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Roof Pitch Calculator

Aniruddh

September 17, 2025

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Roof Pitch Calculator — Convert Rise & Run to Angle, Percent, and Rafter Length

Use this roof pitch calculator to turn quick measurements into clear numbers. Enter rise and run, or pair run with the roof angle, then see pitch in degrees, percent slope, and the rafter length you need. The calculator follows right-triangle trigonometry with plain-English outputs so you can plan materials, estimate time, and talk to your roofer without confusion.

Why roof pitch matters

Pitch affects everything. Water shedding, snow loads, rafter length, even how your home looks from the street. A steeper roof sheds rain faster. A lower slope works with certain membranes. Builders also price jobs by pitch because safety gear and time both change.

You don’t need a framing square and a notebook to get these numbers. The Roof Pitch Calculator translates your measurements into meaningful outputs in seconds. It also standardizes how you communicate with suppliers and contractors.

How the Roof Pitch Calculator works

The calculator treats your roof face as a right triangle. The run is the horizontal distance from wall plate to ridge. The rise is the vertical height. The rafter is the hypotenuse. The angle at the wall plate gives the roof pitch in degrees. With any two of rise, run, and angle, you can solve the rest.

Choose one of three input modes to keep things simple:

• Rise + Run → get angle, rafter length, percent slope, and “x:12” pitch.
• Run + Angle → get rise, rafter length, percent slope, and “x:12”.
• Rise + Angle → get run, rafter length, percent slope, and “x:12”.

Units stay flexible. Pick metric or imperial. Prefer feet and inches as separate fields. The calculator supports both styles and converts behind the scenes so your results stay consistent.

Formulas we use

Everything rests on elementary trigonometry. If you like to peek under the hood, here are the equations:

• Slope \(s = \dfrac{\text{rise}}{\text{run}}\)
• Angle \( \theta = \arctan(s) \)
• Rafter length \( L = \sqrt{\text{rise}^2 + \text{run}^2} \)
• Percent slope \( \% = 100 \times s \)
• Pitch (x:12) \( x = 12 \times s \)

These relationships come from right-triangle trigonometric functions. For a refresher see Trigonometric functions. For the pitch convention “rise in inches per 12 inches of run” see Roof pitch.

Visual guide to rise, run, angle, and rafter

Degrees vs percent vs x:12 — what each means

Different trades favor different formats. Architects speak in degrees. Roofers often use “x:12”. Site crews like percent because it reads like grade.

• Degrees show the angle of the roof relative to level ground. A 26.565° roof equals a 6:12 pitch.
• Percent slope equals rise divided by run then multiplied by 100. A 6:12 roof equals 50% because 6 ÷ 12 = 0.5.
• Pitch x:12 shows inches of rise for every 12 inches of run. This is common in North America for framing and shingle specs.

Swap units freely in the calculator. The outputs update instantly so you can communicate with any stakeholder without doing mental math.

Step-by-step: measure and calculate

1. Pick your mode. For a quick retrofit estimate use Run + Angle since you can read angle with a digital level while standing safely.
2. Measure run. Measure horizontally from the inside face of the exterior wall to the centerline of the ridge. For a full span divide by two.
3. Measure rise or angle. Rise needs a vertical reading from the top plate to the ridge line. Angle comes from a level or inclinometer placed on a rafter or roof deck.
4. Enter values. Leave fields empty unless you have a number. Choose units that match your tape and tools.
5. Read the results. Note the rafter length and the pitch. Save a screenshot for your material takeoff.

Worked examples you can copy

Example 1 — Rise + Run → angle, rafter, and pitch

You measured a rise of 1.5 m and a run of 3.0 m.

• Slope \( s = 1.5 / 3.0 = 0.5 \)
• Angle \( \theta = \arctan(0.5) \approx 26.565° \)
• Rafter \( L = \sqrt{1.5^2 + 3.0^2} \approx 3.354\,\text{m} \)
• Percent = \( 100 \times 0.5 = 50\% \)
• Pitch x:12 = \( 12 \times 0.5 = 6:12 \)

This roof is a classic 6:12. Shingle specs that list a minimum of 4:12 handle this pitch comfortably.

Example 2 — Run + Angle → rise and rafter

Your addition needs a run of 8 ft and you want a 30° roof.

• Rise \( = \text{run} \times \tan(30°) \approx 8 \times 0.57735 = 4.619\,\text{ft} \)
• Rafter \( = \text{run} / \cos(30°) \approx 8 / 0.86603 = 9.237\,\text{ft} \)
• Pitch x:12 \( = 12 \times \tan(30°) \approx 6.928:12 \)
• Percent \( = 100 \times \tan(30°) \approx 57.735\% \)

Order rafters a touch long and trim to fit. Field conditions rarely land at perfect math.

Example 3 — Rise + Angle → run and rafter

You know your attic height gain must be 900 mm and you like a 35° angle.

• Run \( = \text{rise} / \tan(35°) \approx 900 / 0.70021 = 1{,}285.6\,\text{mm} \)
• Rafter \( = \text{rise} / \sin(35°) \approx 900 / 0.57358 = 1{,}569.1\,\text{mm} \)
• Pitch x:12 \( = 12 \times \tan(35°) \approx 8.402:12 \)
• Percent \( \approx 84.02\% \)

This example shows why angle selection matters. A few degrees can swing your rafter length by hundreds of millimeters.

Quick conversion table

Use this responsive table when you need a fast estimate. Values round to two decimals.

Pro tips for accurate measurements

• Zero your level. Calibrate before you climb. A one-degree error throws rafter length off on long spans.
• Measure run on framing. Measure from the inside face of the plate to the ridge centerline. Tape across air, not along the deck.
• Account for birdsmouth. Framing cuts change practical length. Math gives you theoretical center-to-center. Cut layout handles seats and plumb cuts.
• Watch units. Record inches or centimeters consistently. Mixed units sneak errors into takeoffs.
• Work safely. Use a harness on steep slopes. Move methodically. Numbers can wait.

Roof Pitch Calculator FAQ

What is roof pitch in simple terms?

Roof pitch describes how much a roof rises for a given horizontal run. A 6:12 roof rises six inches for every twelve inches of run. The same roof measures about 26.565° in angle and 50% in slope.

Is run the same as span?

No. Span equals the full building width from outside wall to outside wall. Run equals half of that measured to the ridge line. Use run inside the calculator to avoid doubling your rafter length by mistake.

How do I convert pitch to degrees without a calculator?

Use the tangent relationship. For a 6:12 roof compute \( \arctan(6/12) \). The result is about 26.565°. The conversion table above lists common values if you need a quick answer.

How accurate is the rafter length?

The math is exact for the geometry you input. Real framing adds allowances for ridge thickness, seat cuts, and overhangs. Order rafters slightly long then cut to final fit.

Can I switch between metric and imperial?

Yes. Enter numbers in centimeters, meters, inches, or feet. The outputs remain consistent because everything converts to a single internal unit system during calculation.

What about hip or valley rafters?

The calculator shows common rafter length. Hip and valley rafters run longer due to plan angle. Many framer’s guides include multipliers for those members. Calculate the common first then apply plan-angle factors as required.

Does a steeper pitch always mean better drainage?

Steeper roofs shed water and snow faster. Drainage also depends on surface material, underlayment, and climate. Local codes and manufacturer instructions take precedence when you choose a final pitch.

How to use the Roof Pitch Calculator on this page

Open the calculator above. Pick the input mode that matches your measurements. If you see two fields for feet and inches or meters and centimeters, use them when you want exact fractional lengths without converting in your head.

• Enter Rise + Run to solve for angle and rafter length on existing roofs.
• Enter Run + Angle to design an addition around a target look.
• Enter Rise + Angle to meet a height constraint like a dormer window or code limit.

Every result updates as soon as you type. Tap Reset when you want to start a new scenario. On phones the input fields stack with large tap targets so you can work safely with one hand.

Common roof pitches and where you’ll see them

Builders develop preferences by climate and material. You’ll often see:

• Low slopes (2:12 to 4:12). Great for modern lines. Works with qualified membranes and standing seam metal when details follow manufacturer guidance.
• Moderate slopes (5:12 to 8:12). A sweet spot for shingles and many regional styles. Comfortable balance of buildability and drainage.
• Steep slopes (9:12 and up). Strong visual impact. Extra staging and time. Snow slides quickly.

Match pitch with your cladding and local practice. Manufacturers publish installation limits for each product. Check their datasheets before you order.

Behind the scenes: why the calculator returns consistent results

As you change units the math stays stable. The calculator converts every length into meters internally then returns the results in your chosen unit. Angles convert between degrees and radians using fixed constants. You can mix input styles without losing precision.

Precision defaults to a few decimal places so numbers stay readable. Rounding may cause tiny differences when you compare with a longhand calculation. The underlying values still trace back to the same trigonometric identities.

Accuracy checklist before you hit “Order”

• Confirm whether your run measurement includes ridge thickness. If not, adjust rafter length after framing layout.
• Use the same reference for each dimension. Inside face to ridge for run. Plate top to ridge for rise.
• Write a note about overhang. The calculator solves the structural triangle. Eaves add length beyond the wall line.
• Check that your angle reading came from the same roof face you measured for run. Mixing faces introduces errors on asymmetrical designs.

Troubleshooting odd results

• Angle looks tiny. You may have entered span instead of run. Divide span by two and try again.
• Rafter length looks short. Confirm that you measured the horizontal run. If you used the roof deck length by mistake the calculator will under-estimate.
• Percent exceeds 100. That can happen on very steep roofs. Anything steeper than 12:12 reads above 100%.

Why builders still talk in “x:12”

The convention grew from carpentry practice. Inch marks on a square make layout quick. A 6:12 line on a square sets the plumb cut without any calculator. The method survived because crews still carry squares. Our calculator translates that old shorthand into angles and lengths suited to drawings and ordering systems.

When a small change in pitch makes a big difference

Consider a 24-ft span with a 4:12 pitch. Common run equals 12 ft. Rafter length computes to \( \sqrt{(4/12 \times 12)^2 + 12^2} \) which simplifies to \( \sqrt{4^2 + 12^2} = 12.649 \) ft per side. Bump the pitch to 6:12. The rafter grows to \( \sqrt{6^2 + 12^2} = 13.416 \) ft. That extra 0.767 ft per rafter multiplies across a whole roof. Waste planning matters.

Glossary

• Rise — vertical height from the plate to the ridge measured perpendicular to level ground.
• Run — horizontal distance from the plate to the ridge line.
• Rafter — sloped structural member running from plate to ridge.
• Pitch (x:12) — inches of rise per 12 inches of run.
• Percent slope — slope expressed as a percentage.

The Roof Pitch Calculator turns shop-floor measurements into crisp numbers you can act on. You get degrees for drawings, “x:12” for the crew, and precise rafter lengths for ordering. Bring it to site. Use it during design. Save it for the next project. Simple inputs produce dependable results every time.