Plan a simple material order
This roofing calculator is for a rectangular roof footprint covered by a flat plane, one continuous slope, or two equal slopes meeting at a centered ridge. It turns horizontal measurements into the actual sloping surface area, then applies your chosen allowance and the usable coverage of one pack. Use it to compare quantities before discussing an order with a supplier. It does not identify your roof from an address or estimate local prices.
Measure and calculate
Choose the roof shape and the dimension unit first. Measure L along the eave or ridge and W across the entire roof footprint. Both are horizontal distances from outer roof edge to outer roof edge, including overhangs. For a gable, W covers both sides together; do not enter half the width and do not double the result. Measurements taken along the sloping roof surface do not belong in these fields.
Enter the slope as the rise for every 12 horizontal units, or switch to degrees. A 6:12 slope rises 6 units over a horizontal run of 12; it is about 26.57°, not 6° or 50°. Flat mode ignores the slope field. Read the material label and enter its usable coverage per pack in m² or ft² as shown. The 10% allowance is an editable example, not a recommended allowance for every job. Press Calculate, then Copy result to keep the dimensions and estimate together.
How the estimate works
With r = rise/run, surface area A = L × W × √(1 + r²). With an angle θ, the same area is L × W ÷ cos(θ). Flat mode uses r = 0. The symmetric gable already includes both slopes because their two half-widths sum to W. A single slope with the same footprint and angle has the same total area, although its ridge height and construction differ.
Area with allowance = A × (1 + waste/100). Packs = ceiling(area with allowance ÷ usable coverage per pack). Whole packs are rounded up only at the last step, using unrounded area. A roofing square is 100 ft² of surface area; the squares result here excludes the allowance. Coverage, pack counts and roofing squares are different quantities, so never assume a universal number of bundles per square.
Two worked examples
Example 1: a symmetric gable measuring 12 m × 8 m at 6:12 has a 96 m² footprint and about 107.33 m² of roof surface. With 10% allowance the ordering area is 118.06 m². At 3 m² usable coverage per pack, order 40 whole packs. The displayed rounded areas are not used to calculate the pack count.
Example 2: a flat 30 ft × 20 ft rectangle has 600 ft² of surface, or 6 roofing squares. With 5% allowance and 100 ft² per pack, its ordering area is 630 ft² and the estimate is 7 packs. Changing the dimension unit converts the existing lengths and pack coverage together; it does not reinterpret 30 ft as 30 m.
Limits and practical use
Only a rectangular footprint with one uniform slope magnitude is supported. Unequal gables, hips, valleys, dormers, curved roofs, openings and intersecting sections need a separate measured takeoff. Do not try to represent their geometry by selecting the nearest-looking shape. This tool does not size framing, check structural loads, certify waterproofing or determine material suitability.
Use positive lengths no greater than 10,000 m, slope up to 75°, positive pack coverage and an allowance from 0 to 100%. Decimal points or decimal commas are accepted, without thousands separators. Check units and manufacturer coverage if the estimate seems excessive. Numbers normally show up to two decimals; very small nonzero results use scientific notation. Editing an input clears the confirmed result until you calculate again. The diagram is conceptual, with dimension labels rather than a construction drawing.
Frequently asked questions
Should I enter the house floor area?
No. Use the roof’s horizontal outline including its overhangs. Floor area may include multiple stories and cannot describe this footprint. Use plans or measurements you can obtain safely; this calculator does not require climbing onto the roof.
Can I use any roofing product?
Enter the chosen product’s effective coverage, accounting for its specified overlaps. A calculated pack count does not approve that product for the roof slope. Ridge caps, flashing, fasteners, underlayment and other separate items are not automatically included.
Does waste include every extra material?
No. It is simply the percentage you enter applied to the surface area. Cuts, layout, breakage and spare material can change the real order. Ask the supplier or estimator for an allowance appropriate to the actual job.
Why did switching shape leave the area unchanged?
A single slope and an equal gable with the same full horizontal footprint and slope have equal surface area. Their cross sections differ, as shown in the diagram. A flat roof removes the slope multiplier.
What does Copy result include?
It copies the selected shape, dimensions, slope, allowance, pack coverage and calculated values as plain text. Nothing is saved to an account. Clear removes entered numbers and the confirmed result while keeping unit and shape choices. Pale examples return in empty fields after leaving them; they are not your previous measurements.