Roofing Calculator

A roofing calculator works out how much roofing material you need — in squares (100 sq ft each) and shingle bundles — and corrects for your roof's pitch, which a flat footprint measurement misses. It then lists underlayment, nails and drip edge so you order the whole job at once.

Your measurements

Result

The pitch factor is the part DIYers get wrong: a 6:12 roof has about 12% more surface than its footprint, and a 12:12 roof over 40% more. Get that wrong and you're short a pallet.

How it is calculated

Roof area = footprint × slope factor, where slope factor = √(1 + (rise/12)²). Squares = roof area ÷ 100, times the waste multiplier. Shingles come 3 bundles to the square; underlayment ≈ 4 squares per roll; nails ≈ 320 per square for standard 3-tab/architectural shingles.

Steps

Enter the building footprint (length × width or the area).
Pick the roof pitch — the rise per 12 inches of run.
Set a waste allowance (10% simple, 15%+ for valleys/hips).
Read the squares and bundle count.
Order underlayment (≈4 squares/roll) and nails to match.

Worked example

A 50 × 30 ft footprint at 6:12 pitch is ~1,677 sq ft of roof — about 18.4 squares with 10% waste, or 56 bundles of shingles.

Common questions

How many bundles of shingles do I need?

Three bundles per square (100 sq ft) of roof, plus waste. The calculator converts your footprint and pitch into squares and bundles.

What is a roofing square?

100 square feet of roof surface. Roofing is estimated and sold in squares, not square feet.

Why does roof pitch matter?

A pitched roof has more surface than the footprint it covers. The slope factor scales the footprint up — skip it and you under-order.

How much waste should I add?

10% for a simple gable roof; 15-20% for hips, valleys, dormers and lots of cuts.