Roof Pitch Explained: How to Measure, Calculate & Convert Roof Pitch

Roof pitch is a ratio: how far a roof rises for every 12 it runs. Degrees, percentage slope and the multiplier that turns a footprint into roof area are that one ratio written other ways. This guide shows how to measure it, calculate it and convert it, with the arithmetic on the page.

CB·02 roofing·ContractorBuilds Editorial·Published 2026-09-15 · Updated 2026-09-15·14 min read

✓Reviewed 2026-09-14 — independent technical, source and arithmetic review; every chart value derived independently and every worked result reproduced in the named calculator

What roof pitch is, how to measure it on a real roof, and how to turn rise and run into X/12, degrees, percentage slope and the multiplier that gives roof area.

01Quick answer

Roof pitch is rise per 12 of run: a roof that rises 6 in over 12 in of horizontal run has a 6/12 pitch. Rise ÷ run is the slope as a decimal (6 ÷ 12 = 0.5); × 100 is percentage slope (50 %); atan gives the angle (atan 0.5 = 26.57°); and √(1 + 0.5²) = 1.1180 is the multiplier that turns plan area into sloped roof area. From the pitch, the angle or the percentage slope, the calculator gives you the rest.

Convert your roof pitch now

02What is roof pitch?

Roof pitch is how steep a roof is: its vertical rise compared with its horizontal run. It is a pure ratio — no unit survives the division — so inches, feet or metres give the same pitch.

In US customary practice the run is fixed at 12, so a roof is "4/12", "6/12" or "9/12", and pitches compare at a glance: 6/12 has exactly half again the rise of 4/12. The same ratio can be written as a decimal (6 ÷ 12 = 0.5), as an angle from the horizontal, or as a percentage slope; this guide converts between all four.

"Slope" means the same ratio in most roofing contexts. Some older references define "pitch" as rise over the whole span, which halves the number; this guide and the calculators always mean rise per 12 of run.

03Rise, run and X/12 notation

Three words:

• Rise — the vertical height the roof gains, measured straight up.
• Run — the horizontal distance over which it gains that height, measured level.
• Rafter line — the sloped distance, the hypotenuse of the triangle the other two make.

What is a 6/12 pitch? Six of rise for every twelve of run — and "6/12", "6:12" and "6-in-12" are all that one notation. The rise is always per 12 of horizontal run — never per foot of rafter, never per 12 of span. A tape laid along the roof surface measures the rafter line, which is longer than the run, and dividing a rise by it understates the pitch.

Span versus run

On a symmetric gable the span is the full width wall to wall and the ridge sits over the middle, so each side's run is half the span. On a shed roof or an unequal gable, the run is the horizontal distance from the low wall to the high point of that plane.

04How to measure roof pitch

The pitch of a plane is the same everywhere on it, so each method below measures the same number. Roof-surface measurement is for people equipped and permitted to be on a roof; the attic and gable-end methods need no roof access.

In the attic, on a rafter

Hold a 12 in level against the underside of a rafter, one end touching the wood, bubble centred. Measure straight up — vertically, not perpendicular to the rafter — from the 12 in mark to the underside of the rafter. That reading in inches is the rise per 12: 6 in is 6/12.

On the roof surface

The same method works on the roof: level horizontal with one end on the shingles, tape reading the vertical drop at the 12 in mark. With a 24 in level, read the drop at the 24 in mark and halve it.

From the ground at a gable end

At a gable end you can see the whole triangle. Take the rise and the run between two points on the same sloped line — for example the underside of the roof edge at the wall line and at the peak — as a vertical and a horizontal distance (the run is half the span on a symmetric gable), in the same unit. Measuring from the top of the wall to the top of the ridge mixes two lines and overstates the pitch. The pitch is rise ÷ run × 12, worked out in the next section.

With a phone inclinometer

A phone's level app on the roof surface or a rafter reads the angle in degrees; the conversion back to X/12 is below.

05How to calculate roof pitch from rise and run

Divide the rise by the run for the slope as a decimal, then multiply by 12 to write it per 12 of run. Take a roof that rises 6 in over a 12 in run — the example this guide follows throughout.

Reproduce this in the Roof Pitch Calculator with mode Rise and run, rise = 6 in, run = 12 in (or 1.5 m and 3 m): pitch 6:12, angle 26.57°, grade 50 %, slope factor 1.1180.

When the result is not whole — 5 ft over 14 ft is 0.3571… × 12 = 4.2857… per 12 — the trade, and the calculator, write it to the nearest eighth: "4 1/4:12".

From total rise and span

Halve the span first: a 24 ft span with a 6 ft rise is a run of 12 ft, 6 ÷ 12 = 0.5 → 6/12. The calculator's Rise and span mode does the halving and says so.

06How to convert roof pitch to degrees

The angle from the horizontal is the inverse tangent of the slope: angle = atan(rise ÷ run).

Reproduce this in the Roof Pitch Calculator, mode Rise per 12, rise per 12 = 6: angle 26.57°.

From degrees back to pitch

Reverse it with the tangent: rise per 12 = 12 × tan(angle). An inclinometer reading of 30° is 12 × tan 30° = 12 × 0.57735… = 6.9282… per 12; the trade rounds that to the nearest eighth, 6 7/8:12, which is the pitch the calculator's Angle in degrees mode shows. The angle is measured from the horizontal: a 45° roof is 12/12, and 90° would be a wall.

07Roof pitch vs degrees vs percentage slope

Three notations, one ratio — and they do not line up the way intuition expects.

Reproduce this in the Roof Pitch Calculator, mode Rise per 12, rise per 12 = 6: grade 50 %; the calculator calls percentage slope "grade".

Percentage slope is rise over run, not a fraction of 90°: 100 % is a 12/12 roof, which is 45°, not a wall. Doubling the pitch from 6/12 to 12/12 doubles the percentage but not the angle (26.57° → 45°), because the tangent is not a straight line; in the chart below each 1/12 step adds a little less angle than the last.

08Roof pitch chart

Every value here is computed from this guide's formulas — slope = rise ÷ 12, angle = atan(slope), slope % = slope × 100, multiplier = √(1 + slope²) — at the calculators' display precision. If another chart differs in the fourth decimal, trust the formula.

Calculated from the formulas above; values shown at the calculators' display precision.

The multiplier is not the pitch: 6/12 is "half" as a ratio, but the surface is only 11.8 % longer than the plan — it is the hypotenuse, not the rise.

09What is a roof pitch multiplier?

The roof pitch multiplier converts a plan (horizontal) measurement into the same measurement along the slope. It is Pythagoras: for a run of 1 and a rise of (rise ÷ run), the rafter line is √(1² + (rise ÷ run)²).

multiplier = √(1 + (rise ÷ run)²)

Reproduce this in the Roof Pitch Calculator, mode Rise per 12, rise per 12 = 6: slope factor 1.1180.

The same number is called the slope factor, pitch factor or roof slope multiplier; the calculators say slope factor, and the Roof Pitch Calculator prints it as √(rise² + 12²) ÷ 12, the same expression with the 12 inside the root: √180 ÷ 12 = 1.1180.

The hip and valley factor is not the multiplier. A hip or valley rafter runs diagonally across two planes rather than up one, so it has its own larger factor (1.5 at 6/12) that converts a plan run into hip or valley length; the Roof Pitch Calculator reports both.

10How roof pitch affects roof area

A roof is larger than its footprint by exactly the multiplier: every plan length up the slope becomes that length × multiplier, lengths along the eave are unchanged, so the area scales by the multiplier once:

sloped area = plan area × multiplier

Take a roof plan — footprint including overhangs — of 1,000 ft² under a 6/12 roof.

In roofing squares of 100 ft², that is 11.18 squares.

Reproduce this in the Roof Area Calculator: gable preset, 25 ft × 40 ft footprint, no overhangs, pitch 6/12 — sloped area 1,118.03 ft², 11.18 squares.

The multiplier does not care about roof form — a gable and a hip over the same plan area at the same pitch have the same sloped area; only the edges differ — and the plan area must include the overhangs, because the roof covers them too.

Get your roof area

11Worked examples at 3/12, 4/12, 6/12, 8/12 and 12/12

The same formulas, four more pitches, unrounded until the display step.

Reproduce each in the Roof Pitch Calculator, mode Rise per 12, rise per 12 = 3, 4, 6, 8 or 12.

At 3/12 a roof is about 3 % larger than its footprint; at 4/12 about 5 %; at 6/12 about 12 %; at 12/12 over 41 % — which is why pitch matters to a material order.

12Common roof-pitch measurement and calculation mistakes

Measuring the run along the rafter. The rise is vertical; a tape along the roof surface reads the longer rafter line and gives too shallow a pitch. Level horizontal, tape vertical.

Using the span as the run. A 6 ft rise over a 24 ft span is 6 ÷ 12 = 6/12, not 6 ÷ 24 = 3/12. Halve the span first, or use the rise-and-span mode.

Reading a 24 in level as if it were 12. A 12 in drop at the 24 in mark is 6/12, not 12/12. Halve the reading.

Mixing degrees and percent. 50 % is not 50°, and 100 % is 45°, not vertical: slope % = rise ÷ run × 100; angle = atan(rise ÷ run).

Rounding the multiplier before multiplying. 1.12 × 1,000 ft² = 1,120 ft², but 1.1180339… × 1,000 ft² = 1,118.03 ft² — the rounded figure adds about 2 ft² for every 1,000 ft² of plan at 6/12. Carry the full multiplier and round the answer.

Using the hip and valley factor as the multiplier. At 6/12 the slope factor is 1.1180 and the hip/valley factor is 1.5; the second applied to an area overstates it by about 34 %. Area uses the slope factor; hip and valley lengths use the hip/valley factor.

Treating the calculated pitch as a suitability answer. A pitch says how steep a roof is — not whether a product may be installed on it, whether it can be walked, what it does under snow or wind, or whether the framing is adequate. Those depend on the product's instructions, the loads, the code and the jurisdiction.

Where pitch thresholds come from

Pitch limits are not derived from the geometry; manufacturers and codes write them. Two examples of how such limits are stated: Owens Corning's installation instructions for its Oakridge and TruDefinition Oakridge shingles treat 2:12 up to under 4:12 as a low-slope range requiring additional underlayment, 4:12 and steeper as standard, and do not cover application below 2:12 [1] (Manufacturer specification); and the 2021 International Residential Code provides that where the roof pitch is less than 3:12, the members that support rafters — such as ridges, hips and valleys — are designed as beams [2] (Code reference). Two different rules from two different documents; neither is a property of the pitch itself. Reference checks are informational, not compliance determinations.

13Which ContractorBuilds calculator should I use?

Roof Pitch Calculator — pitch, angle and percentage slope from whichever you have, or from rise and run or rise and span, plus the slope factor and hip/valley factor.

Roof Area Calculator — the multiplier applied to a gable, hip, shed or flat footprint, or plane by plane: sloped area, squares and edge lengths.

Rafter Calculator — the geometry continued into a common rafter: line length = run × slope factor (12 ft at 6/12 → 13.42 ft, displayed 13' 5"), then ridge deduction, overhang and the cuts.

Roofing Calculator — the full gable takeoff, footprint and pitch to bundles.

14FAQ

15Sources, methodology and how we label facts

Every number in this guide is either derived in front of you — the formula and arithmetic are on the page, and the named calculator reproduces the result — or labelled with where it comes from. The relationships between rise, run, angle, percentage slope and the multiplier are mathematical facts; nothing in the chart is read from a published table. How the trade measures and writes pitch — level and tape, X/12, eighths — is convention, worded as convention, never as a rule. The two thresholds above are a manufacturer's instruction and a code reference, marked and cited as such; they are examples of where such limits live, not limits this guide sets.

The "How we label facts" box explains the provenance chips used across ContractorBuilds; for how the calculators derive and label every figure, see our methodology.