About the Roof Pitch Calculator
A roof's pitch is how steeply it rises. It decides which roofing materials can be used, how much the roof covering will cost, how easy the roof is to work on, and how much usable space there is in the loft. It is also stated in several different ways, which causes a lot of confusion.
Builders and roofers in North America give pitch as rise in 12 — "a 6/12 roof" rises 6 inches for every 12 inches it runs horizontally. Architects and most of the rest of the world use degrees. Engineers and some product literature use a percent grade or a ratio. They all describe the same right-angled triangle of rise and run.
This roof pitch calculator takes a measured rise and run, or an angle, and converts it to every other form, with the pitch factor used to estimate roof area and a note on what the slope is suitable for.
How to Use the Roof Pitch Calculator
Choose how you have measured the roof: rise and run, or angle in degrees.
For rise and run, enter the vertical rise and the horizontal run over which it was measured. Any unit works — inches, millimetres, feet — as long as both are in the same one, because only their ratio matters.
For angle, enter the angle from horizontal, for example from an inclinometer app.
How the Pitch Is Worked Out
rise in 12 = rise ÷ run × 12
angle = atan(rise ÷ run)
percent grade = rise ÷ run × 100
pitch factor = √(1 + (rise ÷ run)²)Step-by-Step Example
A rise of 450 mm measured over a run of 1000 mm.
Rise in 12: 450 ÷ 1000 × 12 = 5.4 in 12
Angle: atan(0.45) = 24.2°
Grade: 0.45 × 100 = 45%
Pitch factor: √(1 + 0.45²) = 1.097The closest common pitch is 5½ in 12. Working the other way, an angle of 30° is 6.93 in 12 — almost exactly a 7 in 12 roof.
Common Pitches
| Pitch | Angle | Grade | Pitch factor |
|---|---|---|---|
| 2 in 12 | 9.5° | 16.7% | 1.014 |
| 4 in 12 | 18.4° | 33.3% | 1.054 |
| 6 in 12 | 26.6° | 50.0% | 1.118 |
| 8 in 12 | 33.7° | 66.7% | 1.202 |
| 10 in 12 | 39.8° | 83.3% | 1.302 |
| 12 in 12 | 45.0° | 100% | 1.414 |
What Each Slope Suits
Below 2 in 12 (about 9.5°) is a flat or low-slope roof. Water drains slowly, so overlapping materials such as shingles and tiles would leak. These roofs use continuous membranes: EPDM rubber, TPO, PVC or modified bitumen.
2 to 4 in 12 can take asphalt shingles, but manufacturers require a double layer of underlayment or a full ice-and-water membrane underneath.
4 to 9 in 12 is the conventional range. Shingles, concrete and clay tiles, slate and metal all work, and the roof can be walked on with care.
Above 9 in 12 (about 37°) is steep. It sheds water and snow quickly and looks dramatic, but roofers need roof jacks, toe boards or harnesses, and labour costs more per square. Many tile and slate systems have their own minimum pitches, so always check the manufacturer's figure.
Measuring Pitch Safely
You do not need to climb onto the roof.
From the loft: hold a spirit level horizontally with one end touching the underside of a rafter. Mark 12 inches (or 300 mm) along the level from the rafter, then measure straight up from that mark to the rafter. That height over 12 is the pitch.
From the gable end: hold a phone inclinometer app, or a digital angle finder, against the edge of the verge or a rafter tail.
From plans: architectural drawings usually state the pitch in degrees, and elevations can be scaled.
Pitch and Loft Space
A steeper roof encloses much more volume. On a 30 ft (9 m) wide house, a 4 in 12 roof rises only 5 ft (1.5 m) at the ridge, while a 12 in 12 roof rises 15 ft (4.5 m) — enough for a loft conversion. That is one reason older houses with steep roofs are popular for loft rooms.
Pitch, Slope and "Pitch" in the Old Sense
Understanding Your Result
Pitch gives the rise in 12 and the angle.
Grade and ratio gives the percent grade and the 1 : n ratio.
Pitch factor is the multiplier from plan area to roof area.
Slope type says what the roof is suited to.
Worth knowing gives the closest common pitch, or a note on rafter cuts.
When Should You Use This Calculator?
Before ordering roofing, to check the materials suit the slope.
Converting plans from degrees to rise in 12, or the reverse.
Designing an extension, to match an existing roof.
With the roofing calculator, which uses the pitch factor for area.
Common Mistakes
Confusing pitch and slope. Rise in 12 is not the same as degrees.
Measuring run along the slope. Run is horizontal.
Mixing units. Rise and run must be in the same unit.
Ignoring the minimum pitch for tiles, slate or shingles.
Guessing from the ground. Measure it.