How many panels fit on my roof?

Enter an area and a module size and see the count, the array size, and portrait against landscape — with gaps placed between panels, which is where most quick estimates go wrong.

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How many panels fit?

A rectangular area, a rectangular module, and honest arithmetic about the gaps.

The surface

Setbacks — space you must leave clear

Defaults to zero and warns you, because these distances are set locally and no calculator can know yours.

The module

Portrait against landscape

Layout sketch

A scale sketch of the arrangement counted above. It is not a roof plan: it has no obstacles, no vents, no hips or valleys, and no structural meaning.

What would that generate? →
Your data stays in this browser. Everything on this page is calculated on your own device. Your electricity use, roof dimensions, prices and equipment details are never uploaded, stored on a server, or included in analytics.
Planning estimate — not an engineering or installation plan. Results depend on the information and assumptions you enter. Output, savings and equipment behaviour vary with weather, shading, tariffs, temperature, ageing and installation. Verify equipment limits against current manufacturer documentation. Permits, structural capacity, electrical protection, conductor sizing, grounding, rapid shutdown, fire access, utility interconnection and code compliance must be reviewed by qualified professionals and the applicable authorities.
How this is calculated

Usable space is the surface less your setbacks. Then, along each dimension:

count = floor((usableLength + gap) ÷ (moduleLength + gap))

The + gap appears on both sides because gaps go between panels, never outside the first and last one. Four 2 m panels with 20 mm gaps occupy 4 × 2 + 3 × 0.02 = 8.06 m, not 8.08 m. Getting this wrong loses you a panel on most roofs.

Both orientations are calculated independently and the better one is shown. All arithmetic is done in metres internally, so switching units cannot change the answer.

What this deliberately does not do

It fits rectangles into a rectangle. Real roofs have vents, pipes, chimneys, skylights, hips, valleys and dormers, and this knows about none of them. It also cannot tell you what setbacks apply where you live — fire-access pathways and edge clearances vary by country, state and sometimes municipality, and the penalty for guessing is a redesign after inspection. The setback fields exist so you can enter the ones that genuinely apply to you.

Slope length or footprint?

This matters more than people expect. If you measured along the roof surface with a tape, use "along the roof surface". If you took dimensions from a plan, a satellite image or a property map, those are horizontal projections — the actual sloped surface is longer, by roughly 12% on a 6:12 pitch and 41% on a 12:12. Using a footprint measurement as though it were surface length will understate how many panels fit.

Limitations

Plan-view geometry only. No structural assessment, no shading analysis, no wiring or rapid-shutdown considerations, and no claim that any spacing here meets a code requirement. Treat the count as an upper bound to sanity-check a quote against, not a layout to build from.