Roofing Calculator
Squares and bundles from the footprint and the pitch — one multiplier law, three bundles to the square.
Roofing Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Gable-and-hip geometry; mansards and dormers add by hand.
- No ridge cap, starter or flashing take-off.
- Single-plane multiplier — complex plans add in pieces.
In short: A 1,500-ft² footprint at 8/12: × 1.201850 = 1,802.78 ft² of roof → 19.83 squares at 10% → 60 bundles of 3-bundle shingles, 5 rolls of felt.
Formula
multiplier = √(1 + (rise÷12)²)
roof ft² = footprint × multiplier
squares = roof × (1 + waste) ÷ 100
bundles = ⌈squares × bundles/square⌉
[('100', 'ft² per square — roofing’s unit'), ('3', 'bundles per square, standard architectural'), ('432', 'ft² per roll of felt underlayment'), ('the law', 'same multiplier the Pitch page derives')]
Worked Example
- Footprint the plan; square it with the pitch.
- Multiply by √(1+(rise/12)²).
- Waste by shape; divide into squares.
- Bundles at 3 (or 4) per square, ceiled.
- Felt at 432; shield at the valleys.
The 1,500-ft² gable at 8/12: × 1.201850 = 1,802.78 ft² → 19.83 squares at 10% → 60 bundles ($2,280.00 at $38) + 5 rolls of felt. The multiplier is the whole geometry.
Strengths & Limits Of This Model
Where this engine is strong
- One multiplier law shared with the Pitch page
- Waste by roof shape, honestly banded
- Felt and shield named on the receipt
Where it stops
- No flashing take-off
- Complex footprints compute in pieces
Practical Use Cases
Re-roofs
Bundles and squares from the plan.
Insurance claims
Squares the adjuster’s math checks.
Steep pitches
The 1.41 truth about 12/12.
Felt orders
Rolls on the same receipt.
Bid audits
Footprint × law against the quote.
Methodology & Editorial Standards
The pitch multiplier √(1+(rise/12)²) — the same law the Pitch page derives — converts footprint to slope area. Squares divide by 100 after the waste chip; bundles ceiling squares × bundles/square. Felt ceils over 432-ft² rolls. Rise above 24/12 is refused as engineered territory.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Roofing Calculator — 10 Expert FAQs
10 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
How many bundles of shingles per square?
Three for standard architectural shingles — each bundle covers a third of a 100-ft² square. Heavier profiles run four. Order squares × 3 and round up.
How do I convert footprint to roof area?
Multiply by √(1+(rise÷12)²): 1.054 at 4/12, 1.118 at 6/12, 1.202 at 8/12, 1.414 at 12/12. The multiplier is the slope’s price in material.
How much waste for shingles?
Ten percent on a simple gable, fifteen with hips and valleys, twenty when the plan is cut up. Starters, ridge cap and the valley cuts ride that chip.
What is a roofing square?
One hundred square feet of roof — the unit every quote, bundle count and insurance figure speaks. A 2,000-ft² roof at 8/12 is 24.04 squares before waste.
How much does a bundle of shingles cost?
It moves with oil and region — enter your supplier quote; this page asserts no band. The count, not the sticker, is the sourced part.
How much felt do I need?
One 432-ft² roll per 4 squares of roof, roughly — the page ceils it. Valleys and eaves add ice-and-water shield as its own rolls.
Can I shingle over existing shingles?
One overlay is sometimes allowed on flat sound grain — but it hides rot, adds weight and shortens life. Tear-off is the honest base; order for the deck you find, not the roof you saw.
Does pitch change labor too?
Steepness changes everything — staging, harnesses, pace. Material multiplies at 1.2-1.4; installation pricing climbs the same ladder, which is why steep quotes read high.
How are valleys handled?
Open valleys flash in metal; closed valleys weave or cut shingles — all of them eat course ends, which is why cut-up roofs carry the 15-20% chips.
When is the roofing season?
Shingles install in most temperatures within the manufacturer’s band — extreme cold cracks self-seal, extreme heat scars granules. The count does not change; the schedule does.