Free Daylight Factor Calculator

The daylight factor tells you how bright a room will be under an overcast sky, as a percentage of the light outside. This daylight factor calculator uses the average daylight factor formula from the BRE and CIBSE guidance to test a room with several windows and roof lights in seconds. It also gives the window to floor ratio, the window to wall ratio of each facade, the room depth rule and a no-sky line estimate, with a section and plan you can export.

How to use this daylight factor calculator

1. Describe the room. Choose metres or feet, pick the room type, and enter the width along the front wall, the depth, the ceiling height and the working plane height. Set which way the front wall faces, so each wall in the results carries its compass direction.
2. Set the surfaces. Choose light, standard or dark for the ceiling, and light, mid or dark for the walls and floor, or type your own reflectance. Glass reflectance is assumed at 0.1 and can be edited.
3. Add each window or roof light. For every opening, set the position (front, back, left or right wall, flat roof light or pitched roof light), width, height, sill height, frame share, glazing, obstruction angle, obstruction distance and wall thickness. Add window places a new one on a wall with free space and tells you where it went. You can have up to 12.
4. Use the obstruction helper. If you know the distance to the building opposite and its height above the window centre, the helper works out the obstruction angle and applies it, with the distance, to every wall window.
5. Read and export. The average daylight factor, the target checks and the ratios update as you type. Export the section and plan as PNG or SVG, the inputs and results as CSV, or save the room as JSON and load it later. Print gives you the tool on its own, with the drawing at the top.

The calculator runs in your browser and nothing is uploaded. It remembers your last room on this device, and Reset takes you back to the default.

The average daylight factor formula

The calculator uses the formula given in the Code for Sustainable Homes technical guidance (after BS 8206-2 and the Littlefair method), the same expression the Australian NCC uses for its daylight verification method:

DF = T × Aw × θ × M / (A × (1 − R²)), in percent.

• T is the diffuse visible transmittance of the glass. The presets are 0.70 for double clear, 0.60 for double low-E and 0.60 for triple glazing. Single clear (0.82) and tinted double (0.35) are estimates, labelled as such. Use the manufacturer’s figure if you have it.
• Aw is the net glazed area, without the frame. The frame share defaults to 20% of the opening.
• θ is the visible sky angle in degrees from the centre of the window. For a vertical window it is 90 minus the obstruction angle minus the reveal angle, where the reveal angle comes from the wall thickness and the window height. An unobstructed flat roof light sees 180 degrees. For a pitched roof light the tool uses 180 minus the glass tilt minus the obstruction angle.
• M is the dirt factor. On Auto it is 1.0 for vertical glass that is easy to clean, 0.8 for sloping glass and 0.7 for horizontal glass. You can enter your own value instead.
• A is the total area of all room surfaces, ceiling, floor and walls, including the windows.
• R is the area weighted mean reflectance of those surfaces.

Each window is calculated on its own and the results are added, so windows with different glass or obstructions are handled correctly. A table shows each window’s net area, T, θ, M and contribution.

A quick hand check: a 3 × 4 × 2.4 m room has 57.6 m² of surfaces. A 1.2 × 1.2 m window with no frame gives 1.44 m² of glass. With T 0.7, θ 65 degrees, M 1 and a mean reflectance of 0.5, the result is 0.7 × 1.44 × 65 / (57.6 × 0.75) = 1.52%.

The default room, a 4 × 5 × 2.7 m living room with one 2.4 × 1.6 m low-E window facing a 20 degree obstruction, comes out at 1.72%: above the 1.5% living room minimum, below the 2% figure used in Australia.

What is a good daylight factor?

The calculator places every result in one of three bands. Below 2%, a room is likely to look gloomy and need electric light for most of the day. From 2% to 5% it is predominantly daylit, though some supplementary electric light is still needed. At 5% or more it is strongly daylit and electric light is rarely needed, but check glare and heat gain.

For homes, the minimum average daylight factors most often quoted come from BS 8206-2:2008: 2% for a kitchen, 1.5% for a living room, dining room or study, and 1% for a bedroom. An open plan living room and kitchen takes the higher value, 2%. BS 8206-2 has been withdrawn and replaced in the UK by BS EN 17037, but these figures are still widely cited in planning reports, which is why the calculator checks them and says so next to the result.

The results also check the Australian NCC 2022: Volume Two H4P4 asks for an average daylight factor of at least 2% in a habitable room. The EN 17037 lines are marked possible rather than meets, for the reason below. Targets are shown for information: check the edition in force and what your local authority asks for.

Daylight factor and EN 17037

The European standard BS EN 17037 sets its minimum level as 300 lux over half of the reference plane and 100 lux over 95% of it, for at least half of the daylight hours. The daylight factor route turns the 300 lux into a target daylight factor using the median external diffuse illuminance of the site: D_T = 300 / E_v,d,med × 100. The calculator includes four reference cities (Berlin 13,900 lux, Prague 14,900 lux, Bratislava 16,300 lux and Stockholm 12,100 lux) or your own value. For Prague, the default, the target is about 2.0%.

That check is indicative. EN 17037 asks for a value exceeded over half of the plane, while the average daylight factor is a mean, and a mean can be pulled up by a very bright zone next to the window. If the average falls below D_T, the room is unlikely to meet the standard. If it is above, compliance is possible but not proven.

For homes in the UK, the National Annex to BS EN 17037 uses lower targets: 100 lux for bedrooms, 150 lux for living rooms and 200 lux for kitchens, over half of the plane. No UK illuminance value is built in, so choose Custom and enter the figure for your site.

Window to floor ratio and window to wall ratio

The window to floor ratio (WFR) is glass area divided by floor area. The calculator shows it two ways: with the net glass, and with the gross opening including the frame. The window to wall ratio (WWR) is the gross opening area divided by the internal area of that wall, reported for each of the four walls, with the compass direction of each wall taken from the way the front wall faces.

Some codes use a ratio directly. The NCC 2022 Housing Provisions (10.5.1) ask for light transmitting glass of at least 10% of the floor area for windows, or 3% for roof lights, with a mix of the two combined in proportion. The calculator reports this as a score where 100% or more passes.

A ratio alone is not a daylight check. Two rooms with the same ratio can have very different daylight factors once you add an obstruction, low-E glass, a thick reveal or a deep plan, which is why the tool reports both. If you are working through window sizes for a whole project, our door and window schedule guide covers how to record them.

Room depth and the no-sky line

Room depth rule. For a room lit by windows in one wall only, the depth L should satisfy L/W + L/H ≤ 2/(1 − R_b), where W is the room width along the window wall, H is the height of the window head above the floor and R_b is the average reflectance of the surfaces in the rear half of the room. If it fails, the back of the room will look darker than the front, whatever the average says. The calculator shows R_b and the maximum depth the rule allows. With windows on more than one wall, or with a roof light, the rule does not apply and the result says n/a.

No-sky line. The no-sky line divides the part of the working plane that can see the sky through the window from the part that cannot. The calculator uses the section construction from the Code for Sustainable Homes guidance: d = x × h / y, where x is the distance to the obstruction, h is the height of the window head above the working plane and y is the height of the obstruction above the window head. It reports the share of the working plane with direct sky against a target of 80%, and the section drawing shows the line.

The obstruction is treated as continuous and parallel to the window. Roof lights are not included in the no-sky line, so in a room with roof lights the true share with direct sky is higher than reported.

Limits of a hand calculation

The average daylight factor formula assumes a CIE overcast sky, a simple rectangular room and continuous obstructions. It ignores the sun, orientation and climate, so north and south facing rooms with the same windows score the same. The position of a window along its wall does not change the result.

Use it for early design, not as a substitute for simulation. For climate based metrics such as the EN 17037 illuminance method, spatial daylight autonomy or useful daylight illuminance, use validated Radiance based tools; our guide to daylight analysis software compares them. For direct sunlight and orientation, which the daylight factor leaves out, draw a chart with the sun path diagram generator.

Ways to raise the daylight factor

• Raise the window head. A taller window, or the same window moved up, sees more sky, pushes the no-sky line deeper and helps the room depth rule.
• Thin the reveal. A thick wall cuts the visible sky angle. Reducing the wall thickness at the opening increases θ.
• Lighten the surfaces. A white ceiling and light walls raise the mean reflectance, and because R appears squared in the formula, the effect is larger than you might expect.
• Cut the frame. A lower frame share gives more glass in the same opening.
• Add a roof light. In a deep room, a flat roof light with an open sky angle of 180 degrees contributes far more per square metre than a wall window.
• Add a second aspect. A window in a second wall lights the back of the room, even if the average changes less than the evenness does.

Frequently asked questions

How do you calculate the average daylight factor?

With DF = T × Aw × θ × M / (A × (1 − R²)), where T is the glass transmittance, Aw the net glazed area, θ the visible sky angle, M the dirt factor, A the total room surface area and R the mean reflectance. The calculator does it for every window and adds the results.

What daylight factor does a living room need?

BS 8206-2 suggested at least 1.5%, with 2% for kitchens and 1% for bedrooms. The standard has been withdrawn, but many planning reports still quote these figures. In Australia the NCC asks for 2% in any habitable room.

Is the daylight factor the same as EN 17037?

No. EN 17037 uses illuminance targets linked to the local climate, so the target daylight factor depends on the site: about 2% for 300 lux in Prague, for example. The average daylight factor is a mean, so the comparison is indicative only.

What is a good window to floor ratio?

The Australian NCC minimum is 10% for windows and 3% for roof lights. A ratio ignores obstructions, glass and depth, so check the daylight factor too.

What is the no-sky line?

The line on the working plane beyond which no sky can be seen through the window. Areas behind it look gloomy, which is why a common target is direct sky on at least 80% of the working plane.

Can I use this for a planning daylight report?

Use it for early design and to check sketches. Planning reports in the UK normally need a BRE 209 assessment by a specialist, often with simulation.

Is the daylight factor calculator free?

Yes, with no sign up. It runs in your browser and nothing is sent to a server.