Free Roofing Calculator
Calculate shingle bundles, underlayment rolls, and nails for any roof.
What is a roofing calculator?
A roofing calculator is a free online tool that helps you estimate roofing materials including shingles, underlayment, and waste factor from roof dimensions. It runs entirely in your browser — no signup, no app, and no data is ever sent to a server.
A roofing calculator estimates the materials you need for a roofing project: shingles, underlayment, and nails. Start with your roof's footprint (length × width), then apply the pitch factor — a multiplier that accounts for the roof's slope. A 6/12 pitch (the most common) has a pitch factor of 1.118, meaning the actual roof surface is about 12% larger than the footprint. The calculator converts the adjusted area into "squares" (100 sq ft each) — the standard unit for roofing materials.
How it works
The footprint area (length × width) is multiplied by the pitch factor for your selected slope. The waste factor (10% default) is added for cuts, starter strips, ridge caps, and mistakes. The total is divided by 100 to get squares. Each square needs 3 bundles of shingles, 1 roll of underlayment (15-lb felt), and approximately 240 nails (6 lbs per square using 1.25-inch roofing nails).
When to use a roofing calculator
Before ordering materials for a new roof or reroof, you need an accurate material estimate. Roofing contractors use these numbers for bids. Homeowners compare material costs between quotes. The calculator is also useful for estimating repair materials for a single roof section or shed roof.
Common mistakes when estimating roofing materials
The most common mistake I see homeowners make is forgetting the pitch factor entirely and ordering materials based on the footprint area. A 1,500 sq ft footprint at a 6/12 pitch becomes 1,677 sq ft of actual roof surface — that's 16.8 squares versus 15. That discrepancy means you'd be short nearly 2 bundles of shingles and a roll of underlayment. The calculator applies the pitch factor automatically so you get the right material quantities.
Another frequent error is underestimating waste. A 10% waste factor is standard for basic gable roofs, but complex roofs with valleys, hips, and multiple dormers often need 15-20% waste. Every hip and valley creates cut-offs that can't be used elsewhere. I recommend adding an extra 5% if your roof has more than two planes or any penetrations like skylights or chimneys. The calculator's adjustable waste factor lets you account for roof complexity.
Many people also forget that ridge cap shingles and starter strips require extra material beyond the field shingles. Ridge caps typically use one bundle per 20 linear feet of ridge. Starter strips add another half-bundle per square. These aren't included in the standard three-bundles-per-square calculation. The calculator accounts for these extras so your material list is complete when you place your order at the supplier.
How to use this calculator
- Enter your roof dimensions — Enter the length and width of your roof footprint in feet.
- Select your roof pitch — Choose the roof pitch (rise per 12 inches of run) — higher pitches mean more surface area and more materials.
- Add waste factor — Include a waste percentage (10% is standard) for cuts, overlap, and mistakes.
Frequently asked questions
How do I calculate roofing materials?
Measure your roof's footprint (length × width), multiply by the pitch factor to get the actual roof area, then divide by 100 to get squares. Each square needs 3 bundles of shingles, 1 roll of underlayment, and about 240 nails. The calculator does all of this automatically.
What is a roof square?
A roofing square is 100 square feet of roof surface. Roofing materials are priced and sold by the square. A 1,500-square-foot roof (actual surface area, not footprint) is 15 squares. Pitch is the single biggest variable in roof area — a steep 12/12 pitch nearly doubles the footprint area.
How many shingles do I need for a 10×20 roof?
A 10×20 footprint is 200 sq ft. At a standard 6/12 pitch (pitch factor 1.118), the actual roof area is 224 sq ft = 2.24 squares. You need 7 bundles of shingles (3 per square = 6.72, round up to 7) plus underlayment and nails. Add 10% for waste.
How much does a bundle of shingles cover?
A standard bundle of 3-tab asphalt shingles covers approximately 33.3 square feet. Three bundles make one square (100 sq ft). Architectural shingles sometimes cover less — check the bundle label. The calculator uses the standard 33.3 sq ft per bundle.
What type of underlayment should I use?
Standard 15-lb asphalt-saturated felt is the most common and economical choice for most roofs. For low-slope roofs (2/12 to 4/12), use synthetic underlayment or ice-and-water shield. For roofs in cold climates, ice-and-water shield is required along eaves and in valleys by most building codes. The calculator assumes 15-lb felt by default but dimensions work for any roll size.
How many nails should I use per shingle?
Standard practice is 4 nails per shingle for 3-tab asphalt shingles on slopes below 6/12, and 6 nails per shingle for steeper slopes or high-wind areas. Nails should be placed 5/8 inch above the cutout and spaced 12 inches apart. The calculator uses 240 nails per square as the baseline, which covers 4 nails per shingle at standard exposure.