Roofing Calculator

The footprint you can measure, the roof you have to cover

Read this before you enter anything. Sloped area = plan area (including overhangs) x pitch factor. That is exact for a gable and for a hip alike, as long as every plane in the section carries the same pitch, so no roof shape multiplier is applied.

Nobody measures a roof by walking it. What is measurable from the ground, or off a plan, is the footprint: the rectangle the building throws on the site, overhangs included. The shingles do not cover that rectangle, they cover the tilted surface above it, and the ratio between the two is a piece of geometry so plain that it needs no source at all. Rise over run makes a right triangle; the slope length of that triangle, divided by its run, is the number that stretches the footprint into a roof.

That is the whole of it. A roof described as six in twelve rises six inches for every twelve inches it runs, so one unit of run carries the square root of six squared plus twelve squared, over twelve, of covering. The sheet computes it and prints it back to you as the pitch factor, because a number you can see is a number you can check against your own framing square.

The consequence is worth spelling out, because it is where most estimates drift: the factor does not care whether the roof is a gable or a hip. A hip roof slices the same footprint into four planes instead of two, but if all four carry the same pitch, the sloped surface over a given footprint is identical. Hips need more ridge cap — they do not need more shingles per square foot of plan. That is why this sheet has no “roof shape multiplier”: it works the shape out of the geometry you typed rather than applying a coefficient to it.

Pitch factor  F  =  sqrt( rise^2 + 12^2 ) / 12
Sloped area   As =  A x F        A = plan footprint, overhangs in
Squares       S  =  As / 100     (1 square = 100 sq ft)
Order         So =  S x (1 + waste / 100)
Shingles      B  =  ceil( So x bundles per square )
Underlayment  R  =  ceil( So / squares per roll )
Ridge cap     Bc =  ceil( (ridge + hips) / lin ft per bundle )
Starter       Bs =  ceil( (eaves + rakes?) / lin ft per bundle )
Hip length    H  =  (W / 2) x sqrt( 2 + (rise / 12)^2 )   each of four

Four figures, four product sheets

Everything above is geometry. Everything below is a manufacturer’s published yield, and this is where roofing calculators quietly stop citing anything. Of the six pages ranking for roofing calculator when this sheet was audited, three are cost estimators driven by your street address rather than material calculators at all; of the three that do compute a roof, not one traces a coverage figure to a named product. This sheet carries 4 documents, each one a single product, each one shown with the product’s name attached, and each one sitting in an editable field.

• GAF Timberline HDZ, 3 bundles per square. The specification sheet reads: Exposure: 5 5/8" (143 mm) · Bundles/Square: 3 · Pieces/Square: 64 — read it here. That is one laminated shingle’s packing, not a law of roofing: another product’s wrapper can print another number, which is exactly why the field is free.
• GAF Seal-A-Ridge, 4 bundles per 100 linear feet. The sell sheet reads: 6 2/3" (169 mm) exposure · 4 bundles cover 100 linear feet (30 lm) · 45 ridge caps per bundle (15 shingles, each with 3 ridge caps per shingle) — read it here. The default in the ridge cap field is that figure divided out: 100 over 4 is 25 linear feet per bundle, arithmetic rather than a second claim.
• GAF Pro-Start, 120.33 lineal feet per bundle. The sell sheet reads: Bundle count: 19 full sheets (38 individual pieces) · Approx. 120.33 lineal feet (36.69 lm) (when split in half) — read it here. The coverage depends on the sheets being split in half, which the source says and the sheet repeats.
• GAF FeltBuster, a 10-square roll. The sell sheet reads: 10 squares/roll – 1,000 sq. ft. (92.9 m2) (excludes laps) · 48" x 250' (1.23 m x 76.2 m) — read it here. It is also where this page gets the roofing square itself: the same line equates 10 squares with 1,000 sq ft.

GAF FeltBuster prints "10 squares/roll - 1,000 sq. ft. (excludes laps)". The roll count here is roof area divided by that roll: it does not promise 1,000 sq ft of covered deck, because the side and end laps are not in the figure and no lap allowance was found in any document we opened.

Where the linear feet come from

Ridge cap and starter cover linear feet, not squares, so a takeoff that stops at squares leaves both lines to guesswork. The audit of the live results found no page that computes a ridge or hip length from the dimensions you typed, and none that outputs starter at all — one competitor tells the reader outright to budget more ridge cap on a hip roof without putting a number to it.

There is no mystery in it once the form is known. On a gable, the ridge is the length of the building and the eaves are twice that length; the rakes climb the two end walls, so they are the only lengths that take the pitch factor. On a hip, the ridge is shortened by the width — the two hipped ends eat into it from both sides — and four hips run from the ends of that ridge down to the corners, each one diagonal in plan as well as tilted, which is a second square root rather than the first one reused. Eaves go all the way round, and there is no rake at all. On a shed, one plane, one eave, two rakes, no ridge.

Put the same building through both and the point of the sheet shows up: our default gable is 1,118 sq ft with 40 ft of ridge and 2 bundles of ridge cap. Hipped, the identical footprint at the identical pitch is still 1,118 sq ft of shingle — but 90 ft of ridge and hip, and 4 bundles of cap. The eave run goes from 80 ft to 130 ft, so the starter goes from 1 bundle to 2. Nothing about the shingle order changed; the accessories nearly doubled.

Starter along the eaves is what the sheet counts by default. Whether it also runs up the rakes is a product instruction, not a rule this page will invent for you: switch it on and the rake length, already stretched by the pitch factor, joins the starter run. Four sections can be added together for an L-shaped house, a garage wing or a porch, each with its own form, dimensions and pitch, and the sheet keeps the areas and the linear feet apart the whole way through. If the sections disagree about pitch, it stops printing a single pitch factor rather than average one into existence.

Worked example: the default state, line by line

1. Footprint: 40 ft x 25 ft = 1,000 sq ft of plan, overhangs included.
2. Pitch factor at 6 in 12: 1.118.
3. Sloped area: 1,000 x 1.118 = 1,118 sq ft, which is 11.18 squares.
4. Shingles: 11.18 x 3 bundles per square, rounded up = 34 bundles.
5. Underlayment: 11.18 squares over a 10-square roll = 2 rolls, laps excluded.
6. Ridge: 40 ft over 25 linear feet per bundle = 2 bundles of cap.
7. Starter on the eaves: 80 ft over 120.33 lineal feet = 1 bundle.

Squares to bundles, for the sizes people actually search

The question behind most of these searches is not the geometry, it is the shopping list: a number of square feet has been quoted, and the reader wants to know what to put in the truck. The conversion is two multiplications, and it is reproduced here so it can be checked without the form. Read the first column as sloped area — the surface of the roof, not the floor area of the house, which is a different number.

Every row is the same arithmetic the form does: area over 100, then squares times bundles per square, rounded up to whole bundles. Swap the yield for the one on your own wrapper and the column moves with it — that is the reason the sheet asks rather than assumes.

What this sheet refuses to guess

Waste starts at zero. A 10 / 15 / 20 / 25 percent ladder by roof complexity is printed elsewhere; we could not open a document that publishes it, so this sheet asks for your number instead of printing one.

Waste is real; a waste ladder we could open and quote is not. Off-cuts at hips, valleys and rakes consume material, and the number that covers them is a function of your crew, your plan and your product. The field is there, it starts at zero, and whatever you type is applied to the roof area before the bundles are rounded — so you can see what the allowance costs rather than inherit it silently.

Nails, ice barrier, drip edge, valley metal and flashing are not counted: nothing we opened publishes a quantity for them. The Timberline HDZ sheet's "standard 4-nail" line describes a laboratory test, not an installation instruction.

No price is built in. Shingles are quoted by the bundle or the square and the quote moves with the product, the yard and the delivery. Enter your own and the sheet prices your own bundle count.

Frequently asked

How do I calculate how much roofing I need?

Measure the footprint including the overhangs, multiply by the pitch factor, divide by 100 to get squares, then multiply by the bundles per square printed on your wrapper. The default here, 40 by 25 ft at 6 in 12, is 1,118 sq ft, 11.18 squares and 34 bundles.

How many bundles of shingles for a 2000 sq ft roof?

60 bundles of a 3-bundle shingle: 2,000 sq ft of slope is 20 squares, and 20 x 3 = 60, before any waste you add. That is a sloped area, not a floor area — the floor area of a house is a different number.

How many sq ft does 1 sq of shingles cover?

100 square feet. GAF FeltBuster's sheet writes the equivalence on one line, 10 squares/roll – 1,000 sq. ft., which is where this sheet takes it from rather than from folklore.

How much will 3 bundles of shingles cover?

For GAF Timberline HDZ, 3 bundles are one square, so 100 square feet of slope, and the same sheet prints 64 pieces per square. Another product can pack differently: the field on the form is there so you can enter what your wrapper says.

How many square feet of roof on a 1500 sq ft house?

It depends on the pitch and the overhangs, which is why no page can answer it with one number. A 1,500 sq ft footprint at 6 in 12 gives 1,677 sq ft of slope, 16.77 squares. Flatter or steeper moves it by the factor in the table above.

How many bundles of shingles for a 12x16 shed?

A 16 by 12 ft single-slope shed roof at 4 in 12 is 202 sq ft, 2.02 squares, so 7 bundles at 3 per square. Change the form to gable and the sheet re-cuts the ridge, eave and rake lengths with it.

Limits of this sheet

A-102 converts a plan footprint and a pitch into a quantity of shingles, underlayment, ridge cap and starter. It does not measure your roof, design a roof assembly, choose a product, judge whether the deck below it can be reused, or tell you what any of it will cost. Dormers, turrets, curved planes and overlapping roofs are not detected: they are entered as extra sections or they are not in the takeoff at all. Where two sections meet, the valley and the eave they share are not resolved: correct the eave run yourself. Openings for chimneys and skylights are not deducted, which leaves the order marginally long rather than short. Every coverage figure is one manufacturer’s published yield for one product, faithfully quoted and easily wrong for the bundle you are actually buying — check the wrapper, change the field. Codes, minimum slopes, fastening patterns and ventilation are out of scope; your product’s own installation instructions and your local code govern. Quantities are for planning and ordering, not engineering.

Sheet A-102 prepared by TakeoffMetric Editorial; no author, credential or field measurement is claimed.