Roofers price and order by the square – 100 ft² of roof surface. Because a roof slopes, its surface is larger than the footprint it covers; the pitch multiplier converts one to the other, and it is the same for gable and hip roofs with a uniform pitch. Add 10% waste for simple gables and 15% for hips and valleys.
How to use the Roofing Calculator
- Building length
- Building width
- Overhang
- Roof pitch (rise per 12)
- Roof style / waste
- Price per square (installed)
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the building footprint, overhang and pitch, the waste allowance and price per square. The calculator finds the sloped area, squares, bundles, underlayment and drip edge and the cost.
Tips for an accurate estimate
- Measure the footprint from the ground and apply the pitch multiplier rather than climbing on the roof.
- Add 10% waste for simple gable roofs and 15–20% for hips, valleys and dormers.
- Confirm design snow and wind loads with your local building department before changing framing or roofing.
- Always work with fall protection; roofing is one of the most dangerous trades.
Common mistakes to avoid
- Using the footprint area instead of the sloped roof area when ordering materials.
- Forgetting starter strips, ridge caps, drip edge and underlayment.
- Mixing up pitch (rise per 12 in of run) with the roof angle in degrees.
Inputs explained
| Field | What to enter |
|---|---|
| Building length | Units: ft, m, in, yd, cm |
| Building width | Units: ft, m, in, yd, cm |
| Overhang | Units: in, cm, mm, ft, m |
| Roof pitch (rise per 12) | In /12 |
| Roof style / waste | Choose one: Simple gable – 10%, Hip roof – 15%, Complex (valleys, dormers) – 20% |
| Price per square (installed) | Units: /square |
What each result means
- Roof area = ([Building length] + 2×[Overhang])×([Building width] + 2×[Overhang])×sqrt(1 + ([Roof pitch (rise per 12)]÷12)^2)
- Squares = [Roof area]×(1 + [Roof style ÷ waste]÷100)÷9.290304
- Shingle bundles (3 per square) = ceil([Squares]×3 – 0.000001)
- Underlayment rolls (10 squares) = ceil([Squares]÷10 – 0.000001)
- Drip edge = 2×([Building length] + [Building width] + 4×[Overhang])
- Estimated cost = [Squares]×[Price per square (installed)]
Example values
How the main result changes when Building length changes and everything else stays at the example values above:
| Building length (ft) | Squares |
|---|---|
| 20 | 8.12 |
| 30 | 11.81 |
| 40 | 15.5 |
| 50 | 19.19 |
| 60 | 22.87 |
| 80 | 30.25 |
Formula
Roof area = (L + 2o)(W + 2o) × √(1 + (pitch/12)²)Squares = area × (1 + waste) ÷ 100 ft²; bundles = squares × 3Underlayment = squares ÷ 10 (synthetic rolls); drip edge = perimeterWorked example
Inputs — Building length: 40 ft; Building width: 28 ft; Overhang: 12 in; Roof pitch (rise per 12): 6 /12; Roof style / waste: Simple gable – 10%; Price per square (installed): $450 /square.
Results — Roof area = 1,409 ft²; Squares = 15.5; Shingle bundles (3 per square) = 47; Underlayment rolls (10 squares) = 2; Drip edge = 144 ft; Estimated cost = $6,973.18.
Key terms
- Square
- 100 ft² (9.29 m²) of roof.
Frequently asked questions
How do I measure a roof without climbing on it?
Measure the footprint including overhangs from the ground and multiply by the pitch multiplier.
What is a roofing square?
100 ft² of roof surface – three bundles of standard shingles.
How much does a new roof cost?
Asphalt shingle roofs typically cost $400–$700 per square installed in the U.S.
What formula does the Roofing Calculator use?
Roof area = (L + 2o)(W + 2o) × √(1 + (pitch/12)²).