Roofing Calculator: 40 × 40 ft footprint
Estimate roofing squares, shingle bundles and underlayment from your roof footprint and pitch, with waste and rough cost.
// Bill of materials
Costs are rough, materials only — edit the price to match your quote.
Pre-filled for a 40 × 40 ft footprint. Change any value to recalculate, or start from scratch.
How it's calculated
Roof area = footprint area × a pitch multiplier (a steeper roof has more surface than its footprint). We add waste, divide by 100 to get roofing 'squares', then use 3 bundles per square for shingles and 1 roll of felt per 2 squares for underlayment.
Worked example — 40 × 40 ft footprint
- Footprint length
- 40 ft
- Footprint width
- 40 ft
- Roof pitch
- 4:12
- Waste allowance
- 10 %
- Price per bundle
- 35 $
Nearby sizes
Tips for buying roofing
- Buy 10% extra shingles for hips, valleys and cuts — 15% if the roof is complex or steep.
- One roofing 'square' is 100 sq ft. Architectural shingles run about 3 bundles per square; check your brand.
- Order starter strip and ridge cap separately — they're not counted in field bundles.
- Measure the footprint from the ground and apply the pitch multiplier; don't guess the slope area directly.
- Underlayment coverage varies a lot — synthetic rolls cover far more than 30 lb felt, so check the roll.
- Re-roofing over one existing layer is common, but two layers usually means a tear-off; check local code.
Frequently asked questions
How many bundles of shingles do I need?
About 3 bundles per roofing square (100 sq ft) for architectural shingles. This calculator multiplies your pitched roof area by 3 bundles per square and adds waste.
What is a roofing square?
A square is 100 square feet of roof surface. Roofers price and order almost everything by the square, so we convert your area into squares first.
How does roof pitch change the amount?
A steeper roof has more surface than the footprint under it. We multiply the footprint by a pitch factor — a 12:12 roof is about 1.41× its footprint, while a low 3:12 is only about 1.03×.
Do I need underlayment?
Yes — underlayment goes down before shingles. We estimate one roll of 30 lb felt per two squares; synthetic underlayment covers more, so adjust to your product.