Soffit and fascia calculator

Fascia linear feet from the roof edge — split into horizontal eaves and pitch-corrected rakes — plus eave soffit area, board count, and panel count. Gable or hip, imperial or metric.

01 How the math works

Fascia runs along the roof edge, so its length is the roof-edge perimeter — but not every edge is horizontal. The eaves (the edges along the side walls) run level; the rakes (the edges up a gable end) run on the slope. A sloped rake is the hypotenuse of its horizontal run, so its true length is the run multiplied by the pitch factor.

The pitch factor is √(12² + rise²) ÷ 12 — 1.1180 at 6/12, 1.2019 at 8/12. Overhang is the horizontal distance the roof projects beyond the wall face, which is also the width of the soffit under the eave. A gable roof has horizontal soffit on its two eave sides only; a hip roof wraps soffit around all four, so its soffit area is the exact overhang band, (L + 2·oh)(W + 2·oh) − L·W.

02 Worked example

Take a rectangular 40 ft × 28 ft single-story house (12.19 m × 8.53 m), gable roof at 6/12 pitch with the ridge running the 40 ft length, and a 12 in (305 mm) overhang all round.

The two eaves run 2 × (40 + 2 × 1) = 84 lin ft (25.60 m), level. The four rakes each have a horizontal half-width of 28 ÷ 2 + 1 = 15 ft; on the 6/12 pitch factor of 1.1180 that's 4 × 15 × 1.1180 = 67.08 lin ft (20.45 m) of sloped rake. Total fascia: 151.08 lin ft (46.05 m). A flat footprint-perimeter estimate would call it 144 ft — 7.08 ft short, because it treats the rakes as level.

Eave soffit is on the two eave sides only, 12 in wide: 2 × 42 × 1 = 84 ft² (7.80 m²). The same house with a hip roof would carry soffit on all four sides — (42 × 30) − (40 × 28) = 140 ft² (13.01 m²), two-thirds more. Fascia at 16 ft stock with 10% waste is ceil(151.08 × 1.10 ÷ 16) = 11 boards; soffit at 12 ft² per triple-4 panel with 10% waste is 8 panels.

03 When this calculator is wrong

The result is a clean-rectangle takeoff with a uniform overhang. Real rooflines break that assumption in a few specific ways, and the pitch factor is the single most underestimated input in DIY roofing math — the roof edge is always longer than the footprint by the pitch factor, and estimates that skip it under-order by 10–30%.

• It assumes one overhang everywhere. Many houses run a wider eave overhang than rake overhang (or a flush rake with no overhang at all). If your eave and rake projections differ, run the eave and rake numbers separately and add them.
• It counts eave soffit only. A closed or "flush" rake has fascia but no soffit; a boxed rake return does. The soffit area above is the horizontal eave soffit — the part almost every roof panels. Boxed rake overhangs add sloped soffit this figure doesn't include.
• Dormers, valleys, and returns add edge. Every dormer cheek, gable return, and porch tie-in adds fascia and soffit the footprint never shows. On a cut-up roofline, step the waste factor up from 10% toward 15%, the same way roofing and siding scale with complexity.
• Panel coverage is a manufacturer figure. The 12 ft² default is a common triple-4 vinyl soffit panel (12 in coverage × 12 ft). Coverage width and length vary by profile — read the carton and change the input before you order.

04 What to do with the result

Order fascia by the linear foot at the longest stock length that fits your run — splices land on the rake and at inside corners, and fewer of them means fewer seams to seal. Buy the total-with-waste length, not the raw perimeter: a fascia run finished short leaves a gap right where water runs off. For soffit, figure the eave soffit area, add waste, round up to whole panels, and confirm the carton's coverage matches the 12 ft² default before you commit. If the roof has a hip on one end and a gable on the other, run each end on its matching roof type and sum the two.

05 Common questions

How do you calculate linear feet of fascia?

Measure the roof edge, not the wall. Eaves run level at the building length plus twice the overhang; rakes run up the gable on the slope, so multiply their horizontal run by the pitch factor (1.1180 at 6/12). A gable roof is two eaves plus four rakes; a hip roof is the full level perimeter with no rakes.

What's the difference between soffit and fascia?

Fascia is the vertical board that caps the ends of the rafters or trusses along the roof edge — it carries the gutter. Soffit is the horizontal underside that closes the gap between the fascia and the wall. Fascia is counted in linear feet; soffit is counted by area (eave run × overhang width).

How wide is the soffit?

The soffit width equals the horizontal overhang — the distance the roof projects beyond the wall face, typically 12–24 in (305–610 mm) on residential work. Measure from the wall to the back of the fascia; that's the figure to enter as overhang.

Why is rake fascia longer than the wall it runs over?

Because the rake is sloped. A gable end that's 28 ft wide isn't 28 ft of rake — the two rakes climb to the ridge, so each is its horizontal run times the pitch factor. At 6/12 that's 11.8% longer than the flat distance; skip it and the rake fascia comes up short.

How many soffit panels do I need?

Divide the eave soffit area (with waste) by the coverage per panel. A common triple-4 vinyl panel covers about 12 ft² (12 in wide × 12 ft long), so the 84 ft² eave soffit in the example above takes 8 panels at 10% waste. Coverage varies by product — read the carton.