How to use the roofing calculator
You don't need to climb on the roof. Measure the outside of the house, add the overhang, and enter the pitch. For a roof with one pitch throughout, the sloped area is simply the plan area times the pitch multiplier, which is how roofers take off squares from satellite images and plans. The calculator then adds waste and turns squares into bundles, and works out the edge lengths for starter strip, ridge cap and drip edge.
Roofs with dormers, valleys, different pitches or attached porches: run each section separately and add them up.
The formulas
plan area = (length + 2 × overhang) × (width + 2 × overhang)
roof area = plan area × √(1 + (pitch ÷ 12)²)
squares = roof area ÷ 100
bundles = squares × (1 + waste) × bundles per square
Worked example: a 40 × 30 ft house, 12 in overhang, 6/12 gable
- Plan with overhang: 42 × 32 = 1,344 ft².
- Roof area: 1,344 × 1.118 = 1,503 ft², which is 15.0 squares.
- With 10% waste: 16.5 squares × 3 = 49.6, so 50 bundles.
- Edges: eaves 2 × 42 = 84 ft; rakes 4 × 16 × 1.118 = 71.6 ft; ridge 42 ft; drip edge 156 ft.
What goes on a roof
| Item | Where | Measured by |
|---|---|---|
| Underlayment | Whole roof deck | Squares |
| Ice and water shield | Eaves (cold climates), valleys, around penetrations | Lineal ft |
| Drip edge | Eaves and rakes | Lineal ft |
| Starter strip | Eaves, often rakes too | Lineal ft |
| Field shingles | Everything else | Squares / bundles |
| Ridge cap | Ridges and hips | Lineal ft |
Check the wrapper on your starter and ridge cap products for lineal feet per bundle and enter them above to get bundle counts.
Tips
- Tear-off or overlay? Most codes allow at most two layers of shingles, and many roofers always tear off. Tear-off lets you inspect and repair the deck.
- Low slopes. Between 2/12 and 4/12 the code requires double underlayment for asphalt shingles, and below 2/12 shingles aren't allowed.
- Order a bundle or two extra of the exact color lot. Matching later is hard.
- Ventilation. Balanced intake (soffit) and exhaust (ridge) ventilation protects the shingle warranty. Ask your supplier about ridge vent length.
Questions people ask
How many bundles of shingles are in a square?
Most 3-tab and architectural asphalt shingles come three bundles to a square (100 square feet). Owens Corning lists 3 bundles per square for both its Supreme 3-tab and TruDefinition Duration architectural shingles. Some heavy designer shingles take 4 or 5, so check the wrapper.
How do I calculate roofing squares?
Work out the roof's plan area (the footprint plus overhangs), multiply by the pitch multiplier, and divide by 100. A 42 × 32 ft footprint is 1,344 square feet; at 6/12 pitch (multiplier 1.118) that's 1,503 square feet, or 15.0 squares.
How much waste should I add for shingles?
About 10% for a simple gable roof and 15% for hip roofs or roofs with valleys, dormers and lots of cuts. Starter strip and ridge cap are counted separately.
Does a hip roof need more shingles than a gable roof?
For the same footprint and pitch, the roof area is the same. A hip roof needs more waste allowance because every hip edge is cut at an angle, and more ridge cap because hips are capped too.
What else do I need besides shingles?
Underlayment for the whole roof area, starter strip along the eaves (and often the rakes), ridge cap for ridges and hips, drip edge on every edge, ice and water shield at eaves and valleys in cold climates, roofing nails, and vents.
Sources and assumptions
- Owens Corning TruDefinition Duration: 3 bundles per square, 98.4 ft² per square
- Owens Corning Supreme 3-tab: 3 bundles per square
- Roof area = plan area × √(1 + (pitch ÷ 12)²) for roofs with one pitch throughout. Waste of 10% (gable) and 15% (hip) are common estimating allowances. Starter, ridge cap and underlayment coverage vary by product, so enter the figures from your wrapper.
Last reviewed 27 October 2026. Every formula on this page is checked against the worked example above in our automated tests. Found a problem? Tell us.