Roofing Calculator
Roof area, squares, bundles, and cost from footprint and pitch.
What it does
Shingles sell by the square, one hundred square feet of roof, with three bundles per square. The roof surface exceeds the house footprint because slopes stretch it, and the pitch factor measures that stretch from rise over run.
The tool above takes footprint, pitch, and waste in imperial or metric. It returns roof surface, squares, bundles, and cost at your price.
Complexity costs more than slope. Plain gables waste near ten percent in cuts, while valleys, dormers, and hips push fifteen or more and slow the crew, which labor quotes reflect.
Underlayment, ridge caps, flashing, and vents sit outside shingle math. Bundles cover the field; the accessories finish the roof and belong in every real quote.
How to use
- Switch the unit system first: imperial for feet, metric for meters.
- Measure house length and width outside walls.
- Pick pitch from plans or measure rise over a 12 inch run.
- Set waste by complexity and buy full bundles.
Formula
- rise/12
- Pitch slope per foot of run. 6/12 rises 6 inches per foot.
- factor
- Slope stretch over flat footprint, from Pythagoras.
- 100 ft2
- One roofing square of coverage.
- x 3
- Standard bundles per square.
Gable ends add small triangles outside this rectangle math, usually inside the waste share. Hip roofs measure each plane separately for best accuracy.
Worked example
Example: 40 x 30 ft, 6/12 pitch, 10% waste
Footprint 40 by 30, pitch 6/12, waste 10%, $150 per square.
- Factor: sqrt(1 + 0.5²) = sqrt(1.25) = 1.11803.
- Surface: 40 x 30 x 1.11803 = 1,341.64 ft2, shown as 1,342.
- Squares: 13.4164 x 1.10 = 14.76.
- Bundles: 14.76 x 3 = 44.27, round up to 45.
- Cost: 14.76 x $150 = $2,213.71 before caps and felt.
Reference table
| Pitch | Factor | Extra over flat |
|---|---|---|
| 3/12 | 1.031 | 3% |
| 4/12 | 1.054 | 5% |
| 6/12 | 1.118 | 12% |
| 8/12 | 1.202 | 20% |
| 12/12 | 1.414 | 41% |
Limits
- Rectangle footprint assumed. Wings and hips need plane by plane math.
- Gable triangles folded into waste, roughly.
- Caps, felt, flashing, and labor excluded.
- Steep pitches slow crews and raise labor quotes.
Common mistakes
- Ordering footprint squares without the pitch factor.
- Buying exact bundles with no cuts share.
- Forgetting ridge caps, which run separate pieces.
- Mixing shingle colors across lots.
- Ignoring decking repair hiding under old shingles.
Try it yourself
Quick checks. Answers sit tucked under each question.
30 x 30 ft footprint, 4/12 pitch, no waste. Squares?
900 x 1.054 = 948.7 ft2. That is 9.49 squares.
Bundles for 9.49 squares?
9.49 x 3 = 28.47, so 29 bundles.
Questions
What is a roofing square?
100 square feet of roof surface. Shingles price and sell by it.
How many bundles per square?
Three standard bundles. Metric packs state m2 coverage instead.
How do I measure pitch?
Level 12 inches from the roof and measure the rise. Six inches up is 6/12.
How much waste should I add?
Ten percent for simple gables, fifteen with valleys, dormers, or hips.
Do I include overhangs?
Yes, measure to the drip edge. Footprint plus eaves is the roofing rectangle.
When do roofs need replacing?
Asphalt lasts 15 to 30 years. Curling, bald spots, and leaks decide, not age alone.