About, and how these numbers are checked

The geometry

Every tool on this site is plain trigonometry applied to a building plan. A roof plane is a rectangle or a triangle that has been folded up along the eave; its sloped area is its plan area multiplied by a slope multiplier, and that multiplier is the hypotenuse of a right triangle with legs of 12 and the rise. Everything else — ridge length, hip length, rafter length, eave perimeter — is another side of the same kind of triangle.

That is why a hip roof calculator can be exact rather than approximate. Two trapezoids plus two triangles, added algebraically, reduce to length × width × multiplier. There is no empirical constant anywhere in it, and no manufacturer's table that can go out of date.

How the numbers are verified

Each published table declares the formula it came from, and the build recomputes every row from that formula before the page is allowed to ship. If a hand-entered multiplier is off by more than a thousandth, the build fails and the page does not go out. This catches the errors that actually happen: a row copied one cell off, a slope entered as 10 when it should be 12, rounding that is inconsistent between columns.

What this check cannot catch is a formula that is wrong in the first place. That is a human review step, and it is why the derivation for each tool is written out on the page rather than hidden behind a script tag. If the derivation looks wrong to you, it probably is — check it and tell us.

What these tools deliberately do not do

• No cost estimates. Material prices vary by region, supplier and week. A dollar figure would be the least reliable number on the page.
• No structural sizing. Rafter spans, ridge beam sizes and nailing schedules depend on species, grade, snow load and local code. Those are engineering decisions, not geometry.
• No single-pitch assumptions hidden. Where a tool assumes one uniform pitch, the page says so, and says what to do instead when your roof is not that shape.
• No accounts, no email capture, no ads. The calculators run entirely in the browser. Nothing you type is sent anywhere.

Using the results

Treat every number here as a geometric quantity to be checked against the material you are actually buying. Shingle coverage per bundle, panel widths, underlayment laps and cap lengths all come from the manufacturer's wrapper or data sheet, and those are the numbers your order should be built on. The calculator tells you how much roof you have; the supplier tells you how their product covers it.

Estimates only. Verify quantities and dimensions against your local building code and, for structural work, a licensed engineer or contractor. RoofGeometry is not responsible for material ordered or work performed on the basis of these figures.