Roofing Calculator: How Many Shingles Do I Need?
Measure the footprint your roof covers and tell the calculator how steep it is — it works out the actual sloped area from there. A 6/12 roof has about 12% more surface than the rectangle it sits on, and missing that is the classic way to come up short.
Roof measurements
Measure the footprint from the ground, or the roof itself if you have safe access.
This estimates covering material only. It says nothing about structure, decking, underlayment, ventilation or code compliance — a roof is one of the places where getting it wrong is expensive and dangerous.
How many shingles do I need?
Work out the sloped area of the roof, add waste for cutting and starter courses, then convert to squares. A roofing square is 100 square feet by definition, and shingles are sold by the bundle at three bundles to the square for most architectural products.
If you have measured the roof itself rather than the ground beneath it, switch the mode. Applying the pitch to an already-sloped measurement counts it twice and will have you ordering a fifth more material than you need.
How pitch changes the area
| Pitch | Multiplier | Extra area over the footprint |
|---|---|---|
| 4/12 | 1.054 | 5% |
| 5/12 | 1.083 | 8% |
| 6/12 | 1.118 | 12% |
| 8/12 | 1.202 | 20% |
| 10/12 | 1.302 | 30% |
| 12/12 | 1.414 | 41% |
The multiplier is √(1 + (rise/12)²) — the slope length over the horizontal run.
You can measure pitch from inside the loft: hold a level 12 inches out horizontally from a rafter and measure the drop to the rafter at that point. That drop is the rise.
How much waste to allow
| Waste | Good starting point for |
|---|---|
| 10% | A straightforward gable roof |
| 15% | Hips, valleys, dormers and penetrations |
| 20% | Many facets, or shingles that need cutting to a pattern |
Waste on a roof is mostly hips, valleys and starter courses. A plain gable roof wastes very little; a roof with dormers, several valleys and a chimney to cut around wastes a great deal more.
How to measure a roof
Length by width of the area the roof covers, including the overhang.
An L-shaped house is two rectangles; add them as separate sections.
If you have safe access and a real measurement of the slope, switch the mode and skip the pitch.
Common mistakes
- Using the ground footprint as the roof area and ignoring the pitch.
- Applying the pitch to a measurement already taken on the slope.
- Using a simple-gable waste allowance on a roof full of valleys.
- Treating a material estimate as a specification for the roof itself.
Insulating the roof space?
Work out the cavity area while the roof is open.
Open the Insulation Calculator →