Roof Shingles Calculator — Squares and Bundles
Measure the footprint on the ground, pick your pitch, and this works out the true sloped area — then the bundles, starter, ridge cap and nails. No ladder needed for the measuring.
Take this list to the store
We'll email the quantities above, what you typed in, and a link that reopens this calculator already filled in — so you can adjust it on your phone in the aisle.
How to work it out
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Measure the footprint on the ground
Measure the outline of the house, including the overhang at the eaves. You do not need to climb anything — the calculator converts the flat footprint to the true sloped area using the pitch.
An L-shaped house is two rectangles. Add a section for each.
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Find the pitch from inside
Hold a level against a rafter in the loft, mark 12 inches along it, and measure straight down to the rafter at that mark. That drop in inches is the pitch — a 6 inch drop is a 6/12 roof.
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Measure the ridge and the eaves
Ridge cap and starter strip are separate products sold by the linear foot, and both are easy to forget until the day of the job. Pace them out at ground level if you have to.
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Add waste for the shape, not the size
Ten per cent on a simple gable roof with two planes. Fifteen once there are hips, valleys or dormers, because every one of those means cutting shingles at an angle and binning the cut-off.
How pitch changes the area
For a 1,500 sq ft footprint. Bundles include 10% waste.
| Pitch | Multiplier | True roof area | Squares | Bundles |
|---|---|---|---|---|
| 3/12 | 1.031 | 1,546 sq ft | 15.5 | 54 |
| 4/12 | 1.054 | 1,581 sq ft | 15.8 | 54 |
| 6/12 | 1.118 | 1,677 sq ft | 16.8 | 57 |
| 8/12 | 1.202 | 1,803 sq ft | 18.0 | 60 |
| 10/12 | 1.302 | 1,953 sq ft | 19.5 | 66 |
| 12/12 | 1.414 | 2,121 sq ft | 21.2 | 72 |
A 12/12 roof has 41% more surface than its footprint. Ignoring the pitch is the single biggest error in roofing estimates.
What else the job needs
| Item | Coverage | Note |
|---|---|---|
| Shingle bundle | 33.3 sq ft | Three bundles make one square of 100 sq ft. |
| Starter strip | 100 ft per roll | Along every eave and rake edge. |
| Ridge cap | About 20 ft per bundle | Ridges and hips. Cut-up 3-tab also works. |
| Underlayment | 400 sq ft per roll | Synthetic. Felt rolls are usually 200 or 400 sq ft. |
| Roofing nails | 320 per square | About 2.5 lb per square. Buy a full box. |
| Drip edge | 10 ft per length | Every eave and rake. Required by most codes. |
Ice and water shield is required at the eaves in cold climates — usually the first 24 in. past the inside wall line.
How many bundles of shingles for a 1,500 sq ft roof?
Careful — 1,500 sq ft of house is not 1,500 sq ft of roof. On a typical 6/12 pitch the true roof area is 1,677 sq ft. Add 10% waste and that is 18.4 squares, or 56 bundles.
If the 1,500 figure is already the measured roof area, it is 15 squares plus waste — about 50 bundles.
What a roofing square actually is
One hundred square feet. It is the unit the entire trade prices and orders in, and it has nothing to do with a square foot or a square yard.
Shingles come three bundles to the square for standard architectural and 3-tab types. Heavier designer shingles can be four or even five bundles per square, so check the wrapper before dividing.
Finding your pitch without a ladder
Go into the loft with a spirit level and a tape. Hold the level horizontally against the underside of a rafter, mark exactly 12 inches along it, then measure vertically from that mark up to the rafter.
A 6 inch measurement means a 6/12 pitch. It is the same slope you would measure on the outside, and considerably safer.
When to tear off and when to go over
Most US codes allow two layers of shingles. Going over an existing layer saves the tear-off cost, but it hides any rotten decking, adds weight, and the new shingles never lie quite flat over the old tab lines.
If there is already one layer and the decking is sound, a second is legal. If there are two, or you suspect rot, it is a tear-off — and while the deck is bare is the only time to fix it.
Ventilation is why roofs fail early
Shingles are rated for twenty or thirty years in laboratory conditions. The ones that fail at twelve almost always sat over a badly ventilated loft.
Air has to enter low at the soffits and leave high at the ridge. Without that flow the loft cooks in summer — cooking the shingles from underneath — and in winter warm damp air condenses on the cold underside of the deck.
The usual target is one square foot of net free vent area per 300 square feet of loft floor, split roughly half low and half high. Ridge vent plus continuous soffit vent is the standard pairing.
The classic mistake is insulating the loft and burying the soffit vents at the eaves. Baffles cost very little and keep the airway open.
Valleys, flashing and where leaks actually start
Roofs very rarely leak through the middle of a shingle field. They leak at the joins.
- Valleys — where two roof planes meet, carrying the most water on the roof. Ice and water shield the full length, then either a metal valley or a woven shingle valley.
- Chimneys and walls — step flashing, one piece per shingle course, tucked behind the cladding or into a mortar joint. Never a single long strip, and never relying on sealant.
- Pipe boots — the rubber collar perishes and splits long before the shingles wear out. On a re-roof, replace every one, even if it looks fine.
- Eaves — ice dams push water back up under the shingles. Ice and water shield from the edge to at least 24 in. past the inside of the exterior wall.
None of this is in the bundle count, and all of it is what determines whether the roof stays dry.
How this is worked out
No black box — here is the arithmetic the calculator runs, so you can check it against your own numbers.
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footprint = length × width, summed over every sectionMeasured on the ground, including the eaves overhang. -
× pitch multiplier√(1 + (rise/12)²). A 6/12 roof is 1.118 — 12% more surface than the footprint. -
× (1 + waste ÷ 100)10% for a plain gable, 15% with hips, valleys or dormers. -
squares = area ÷ 100The trade unit. One square is 100 sq ft. -
bundles = squares × 3, rounded upThree bundles per square for standard shingles. -
ridge cap = ridge length ÷ 20About 20 linear feet per bundle of cap. -
underlayment = squares ÷ 4A synthetic roll covers about 400 sq ft, or 4 squares.
Common questions
How many bundles of shingles in a square?
Three for standard architectural and 3-tab shingles, each covering about 33.3 sq ft. Heavier designer shingles can be four or five bundles per square — check the wrapper.
How do I measure my roof without getting on it?
Measure the footprint of the house at ground level, including the eaves overhang, then multiply by the pitch factor. Find the pitch from inside the loft with a level and tape. That is exactly what this calculator does.
What is a roofing square?
One hundred square feet of roof surface. It is the unit roofers quote and order in, and it is unrelated to a square foot or square yard.
How much waste should I allow for shingles?
Ten per cent on a simple gable roof. Fifteen once there are hips, valleys or dormers, since every one means angled cuts and unusable cut-offs.
How many nails per square of shingles?
About 320 — four per shingle in normal conditions, six in high wind zones, which takes it to roughly 480. That is around 2.5 lb per square.
Do I need underlayment under shingles?
Yes, and code requires it. Synthetic underlayment is now standard and comes in 400 sq ft rolls. Cold climates also need ice and water shield at the eaves, typically 24 in. past the inside of the wall.
Can I shingle over my existing roof?
Most codes allow two layers total. It saves tear-off cost, but it hides rotten decking, adds weight and never lies perfectly flat. If there are already two layers, or you suspect rot, it has to be a tear-off.
How long do asphalt shingles last?
Twenty to thirty years for architectural shingles in a temperate climate, less in intense sun or with poor attic ventilation. Ventilation matters more than most people realize — a hot loft cooks the shingles from underneath.
Where these figures come from
Every figure below is traceable. How we check the numbers explains how they are verified, and the coverage reference collects them all in one place.
- Asphalt Roofing Manufacturers Association — the square unit, bundles per square and coverage
- GAF and Owens Corning product data — bundle coverage, nail counts and starter strip lengths
- International Residential Code R905.2 — underlayment, drip edge and layer limits for asphalt shingles
Think a number here is wrong? Tell us — we check every report, correct the page, and date the change. Last checked September 11, 2026.