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Roofing & Siding

Roof Pitch Calculator

Convert a roof's rise and run into the three ways pitch gets quoted: as a ratio, as a percentage, and as an angle in degrees. Useful for ordering materials, checking a spec, or working out whether a roof is safe to walk.

Rise and run

Measure the vertical rise over a fixed horizontal run.

Measurements
Roof pitch is conventionally quoted over a 12 in run, so leaving the run at 12 gives the familiar "6 in 12" form.
Optional
Leave blank if you only want the pitch itself.
Rise over run Run — 12 in Rise 6 in 26.6° Roof slope
Pitch is the rise over a fixed horizontal run — not along the slope.

Your result

Sample values

6:12

Pitch ratio

50.0%

Pitch percentage

26.57°

Pitch angle

1.118

Slope factor

A 6:12 roof is walkable for most roofers in dry conditions.

Common pitches

Ratio Angle
2:12 — low slope9.46°
4:12 — common18.43°
6:12 — common26.57°
8:12 — steep33.69°
12:12 — 45 degrees45.00°

How to use this calculator

  1. Hold a level horizontally against the roof and mark 12 inches along it.
  2. Measure straight down from that mark to the roof surface. That distance is the rise.
  3. Enter both figures. Leave the run at 12 unless you measured over a different distance.
  4. Add the building span if you also want the rafter length.

Measuring from inside

If the roof is unsafe to climb, take the measurement in the loft against a rafter instead. The pitch is the same, and you avoid working at height entirely — worth doing on anything above about 6:12.

Formula

Ratio = Rise : Run

Percentage = (Rise ÷ Run) × 100

Angle = arctan(Rise ÷ Run)

Slope factor = √(Rise² + Run²) ÷ Run

Run
The horizontal distance, conventionally 12 in
Slope factor
Multiply flat area by this to get actual roof area

Percentage and angle are two descriptions of the same slope: 100% is 45°, not vertical.

A 6 in rise over a 12 in run

The standard way pitch is quoted on a residential roof.

  1. Ratio = 6 : 12, spoken as “six in twelve”
  2. Percentage = (6 ÷ 12) × 100 = 50%
  3. Angle = arctan(0.5) = 26.57°
  4. Slope factor = √(36 + 144) ÷ 12 = 1.118

A 1,500 sq ft footprint becomes 1,677 sq ft of actual roof surface.

Understanding your results

The three pitch figures describe the same roof for different audiences. Roofers and material suppliers work in the ratio. Building control and drainage specifications often use percentage. Anyone cutting timber or setting out a frame needs the angle.

The slope factor matters most for ordering

Roof area is always larger than the footprint it covers, and the steeper the roof the bigger the gap. The slope factor is the multiplier: at 6:12 it is 1.118, so a roof covering 1,500 square feet of ground has about 1,677 square feet of surface to cover. Ordering shingles against the footprint is one of the most common ways to come up short.

Pitch and material choice

Most asphalt shingles are rated down to 4:12 in standard application, and between 2:12 and 4:12 only with a doubled underlayment. Below 2:12 you are into membrane roofing rather than shingles. If your result lands near one of those thresholds, check the manufacturer's installation instructions before ordering — the warranty usually depends on it.

Questions

About this calculator

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As a rough guide, up to about 6:12 most roofers will walk in dry conditions with suitable footwear. From 7:12 to 9:12 usually needs roof jacks or a harness, and anything steeper is scaffold and fall arrest work. Wet, mossy or frosted surfaces drop those thresholds sharply.

Horizontally. The run is the flat distance covered, not the length of the rafter. Measuring along the slope by mistake gives a shallower pitch than the roof actually has.

In everyday use they are treated as the same thing, and this calculator follows that. Strictly, some references define pitch as rise over the full span and slope as rise over run, which halves the pitch figure. If you are reading an older drawing, check which convention it uses.

Because percentage is rise divided by run, not a proportion of vertical. When rise equals run the result is 100%, which is a 45 degree roof. A vertical wall has an infinite percentage, which is why the ratio and angle forms are more practical at steep pitches.

Multiply half the building span by the slope factor, then add the eaves overhang. Enter the span above and the calculator will do it for you. Cut lengths still need the ridge board thickness deducted and the birdsmouth set out.

Keep going

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Estimates, not specifications. Results are based on the values you enter and on typical material properties. Densities, coverage rates and product sizes vary between suppliers, so confirm quantities against your supplier's data before ordering. Structural work must be signed off by a qualified professional and meet your local building code.