How Many Solar Panels Fit on My Roof? Roof Layout Calculator

How many solar panels fit on my roof? Subtract the required clearances from each roof face, see how many panel rows and columns fit in portrait and landscape, then remove space for vents and skylights. With the defaults, a 40 by 18 foot face with 3 feet of clearance holds 16 panels of 400 W, or 6.4 kW.

How to use the roof calculator

Measure one roof face at a time. A face is one flat plane of the roof, such as the south side of a gable roof.

1. Roof face length (along the eave). The horizontal length of that face. Default 40 ft.
2. Roof face depth (eave to ridge). Measured along the slope, not the floor plan. Default 18 ft.
3. Clear space at edges. Applied to every side as a simple stand-in for fire-code pathways and setbacks. Default 3 ft.
4. Share free of vents and skylights. Reduce it for chimneys, plumbing vents, skylights and shade. Default 90%.
5. Panel wattage. From your quote. Default 400 W.
6. Advanced: Panel length and Panel width. From the spec sheet. Defaults 5.75 ft by 3.6 ft.

The results show the panels that fit, the system size on this face and the counts for portrait and landscape layouts, with a bar chart comparing the two. A scenario table reruns your roof with 1.5 ft of clearance, your own clearance and 4 ft of clearance.

How the calculation works

The tool shrinks the roof face by the clearance on all four sides, packs panels in a simple grid with a 1-inch gap, tries both orientations and keeps the better one.

Usable length = roof length − 2 × clearance
Usable depth  = roof depth − 2 × clearance
Fit(a, b)     = floor((usable length + gap) ÷ (a + gap)) × floor((usable depth + gap) ÷ (b + gap))
Portrait      = Fit(panel width across, panel length up the slope)
Landscape     = Fit(panel length across, panel width up the slope)
Panels        = floor(max(portrait, landscape) × usable share)
System kW     = panels × panel watts ÷ 1,000

Assumptions: a rectangular face, a uniform clearance on every side, a 0.08 ft (about 1 inch) gap between panels, no mixed orientations, and obstacles handled as a percentage rather than placed on the roof.

Worked example

Default inputs: 40 ft by 18 ft face, 3 ft clearance, 90% usable, 5.75 by 3.6 ft panels, 400 W.

• Usable area: 34 ft by 12 ft.
• Portrait: 9 panels across × 2 rows = 18. Landscape: 5 across × 3 rows = 15.
• Panels that fit: 18 × 90% = 16.2, rounded down to 16 panels, or 6.4 kW in portrait.

Second example, a smaller ranch roof: 34 ft by 16 ft, 1.5 ft clearance, 85% usable, 430 W panels of the default size.

• Usable area: 31 ft by 13 ft. Portrait fits 16 and landscape 15.
• Panels that fit: 16 × 85% = 13.6, rounded down to 13 panels, or 5.59 kW.
• With 4 ft of clearance, the same face drops to 6 panels (2.58 kW).

How clearance changes the default roof

Clearance has a big effect because it removes space from all four edges, and a lost row of panels is a lot of capacity.

What your result means

Compare the kW that fits with the size you need from how many solar panels do I need. If one face is not enough, add other faces that get good sun. The median US home system installed in 2025 was 7.7 kW (LBNL, 2026 (opens in a new tab)), so many homes use two faces.

If roof space is tight, higher-wattage panels raise kW per square foot. Our guide to how much solar panels cost explains how that affects price, and are solar panels worth it covers whether a smaller system still pays off.

Rules that change the answer

• Fire-code access. Many counties adopt IRC R324.6: at least two 36-inch pathways from eave to ridge, and ridge setbacks of 18 inches when the array covers up to 33% of the roof or 36 inches above that, with more room allowed for sprinklered homes (Leon County (opens in a new tab); Spokane County (opens in a new tab)). Local amendments vary.
• Orientation. DOE says south-facing roofs with a 15 to 40 degree slope generally perform best (DOE (opens in a new tab)). Check your pitch with the solar panel angle calculator.
• Hot climates. In places like Phoenix, an east or west face can still produce well; see solar panels in Arizona.
• HOA rules. Some states limit HOA restrictions on solar, but placement rules may still apply.

Common mistakes

• Measuring depth from the floor plan instead of along the slope.
• Counting a shaded face as usable.
• Forgetting that pathways and ridge setbacks remove whole rows.
• Assuming panels can run to the roof edge.
• Using one roof face's result as the whole-house answer.

Limits of this calculator

This is an educational layout estimate, not a design or code review. It assumes a rectangular face and a uniform clearance, and it does not place obstacles, check structural capacity, wind and snow loads, or your local fire department's exact rules. An installer's site survey and the permit drawings decide the final count.