How to Calculate Roofing Squares in 5 Simple Steps

Roofing squares in one line

Roofing squares are roof length, times roof width, divided by 100, times bundles per square, plus waste, rounded up to whole bundles. A 40 ft by 30 ft roof is 1200 square feet, or 12 squares; at 3 bundles per square that is 36 bundles exact, and with 10% waste the order is 40 bundles. The rest of this guide is about getting those three inputs honest: the roof area, the bundles per square, and the waste allowance.

Step 1: Measure the roof on the slope

Measure the length and width of each slope face, in feet, on the roof itself — not the ground footprint beneath it, since a pitched roof is larger than the floor plan it covers. The worked example is one rectangular face, 40 ft by 30 ft. If the roof has two faces, measure both and add them: a 40 ft by 30 ft face plus a matching face is 2400 sq ft, not 1200. The roof pitch chart converts a footprint measurement to slope area when the roof cannot be measured directly.

Step 2: Divide by 100 to get squares

Roofers count in squares of 100 square feet, so divide the area by 100: 1200 ÷ 100 = 12 squares. Squares are a trade unit, not a shape — a 12-square roof is not twelve 10 ft by 10 ft patches, it is 1200 square feet in whatever shape the roof takes. Keep the division honest: a roof measured on the ground footprint gives fewer squares than the slope faces actually need, and the shortfall shows up at the last course.

Step 3: Multiply by bundles per square

Multiply the squares by the bundles per square for the shingle you are buying: 12 × 3 = 36 bundles exact for three-tab shingles. This is the step most estimates get wrong, because the bundle figure belongs to the shingle, not to the roof: an architectural shingle at 4 bundles per square takes the same 12-square roof to 48 bundles exact. Read the figure off the bundle you are buying, not off a habit, and keep it separate from the waste step — exact first, waste second.

Step 4: Add waste, then round up

Add waste for cutting, valleys and breakage: 10% for a plain gable roof takes 36 bundles to 39.6, and bundles are sold by the bundle, so round up to 40. A roof with many valleys, hips and dormers wants 15%, which would take the same roof to 42 bundles. Waste is not spare stock — it is the bundle the job consumes in valley cuts and in rejecting damaged shingles, and a roof ordered without it runs short at the last course.

Step 5: Order whole bundles, and count the extras

Order 40 bundles, since bundles are sold by the bundle and 40 is already whole — the rounding happened at the waste step, not at the merchant. Then count what bundles do not cover: the underlayment beneath them, the ridge cap along the top, the flashing at valleys and chimneys, and the fasteners — none of these come in the bundle count. Keep a few spare bundles after the job: they patch storm damage in the shingle's own colour.

Roofing squares in numbers

Suppose the roof is 40 ft by 30 ft, three-tab shingles at 3 bundles per square, 10% waste. Area: 40 × 30 = 1200 sq ft. Squares: 1200 ÷ 100 = 12. Bundles exact: 12 × 3 = 36. With waste: 36 × 1.10 = 39.6, rounded up to 40 bundles. Order: 40 bundles, plus underlayment, ridge cap and flashing bought separately.

Common mistakes

The most common mistake is measuring the ground footprint instead of the slope faces, which under-orders on every pitched roof. The second is using the bundles-per-square figure from a different shingle, which moves the order by a dozen bundles at a stroke. The third is skipping the waste allowance, which under-orders by exactly the amount the valley cuts consume. The fourth is adding waste before multiplying by bundles per square, which rounds twice and can shift the count by a bundle. The fifth is counting ridge cap and underlayment as bundles, which are separate counts, not waste. The sixth is ordering the exact bundles and rounding down, which leaves the last course short.

Worked example, start to finish

A roof, 40 ft by 30 ft, three-tab shingles at 3 bundles per square, 10% waste. Area: 1200 sq ft. Squares: 12. Bundles exact: 36. With waste: 40 bundles. Result: order 40 bundles, plus underlayment, ridge cap and flashing bought separately, and keep a few spare bundles for storm repairs. If the same roof used architectural shingles at 4 bundles per square instead, the exact count would be 48 bundles and the order 53 with waste — the shingle alone would add 13 bundles, which is why step three, reading the bundle, matters most.

What changes the answer most

The bundles-per-square figure changes the answer most: moving from 3 to 4 adds 13 bundles at a stroke, since it multiplies every square of roof. The roof area is second: area moves the count, but only across a bundle rounding does the order change. The waste allowance is third: 10% is small beside the bundle figure, but it is the difference between a finished last course and an open one. The roof shape is fourth, and it changes only the waste, never the squares.

Frequently asked questions