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Roof slope calculator

Some plans say 6/12. Some inspectors say degrees. Some drainage notes say percent. This page converts between them and gives the slope factor you multiply by footprint to estimate surface area. Prefer rise/run → X-in-12 language? Use the pitch calculator — same math family, different entry words. Neither page is a contractor bid.

in
in
sq ft
× factor ≈ surface
ORDER ESTIMATEROOF SLOPE
Pitch6.0/12
Degrees26.6°
Slope percent50.0%
Slope factor1.118
How it calculates
Normalize to rise/run; pitch on 12 = ratio×12; degrees = atan(ratio); percent = ratio×100; factor = √(1+ratio²); surface ≈ footprint × factor.

Geometry only — not a roofing quote. Pick one path with the pitch page.

When to use

Reading a plan that mixes units; estimating surface area from footprint × factor; checking whether a slope is “walkable” in degree terms (still not safety advice).

When not to

Structural rafter sizing. Multi-plane roofs with hips/valleys as one rectangle — split planes. Buying shingles by the bundle → shingle calculator after you have area.

Assumptions

Slope ratio rise/run; degrees = atan(rise/run); percent = (rise/run)×100; factor = √(1 + (rise/run)²). Optional footprint → surface ≈ footprint × factor. No hip/valley waste here (that lives on roofing/shingle pages).

Worked examples

Example 1

6/12
→ pitch 6-in-12, ≈ 26.6°, ≈ 50% slope, factor ≈ 1.118

Same family as the pitch page — this entry prefers angle/% labels.

Example 2

30° roof, footprint 1,000 sq ft
→ factor ≈ 1.155 → surface ≈ 1,155 sq ft before waste

Add roofing waste on the roofing page.

How it calculates

Enter rise/run, degrees, or percent — read the other units and slope factor.

  1. Pick input mode: rise/run, degrees, or percent.
  2. Optional footprint for surface estimate.
  3. Read pitch, degrees, percent, and factor.
normalize to rise/run
degrees / percent / factor from the ratio
optional surface = footprint × factor

Common traps

  1. Percent vs degrees — 100% slope is 45°, not vertical.
  2. Footprint ≠ shingle order — still add waste, hips, ridges.
  3. Low slope membranes have different product rules than steep asphalt.
  4. Do not double-count pitch page and slope page in a takeoff — pick one path.

Next tool

Rise/run → X/12 focus → roof pitch calculator.

Squares & waste → roofing calculator.

Bundles → shingle calculator.

FAQ

Same triangle; pitch is often X-in-12, slope often degrees or percent.

Related

Estimate only — not a contractor bid or engineered takeoff. Confirm bag yields and local codes before you pour or order.