Daylight Factor (DF%) Calculator

Introduction: what daylight factor measures and why it survives

The daylight factor (DF) is the ratio of the illuminance at a point indoors to the simultaneous illuminance on an unobstructed horizontal plane outdoors, expressed as a percentage. Its defining trick is that it is a ratio under a CIE Standard Overcast Sky, so the absolute brightness of the day cancels out. Whether the sky delivers 5,000 lux or 20,000, a given room returns roughly the same DF, which is what makes it comparable between buildings, between design options, and between a measurement taken in March and one taken in November.

That robustness is also its limitation. Because the overcast sky is assumed uniform in azimuth, DF is blind to orientation: a north-facing and a south-facing window of identical size and obstruction return identical numbers. Annual climate-based metrics have largely replaced it for performance prediction, yet DF remains embedded in planning guidance, in BREEAM and Housing Quality Mark credits, and in the pocket of every surveyor with a lux meter, because it is the only daylight metric you can verify on site in under a minute.

This page runs both of the ways DF is used in practice. Measured mode takes two lux readings and returns the point daylight factor. Design mode implements the BRE and BS 8206-2 average daylight factor from window geometry, glazing transmittance, room surface area and reflectance — the calculation you need before the building exists and there is nothing to point a meter at.

How to use the daylight factor calculator in either mode

Pick a mode at the top of the form; the fields change to match.

Measured mode, from two lux readings

The tool only asks for two numbers, but the order you collect them in matters more than the arithmetic. Take the outdoor reading first with the sensor flat and the sky in full view, then get inside and take the indoor reading within a few seconds — daylight can shift 20 % between one passing cloud and the next, and a lagging outdoor number quietly inflates or deflates the whole ratio.

1. Enter the outdoor illuminance in lux — the roof, terrace or open-ground reading with nothing shading the sensor.
2. Enter the indoor illuminance in lux at your chosen workplane point, taken with the electric lights off so you are measuring daylight alone.
3. Press Compute daylight factor. Re-run at two or three points across the room and compare, because one reading never speaks for the whole space.

Design mode, from window geometry

1. Enter the room dimensions; the calculator derives the total internal surface area from them, which is what the formula needs rather than the floor area alone.
2. Enter the net glazed area — glass only, with the frame subtracted. On a typical domestic window the frame takes 20–30 % of the structural opening, so netting it out matters.
3. Enter the angle of visible sky in degrees, measured in section from the centre of the window: 90° for a window with clear sky above the horizontal, less where a facing building, balcony or deep reveal cuts into the view. This is the single input that captures site context.
4. Set glazing transmittance (about 0.9 for single clear glass, 0.7–0.8 for double, 0.6–0.7 for low-e triple), a maintenance factor for dirt (0.9 clean vertical glazing, down to 0.7 in dirty urban or industrial settings), and the average surface reflectance.
5. Choose the room use so the result is checked against the right BS 8206-2 target.

Design mode also reports the net glazed area you would need to reach the target, which is usually the number you actually wanted.

Formulas: the point ratio and the BRE average daylight factor

The measured form is a plain ratio. With the indoor illuminance at a point and the simultaneous unobstructed outdoor horizontal illuminance, both in lux:

The design-stage form is the Lynes expression codified in BRE Digest 309 and BS 8206-2, which estimates the average daylight factor across the whole working plane rather than the value at one point:

where is the maintenance factor for dirt on the glazing, the diffuse visible transmittance of the glass, the net glazed area in m², the angle of visible sky in degrees, the total area of every internal surface including the windows, and the area-weighted average reflectance of those surfaces. Expressing in degrees is what makes the result come out directly as a percentage; there is no separate factor of 100.

For a rectangular room the surface area is worth writing out, because using floor area by mistake inflates the answer by a factor of four or more:

The denominator is the interreflection term, and it bites hard. Raising average reflectance from 0.4 to 0.6 divides the denominator by 1.31, lifting ADF by 31 % for no change in glazing at all — which is why painting a room white is the cheapest daylighting intervention there is. Because ADF is strictly linear in , the glazed area needed to hit a target follows by inversion:

Worked examples: a desk reading and a living room on paper

Measured: an open-plan desk on a grey afternoon

Stand a colleague on the roof terrace with a lux meter facing the sky while you crouch at a desk two rows back from the window, both calling out numbers on a three-count so the readings line up:

• Outdoor illuminance, = 12,000 lux
• Indoor illuminance at desk height, = 360 lux

DF = (360 ÷ 12,000) × 100 = 3.0 %. That lands squarely in the range most designers are happy to see on a workplane: enough diffuse light to read and work without reaching for the switch on a dull day, without the desk beside the glass becoming a glare trap. Before signing it off, walk the meter back toward the rear wall — desks near the core routinely drop below 1 % even when the front row measures a comfortable 3 %, and that falloff is the real story the single reading hides.

Design: a 4 m by 4 m living room with a facing terrace

Room 4.0 × 4.0 m with a 2.5 m ceiling, so  m². One window with 2.0 m² of net glass, double glazed at , maintenance factor 0.9, and a building opposite that cuts the visible sky to . Surfaces average .

Numerator: 0.9 × 0.7 × 2.0 × 65 = 81.9. Denominator: 72 × (1 − 0.25) = 54.0. So ADF = 1.52 %, which just clears the BS 8206-2 living-room target of 1.5 % and falls well short of the 2 % a kitchen would need.

To reach 2 % the glass would have to grow to 2.0 × 54.0 ÷ (0.9 × 0.7 × 65) = 2.64 m², a 32 % increase. Alternatively, lifting average reflectance from 0.5 to 0.65 — white walls and ceiling, pale floor — drops the denominator to 72 × 0.5775 = 41.6 and raises ADF to 1.97 % with no change to the window at all. Comparing those two routes is exactly what design mode is for.

Interpreting the number against published targets

BS 8206-2 sets minimum average daylight factors by room use, and BREEAM and the Housing Quality Mark share the same calculation procedure. CIBSE summarises the standard's headline positions: an average daylight factor should be at least 2 %, and where it reaches 5 % electric lighting is not normally needed during the daytime, provided uniformity is satisfactory.

For a point daylight factor from measured mode, read the same numbers as rough guidance rather than compliance: below 2 % the point is dim for daylight-only tasks, 2–5 % is generally useful, and above 5 % you are close enough to the glass that glare and solar gain deserve a look. DF is point-specific, so a single reading never describes a room; designers take readings on a grid to judge distribution and uniformity.

Daylight factor against the climate-based metrics

DF is easy to measure and compare, but it is not the only way to evaluate daylighting, and modern practice increasingly prefers metrics that know about orientation and weather:

• Daylight Autonomy (DA) and spatial Daylight Autonomy (sDA): the fraction of occupied hours across a year that a point, or a proportion of the floor area, meets a target illuminance. Requires simulation against a climate file.
• UDI (Useful Daylight Illuminance): the fraction of hours daylight sits inside a useful band, penalising both gloom and excess.
• ASE (Annual Sunlight Exposure): flags over-exposure to direct sun, the glare and overheating risk that DF is structurally incapable of seeing.

BS 8206-2 itself was withdrawn in June 2019 and superseded by BS EN 17037, which introduced target illuminance methods. The average daylight factor nonetheless persists in planning practice, in BREEAM and HQM assessments, and in the AECB and Passivhaus workflows, which is why it is worth computing correctly.

Assumptions and limitations to read before quoting a number

• Overcast-sky assumption. The definition presumes a CIE Standard Overcast Sky. Sunny conditions and broken cloud yield DF values that are unstable and not comparable.
• Blind to orientation. Because the overcast sky is uniform in azimuth, a north-facing and a south-facing window return the same DF. Any question about solar gain, glare or seasonal variation is outside what this metric can answer.
• Simultaneous readings required. If outdoor lux changes between measurements, the ratio is wrong even when both numbers look reasonable.
• Point versus average. Measured mode gives DF at the exact sensor position; design mode gives an area average. They are different quantities and should not be compared to each other directly.
• The Lynes formula is a first approximation. It assumes a simple rectangular room, a single glazing plane, and ignores the distribution of light within the space. Deep-plan rooms, light shelves, clerestories and atria all need simulation.
• Meter accuracy and cosine response. Low-cost sensors, including most phone apps, often have poor cosine correction and calibration. An uncorrected sensor can be 20 % out at oblique incidence.
• Electric lighting must be off. Any artificial light raises the indoor reading and inflates DF.
• Exterior obstructions matter. Facing buildings, balconies, deep reveals and trees all reduce the visible sky angle. In measured mode they are already baked into the reading; in design mode you must estimate honestly from a section drawing.
• Not a comfort guarantee. A high DF can still mean glare; a low DF can still be comfortable with good electric lighting design.

Sources. The average daylight factor expression and its variable definitions follow BS 8206-2 and BRE Digest 309, as implemented in the AECB Daylight Standard, whose calculator states it is “based upon BS 8206-2, and informed by BRE BR209, BRE IP 15/88 and BRE Digest 309” and which tabulates the minimum daylight factors used above (kitchen and utility >2 %, living, dining and study >1.5 %, bedroom >1 %, non-residential occupied spaces >2 %). The “at least 2 %” and “5 % means electric lighting is not normally needed during the daytime” statements are quoted from BS 8206 in CIBSE, Daylighting and Compliance. The overcast-sky basis of the daylight factor definition is the CIE Standard Overcast Sky. BS 8206-2 was withdrawn in June 2019 and superseded by BS EN 17037, Daylight in Buildings. Last reviewed July 2026.

Daylight factor questions that come up on site