SettingsSettings for this calculationUS
The interior width of the light well shaft.
Measure the horizontal interior dimension of the shaft connecting the roof skylight opening to the ceiling opening below.
The interior length of the light well shaft, perpendicular to the width.
Measure the horizontal interior dimension perpendicular to the width.
The vertical distance from the roof opening down to the ceiling opening.
This is the depth of the shaft, not the room's ceiling height — measure from the underside of the roof deck/curb down to the finished ceiling plane.
Light well wall surface area
117 sq ft
This assumes a straight (non-splayed/non-tapered) light well — a splayed well (wider at the ceiling than the roof, common for maximizing daylight spread) needs a per-wall trapezoidal area calculation instead of this simplified straight-shaft method.
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Skylight Light Well Surface Area Calculator: 117 sq ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Light well wall area = 2 × (Width + Length) × Well Height — a straightforward surface-area takeoff for a straight (non-splayed) light well/shaft connecting a roof skylight to a ceiling opening below, useful for estimating reflective finish material needed to maximize daylight transmission.
Inputs used
- Light Well Width
- 4 ft
- Light Well Length
- 5 ft
- Light Well Height (Roof to Ceiling)
- 6.5 ft
Confidence note: This assumes a straight (non-splayed/non-tapered) light well — a splayed well (wider at the ceiling than the roof, common for maximizing daylight spread) needs a per-wall trapezoidal area calculation instead of this simplified straight-shaft method.
What this calculation does not cover
- Measures the finish, not the daylight. How much light reaches the room is governed by the well index, the shaft's depth set against its width: a 2 m (6.5 ft) shaft over a 600 mm (24 in) opening delivers a fraction of what the same skylight gives through a 200 mm (8 in) ceiling. No amount of reflectance rescues a shaft that is too deep and too narrow — widening the ceiling opening or splaying the walls is what does.
- Counts one side of each wall. The shaft passes through the roof void, so those walls are part of the thermal envelope and need insulation on their outer face plus a continuous air seal where they meet the ceiling plane — the same dimensions, a different material, and none of it in this area. The well that skips it is the one that grows a black line of mold around the ceiling opening every winter.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Light well wall area = 2 × (Width + Length) × Well Height — a straightforward surface-area takeoff for a straight (non-splayed) light well/shaft connecting a roof skylight to a ceiling opening below, useful for estimating reflective finish material needed to maximize daylight transmission.
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