Roofing Calculator
Ordering roofing material starts with one number: how many squares your roof covers. This roofing calculator turns your building footprint, eave overhang, and roof pitch into the sloped roof area, then converts it to roofing squares (100 ft² each) plus the underlayment rolls, drip edge, and ridge cap a full material takeoff needs. Pick gable or hip geometry — hip roofs get an extra waste allowance for the cutting they demand.
Longest outside wall-to-wall dimension.
Shortest outside wall-to-wall dimension.
Rise in inches per 12 inches of run — 6 means a 6/12 pitch.
Horizontal overhang past the wall, measured to the fascia.
Hip roofs need extra cutting waste around the hips.
Roofing squares
15.03sq
- Roof area
- 1,503ft²
- Underlayment rolls400 ft² per roll (rule of thumb)
- 4rolls
- Drip edge
- 148ft
- Ridge capGable ridge ≈ building length
- 40ft
How it was calculated
- Pitch multiplier = √(1 + (6/12)²) = 1.1180
- Roof footprint = (40 + 2×1) × (30 + 2×1) = 42.0 × 32.0 ft
- Roof area = 1344 × 1.1180 = 1503 ft²
- Squares = 1503 ÷ 100 = 15.03
How it works
A sloped roof is bigger than the floor plan it covers. The pitch multiplier, √(1 + (rise/12)²), converts horizontal footprint area into true slope area — a 6/12 pitch multiplies the footprint by 1.118, while a 12/12 pitch multiplies it by 1.414.
The footprint itself grows with the eave overhang: each side extends by twice the overhang, so footprint = (length + 2 × overhang) × (width + 2 × overhang). Roof area is then footprint × pitch multiplier.
Roofing material is sold by the square — one square covers 100 ft². Squares = roof area ÷ 100. Gable roofs use this directly; hip roofs add a 15% waste factor because every course must be cut around the hips.
Underlayment is estimated at 400 ft² per roll, drip edge from the footprint perimeter (eaves plus rakes), and ridge cap from the building length on a gable — or roughly length minus width on a regular hip roof, where the ridge only spans the middle section.
Worked examples
Gable roof on a 40 × 30 ft house
A 40 × 30 ft house with a 6/12 pitch and 12 in overhangs: the multiplier is √(1 + 0.25) = 1.118, and the footprint is 42 × 32 = 1,344 ft². Roof area = 1,344 × 1.118 = 1,503 ft², or 15.03 squares. You would order 4 underlayment rolls (1,503 ÷ 400, rounded up), 148 ft of drip edge, and 40 ft of ridge cap.
Hip roof on a 36 × 28 ft house
A 36 × 28 ft hip roof at 8/12 with 18 in overhangs: the multiplier is √(1 + (8/12)²) = 1.202, and the footprint is 39 × 31 = 1,209 ft². Roof area = 1,209 × 1.202 = 1,453 ft², which is 14.53 squares before waste. The 15% hip waste factor brings it to 16.71 squares, with 4 underlayment rolls, 140 ft of drip edge, and about 8 ft of ridge cap (36 − 28).
How to measure
- Measure the building footprint wall-to-wall on the outside, not the interior room sizes — the roof covers the walls too.
- Measure the overhang horizontally from the wall face to the fascia board; typical homes run 12–18 in.
- Find the pitch from inside the attic with a level and tape, or measure at the gable-end eave — you need the rise over a 12 in run.
- For L-shaped or multi-gable houses, measure each rectangular section separately and add the results.
- Measure the ridge along the peak where cap shingles will sit; on a hip roof, sketch the footprint so the length-minus-width shortcut makes sense.
- Double-check valleys, dormers, and skylights — they raise the waste percentage beyond what this calculator assumes.
Assumptions & limitations
Planning estimate
- Roof area uses gable geometry for both roof types; hip roofs get a 15% waste factor on squares as a rule of thumb, not a per-plane hip calculation.
- Hip ridge length ≈ building length − building width assumes regular hip geometry (equal pitch on all sides) — a rule-of-thumb simplification.
- Underlayment at 400 ft² per roll and the 15% hip waste factor are rules of thumb, not manufacturer specifications.
- Overhang is assumed equal on all four sides; drip edge uses the footprint perimeter as a planning figure.
- Valleys, dormers, skylights, starter strips, and flashing are not itemized — budget extra material for complex roofs.
- This is a planning estimate only — not a substitute for code compliance or professional design.
Frequently asked questions
A roofing square is 100 ft² of roof area — the standard unit shingles and roofing contractors use. A 1,500 ft² roof is 15 squares. Material is ordered in squares because bundle counts, underlayment, and labor quotes all scale from it.
Steeper roofs have more surface area over the same footprint. A 6/12 pitch adds about 12% over the flat footprint area, while a 12/12 pitch adds about 41%. The calculator applies the pitch multiplier √(1 + (rise/12)²) so you are not under-ordering on steep roofs.
Every shingle course on a hip roof must be cut at an angle where it meets the hips, and those triangular offcuts are mostly unusable. The 15% waste factor here is a rule-of-thumb allowance for that cutting — complex hip roofs with dormers may need more.
The calculator already rounds underlayment rolls up and adds hip waste, but most roofers still order 5–10% extra shingles for a simple gable and 10–15% for hips and valleys. Leftover bundles are cheap insurance against color-lot mismatches on future repairs.
No — it is a material takeoff only: squares, underlayment rolls, drip edge, and ridge cap. Labor, flashing, vents, ice-and-water shield, and tear-off disposal are separate line items in any real roofing quote.
The geometry works down to a 0/12 pitch, but low-slope roofs (under 2/12) usually need membrane or rolled roofing instead of shingles, and the underlayment and fastening rules change. Treat the numbers as a rough area guide and get a low-slope roofing quote.