# Roofing Calculator

> Add each part of the roof as a footprint with its pitch. Pick a waste percent for the roof type. You get squares, shingle bundles, underlayment rolls, drip edge and cost. A 40 × 25 ft house plus a porch needs 41 bundles.

Canonical: https://receiptscanner.co/tools/roofing-calculator
Updated: 2026-10-28 · Sources re-checked by: 2027-10-28

This is a free interactive calculator that runs in the browser. Nothing you type is stored or sent. Results are estimates.

## How it works

Area of a section = footprint length × width × slope factor, where slope factor = √(1 + (rise ÷ 12)²), times the quantity. Squares = total area ÷ 100. Bundles = squares × (1 + waste) × 3, rounded up, because asphalt shingles are commonly packed 3 bundles to a square. Underlayment rolls = area ÷ the coverage printed on the roll, rounded up. Drip edge pieces = (eave length + rake length) ÷ 10 ft, rounded up. Shingle cost = bundles × the price per bundle.

**Shingle bundles for different roof areas (10% waste)**

| Roof area | Squares | Bundles |
| --- | --- | --- |
| 1,000 sq ft | 10 | 33 |
| 1,500 sq ft | 15 | 50 |
| 2,000 sq ft | 20 | 66 |
| 2,500 sq ft | 25 | 83 |
| 3,000 sq ft | 30 | 99 |

**How much waste to allow by roof type**

| Roof | Waste | Why |
| --- | --- | --- |
| Simple gable | About 10% | Few cuts, long straight runs |
| Hip roof | About 15% | Every hip and ridge needs angled cuts |
| Complex roof with valleys and dormers | About 20% | Many short pieces and offcuts |

**Slope factor for common pitches**

| Pitch | Slope factor | Roof area for a 1,000 sq ft footprint |
| --- | --- | --- |
| 3/12 | 1.031 | 1,031 sq ft |
| 6/12 | 1.118 | 1,118 sq ft |
| 9/12 | 1.25 | 1,250 sq ft |
| 12/12 | 1.414 | 1,414 sq ft |

## Worked example: a 40 × 25 ft house at 6/12 and a 12 × 8 ft porch at 3/12

1. House: 40 × 25 = 1,000 sq ft × 1.118 = 1,118 sq ft. Porch: 12 × 8 = 96 × 1.031 = 99 sq ft.
2. Total roof: 1,217 sq ft = 12.17 squares.
3. Bundles with 10% waste: 12.17 × 1.10 × 3 = 40.2, so 41 bundles, which is 13.67 squares.
4. Underlayment at 400 sq ft a roll: 3.04, so 4 rolls. Drip edge: (80 + 52) ÷ 10 = 13.2, so 14 pieces.
5. Shingles at $35 a bundle: 41 × 35 = $1,435.00.

## FAQ

### How do I calculate how many shingles I need?
Work out the roof area from the footprint and pitch, divide by 100 for squares, add waste, and multiply by three bundles to a square. Round up to whole bundles.

### How many bundles of shingles are in a square?
Three for most asphalt shingles, but some products use four or more per square. Check the label of the shingle you buy and change the pack size when you compare quotes.

### How much waste should I add?
About 10% for a simple gable, 15% for a hip roof and 20% for a complex roof. The chips on the page set those values, and you can type your own.

### How do I handle a roof with several sections?
Add a row for each: the main house, a garage, a porch. Each has its own footprint and pitch, so a low porch roof is not counted at the main roof's slope.

### How many rolls of underlayment do I need?
Divide the roof area by the coverage printed on the roll, which differs between felt and synthetic products. Round up. Valleys and eaves may need an extra layer, so ask your supplier.

### What is drip edge and how much do I need?
A metal strip along the eaves and rakes that guides water off the roof. Pieces are commonly 10 ft long, so add the eave and rake lengths and divide by ten.

## Sources

- [Roof pitch: rise over run, slope factor and the usual names for low, conventional and steep roofs](https://en.wikipedia.org/wiki/Roof_pitch) (checked 2026-10-28)
- [Roof shingle: how asphalt shingles are sold and laid (bundles, squares)](https://en.wikipedia.org/wiki/Roof_shingle) (checked 2026-10-28)
- [Underlayment: the layer under the roof covering, and what it does](https://en.wikipedia.org/wiki/Underlayment) (checked 2026-10-28)

This calculator is an estimate for planning. Real roofs have hips, valleys and penetrations that change the count, so measure carefully and confirm quantities with your supplier or roofer before you order.