or an independent measured takeoff.
The combined roof-plane area is multiplied by 1 + allowance/100. One roofing square means 100 ft², which is approximately 9.290304 m². It is an area unit, not a bundle. Whole bundles equal the ceiling of required area divided by the actual product's bundle coverage. The tool keeps material added by your allowance separate from area added by bundle rounding.
GAF explains that roof planes, overhangs and architectural details affect roof measurements, and that bundle coverage must be checked for the actual product. A chosen allowance here is not a complete replacement for a complex-roof takeoff. Starters, hips, ridges, valleys, flashing, underlay, fixings and other accessories are excluded. The calculator does not assess minimum permitted slope, ventilation, loads or waterproofing details.
Exact decimal fractions handle surface-mode ordering. Projection-mode square roots are bounded with high-precision rational arithmetic; package counts use the conservative upper bound when a mathematical boundary is ambiguous. Displayed decimals remain estimates rather than field measurements. Use plans or a qualified measurement service instead of accessing a roof unsafely. Optional price is per whole bundle and excludes labour, tax, delivery and accessories.
Worked example: A horizontal projection with a 5:12 slope
- Input
- 30 × 20 ft horizontal projection; no exclusions; rise 5 per 12 run; 10% allowance; 32.5 ft² per bundle; USD 35/bundle.
- Result
- 650 ft² roof surface; 715 ft² with allowance; 7.15 roofing squares; 22 bundles; zero whole-bundle surplus; USD 770.00.
The example has a simple rectangular projection. It does not include accessories, design approval or a recommended waste percentage.
Common use cases
- Convert separate measured roof planes into a total roof area and roofing squares.
- Estimate simple rectangular horizontal projections with different rise-to-run slopes.
- Compare actual shingle bundle coverage without a universal three-bundles assumption.
- Download a local quantity breakdown showing the measurement basis, slope inputs and order allowance.
Frequently asked questions
How do I calculate roof area from pitch?＋
For a horizontal projected plane, multiply net projected area by sqrt(1 + (rise/12)²). A 5:12 slope has a 13/12 multiplier, so a 600 ft² projection has 650 ft² of roof surface. If dimensions were measured along the roof itself, use measured-surface mode instead.
What is a roofing square?＋
One roofing square is 100 ft² of roof area, about 9.290304 m². It does not mean one bundle, one sheet or 100 m². The output's square count includes your chosen allowance and can have a fractional part.
Are three bundles always one square?＋
No. Product packaging and coverage vary. Enter the actual stated area covered by one bundle. The calculator divides required area by that coverage, rounds to whole bundles and shows the resulting extra coverage.
Should I enter the house's floor area?＋
Not as a direct substitute for roof area. Overhangs, architectural features and separate planes matter. Use measured roof-plane dimensions or suitable horizontal projections, and account for every intended surface without overlaps.
Does this include ridge caps, starters or flashing?＋
No. Those need separate quantities based on edges, ridges, valleys and the chosen system. This output covers area and optional main-product bundles only. It is not a complete installed roof quote or suitability assessment.
Is rise:12 the same as rise divided by the whole span?＋
No. Here rise:12 means vertical rise per 12 units of horizontal run. A whole span can include more than one run, such as the two sides of a gable. Read the measurement basis carefully and use matching units for rise and run.
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