Roofing Calculator

Roofing Calculator: How Many Squares and Bundles?

If you are pricing a new roof or checking a quote, the question is usually the same: how many bundles of shingles do I need? The answer starts with the size of the house on the ground, not the size of the roof in the air. A sloped roof is always bigger than the footprint underneath it, and the steeper it is, the bigger the difference. This page turns your footprint and pitch into roof area, roofing squares and bundles, then shows every formula it uses so you can check the maths yourself before you order anything.

Quick answer

At a 6/12 pitch, with 10% waste included and 3 bundles per square, the starting figures look like this:

House footprint	Roof area	Squares (with waste)	Bundles
1,000 sq ft	1,118 sq ft	12.3	37
1,500 sq ft	1,677 sq ft	18.4	56
2,000 sq ft	2,236 sq ft	24.6	74
2,500 sq ft	2,795 sq ft	30.7	93

Squares are rounded to one decimal place in this table, and bundles are always rounded up to the next whole bundle, because you cannot buy part of one. A steeper pitch, a hip roof or a roof with valleys will push these numbers higher — the calculator below handles all of that.

Estimates only — never climb a roof just to measure it. Measure from the ground or use a pro or drone service. Roofs flatter than 2/12 cannot use standard shingles, and deck condition, wind ratings, building code and permits are contractor matters.

What is a roofing square?

A roofing square is simply 100 sq ft of roof. It is the unit the trade has used for generations because roof numbers get big quickly: a modest 1,800 sq ft roof is much easier to talk about, quote and order as 18 squares than as a long string of square feet. Shingles, underlayment and labour are all commonly priced by the square, so converting your roof into squares is the step that turns a measurement into a price. A bundle of shingles covers about 33.3 sq ft, so it usually takes 3 bundles to cover one square — that is true for standard 3-tab shingles and for the heavier architectural (dimensional) shingles most people fit today. Always confirm on the wrapper of the exact product you are buying, because coverage is what the bundle count depends on.

Pitch multiplier chart

Pitch describes how steep a roof is: a 6/12 roof rises 6 inches for every 12 inches it runs horizontally. The pitch multiplier turns flat footprint area into true sloped roof area, using √(1 + (rise ÷ 12)²). Find your pitch in the chart and multiply your footprint by the number beside it.

Pitch	Multiplier
1/12	1.003
2/12	1.014
3/12	1.031
4/12	1.054
5/12	1.083
6/12	1.118
7/12	1.158
8/12	1.202
9/12	1.250
10/12	1.302
11/12	1.357
12/12	1.414

Worked example

Take a house with a 1,200 sq ft footprint — say 30 ft by 40 ft — and a 6/12 pitch, roofed as a simple gable. Here is the whole calculation, step by step:

• Step 1 — roof area: 1,200 sq ft × 1.118 (the 6/12 multiplier) = 1,342 sq ft of roof.
• Step 2 — squares before waste: 1,342 ÷ 100 = 13.4 squares.
• Step 3 — add waste: 13.4 × 1.10 (10% for a simple gable) = 14.8 squares to order for.
• Step 4 — bundles: 14.8 × 3 bundles per square = 44.3, rounded up to 45 bundles.

So the headline answer for that house is 45 field bundles, plus the starter, ridge-cap and underlayment figures the calculator adds separately. Change the pitch to 9/12 and the same house needs noticeably more, which is why pitch is never a detail to guess at.

Waste by roof type

Waste is the allowance for shingles that get cut, trimmed or damaged during fitting. A simple gable roof — two plain slopes and a straight ridge — needs about 10%, because most shingles go on whole and the cuts are only at the ends of courses. A hip roof needs about 15%: every slope is cut at an angle where it meets the hips, and those angled offcuts usually cannot be reused on the other side. A complex roof with valleys and dormers needs about 20%, because valleys are cut on the diagonal, dormers break the slopes into small fiddly areas, and each break produces partial shingles and extra starter and cap work. Skimping on waste rarely saves money — running short mid-job costs more in delays and mismatched batches than one spare bundle ever will.

Starter, ridge cap and underlayment

Field shingles are only part of the order, which is why the calculator lists the rest separately instead of folding them into the bundle count. Starter shingles run along the eaves — the bottom edges of the roof — and give the first course something solid to seal to, so wind cannot lift it; this calculator allows one bundle per 35 ft of eave. Ridge-cap shingles are the angled pieces that straddle the ridge and hip tops and shed water away from the peak; they are sold and counted separately, at about 30 ft of ridge per bundle here. Underlayment is the protective layer fitted over the deck before any shingle goes on — the roof's second line of defence if water ever gets underneath — and it comes in rolls, allowed here at one roll per 400 sq ft of roof including waste. Leaving any of these off an order is the classic way a roofing job stalls half-finished.

How to measure a roof from the ground (safely)

You do not need to set foot on the roof to size it. Measure the house footprint at ground level — the length and width of the walls — then add the roof overhang: roofs usually extend past the walls at the eaves and gables, so add that overhang on each side before you multiply length by width. Work out the pitch without climbing, using a pitch app on a phone held against the underside of a rafter in the attic, a level and tape from a ladder at the eave only if that is safe, or the pitch a previous roofer recorded. Then apply the pitch multiplier from the chart above. Satellite measuring tools, including the measure-distance feature in Google Maps, are a good cross-check for footprint and ridge length on a clear roof outline. Call a professional — or use a drone measuring service — when the roof is steep, tall, fragile, partly hidden by trees, or made of many intersecting sections, and whenever anything about the deck's condition is in doubt.

UK and Australia: different systems

United Kingdom: British roofs are normally covered in tiles or slates, not asphalt shingles, and they are priced by the square metre (m²), with pitch described in degrees rather than as a rise over 12. The layer under the tiles is called underlay, usually a breathable membrane, rather than underlayment felt. Australia: most Australian homes use metal roofing, such as Colorbond steel, which is ordered in square metres for the sheets and linear metres for flashings, ridge capping and gutters. Bundles and roofing squares do not apply in either country. If you are in the UK or Australia, use the roof area in m² that this calculator shows in Metric mode as your starting figure, and have a local roofer convert it into tiles, slates or sheet lengths for your exact product.

Roofing guides

Want the thinking behind the numbers? These two guides walk through the same maths this calculator uses, step by step:

• How Many Shingles Do I Need?
• Roof Pitch Calculator — Chart, Multipliers & Degrees

Frequently asked questions