What Your Results Mean
Everything starts with the footprint: the outline of the building as seen from directly above, measured exterior wall to exterior wall including the eaves' overhang, because the roof extends past the walls. Walk the perimeter with a tape, or pace it — a calibrated pace is accurate to about 3 percent, which is fine for this job. Attached garages and porches count if they're under roof; on a two-storey house, use the largest roofed outline, not the sum of the floors.
Pitch is stated as rise over 12: a "6/12" roof climbs 6 inches for every 12 inches of horizontal run. You don't need a ladder to read it — in the attic, hold a level against a rafter, mark 12 inches along it, and measure the vertical drop at the mark. That drop is your pitch's first number. The multiplier converts your flat footprint into actual sloped surface; it's the hypotenuse of the rise/run triangle, not a fudge factor. Below 6/12 pitch barely matters; above 8/12 it matters a great deal.
Shape decides two things: where the cap line runs, and how much waste the cuts generate. A gable roof has two flat faces, a ridge running the length of the house, and raked edges at the gable ends — cap material follows the ridge only. A hip roof slopes on all four sides, replacing the gable ends with angled hip lines; every one of those lines needs capping, and every one of them generates angled cuts, which is why hip waste defaults to 15 percent against the gable's 12.
Reading the Order
- Squares: The headline number. One square is 100 square feet of roof surface; suppliers bundle and price around it. The calculator shows the exact figure (waste included) and the rounded-up count to order.
- Shingle Bundles: Squares × bundles per square, rounded up — three bundles per square covers most three-tab and architectural lines; heavier architectural products can run four or five. The bundle label states its coverage; when in doubt, order by squares and let the supplier do this multiplication.
- Underlayment Rolls: Squares divided by the coverage printed on the roll. Classic #15 asphalt felt covers about four squares per roll, #30 about two, and synthetics range from five to ten or more — the wrapper states it, and the input accepts whatever it says.
- Cap Bundles: The ridge and hip line, divided by roughly 33 feet of coverage per bundle when caps are cut from three-tab shingles at a 5-inch exposure. Specialty ridge-cap products often cover less — check the stated coverage before cutting the order.
- Drip Edge & Starter: Linear feet along eaves and rakes. Drip edge metal comes in 10-foot pieces (the piece count is shown); starter strip runs the same edges so the first course of shingles has something sealed to seal against.
- Nails: About 320 per square at four nails per shingle, plus the nails that fasten the caps — roughly 5 per linear foot of ridge and hip. High-wind zones require six nails per shingle; add half again if your area is mapped for them.
What the Calculator Does Not Do
This tool prices materials by quantity, not dollars — shingle prices swing by region and product line, and quotes are the supplier's job. It models one clean rectangle: for L-shaped or cut-up roofs, run each section separately and add the results, then bump the waste allowance toward 20 percent for the valleys and transitions. It also does not size flashing, ice-and-water membrane at eaves (a rolls-per-linear-foot purchase your climate may require), or ventilation — ridge vents are priced per foot of ridge, which the ridge number above gives you. For a binding number, hand your squares figure to two suppliers and let them compete on it.
Order the rounded-up square count, not the decimal. The last square of rounding is the cheapest insurance in construction — unopened bundles go back to the supplier, while a mid-roof shortage costs a second delivery charge and a crew standing around. The roof measurement guide covers the full method behind every number here.