Windows & Doors
Rooflight & Skylight Daylight Area Calculator
Recommended rooflight area for good daylight
Updated July 12, 2026 · Live
What this tool does
Suggests a rooflight or skylight area from the roof area and a target glazed fraction, as a starting point for bright, even daylight.
Enter the roof area and a target glazed percentage; the calculator returns the recommended rooflight area.
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Formula Used
Suggests a rooflight or skylight area from the roof area and a target glazed fraction, as a starting point for bright, even daylight. It runs entirely in your browser and shows the working, so you can check every step against your own figures.
How the rooflight & skylight daylight area calculator works
The recommended rooflight area is the roof (or floor) area multiplied by your target glazed fraction. A fraction of roughly 15–20% of the floor area gives a bright, evenly-lit living space; less suits a utility space, more risks glare and overheating. The tool leaves the fraction in your hands so the rule of thumb fits the room.
What you enter
Enter the roof area and a target glazed percentage; the calculator returns the recommended rooflight area. Every field is editable, and the result recalculates the moment you change a value.
Reading the result
Use the result as a starting glazed area, then split it into standard rooflight sizes. Rooflights admit far more daylight per m² than vertical windows, so a little goes a long way.
Safe by design
This tool sizes glazing, cost and thermal performance — not the structural opening. Sizing a lintel or a load-bearing opening is a structural engineer’s calculation, and this page never attempts it.
Assumptions and limits
This is a daylight rule of thumb, not a daylight-factor calculation. Actual daylight depends on orientation, obstructions, glazing transmittance and room depth. Large rooflights need solar-control glass or blinds to avoid summer overheating.
Using it with the rest of BuildMetricLab
This calculator is one step in planning the job. Pair it with the related tools in this category and across BuildMetricLab to go from a single figure to a full costed plan — a sizing figure feeds a cost estimate, a cost estimate feeds a project budget, and each result carries straight into the next tool. Every BuildMetricLab tool runs entirely in your browser — no sign-up, nothing sent to a server, and the formula is shown on the page so the maths can be audited.
Sources & methodology
Suggests a rooflight or skylight area from the roof area and a target glazed fraction, as a starting point for bright, even daylight. Every result is calculated from the values you enter, all inputs are editable, and no time-sensitive prices or rates are embedded.
Frequently asked questions
How much rooflight is enough?
Around 15–20% of the floor area gives bright, even daylight for a living space. Utility areas need less; too much risks glare and overheating.
Are rooflights better than windows for daylight?
Per m², yes — an overhead rooflight sees more of the sky than a vertical window, so it admits more daylight and spreads it more evenly.
Will a big rooflight overheat the room?
It can. Pair a large glazed area with solar-control glass, blinds or shading to manage summer gain.
Is this a daylight-factor calculation?
No — it is a planning rule of thumb. A full daylight-factor study accounts for orientation, obstructions and glazing transmittance.
Spotted something off?
Calculations or display — let us know.