This daylight calculator is free to use, with no account and no password. It runs entirely in your browser; enter your name and email to reveal your result, and we can send you a copy.
The tool takes a location, a date, a time, a room and its windows, and returns the daylight on the workplane: average, minimum and maximum illuminance in lux, the daylight factor, the uniformity, the share of the floor above two useful thresholds, and the melanopic equivalent daylight illuminance for circadian assessment. It draws the room in plan and in section, shades the workplane by illuminance, and shows where direct sun lands.
Twenty-one city presets are included, from Jakarta and Bandung through Singapore, Kuala Lumpur and Bangkok to London and New York, and any latitude, longitude and time zone can be entered by hand. Every field starts blank and no result appears until the checklist is complete and you press Generate result. There is a step by step walkthrough of every field in the daylight calculator guide.
The calculation runs in three stages: where the sun is, how bright the sky is, and how much of that reaches the workplane.
Sun position comes from the standard solar position algorithm. The day of year and the local time give a fractional year angle, which gives the equation of time and the solar declination. Combined with latitude, longitude and the time zone offset, that yields the hour angle, and from there the solar altitude and the azimuth measured clockwise from true north. Sunrise and sunset are solved for the same day at the conventional 90.833 degree zenith that accounts for refraction and the solar disc.
Sky illuminance follows from the altitude. Direct normal illuminance is attenuated through the atmosphere using the Kasten and Young relative air mass, which is what makes a low sun so much weaker than a simple sine relationship suggests. Diffuse horizontal illuminance is modelled separately for three sky conditions: clear, intermediate and overcast. Under an overcast sky the direct component is zero and the diffuse component carries everything, which is the classic daylight-factor condition.
Interior daylight uses a split-flux model. Each window is divided into a grid of patches, and for every point on the workplane grid the tool sums the contribution of every patch, weighted by the cosine at the window, the cosine at the receiving point and the inverse square of the distance between them. That gives the direct sky component point by point rather than as a single room average. The window is treated as seeing half sky and half ground, with the ground reflecting a fixed fraction of the global horizontal illuminance.
The internally reflected component is added on top. Total flux entering through the glazing is multiplied by the area-weighted mean room reflectance and divided by the total internal surface area and one minus that reflectance. This is the term that makes a light-coloured room measurably brighter at the back than a dark one with identical windows, and it is why the ceiling, wall and floor reflectances are inputs rather than assumptions.
Direct sun is handled separately. For each grid point the tool traces a ray back along the solar vector and tests whether it passes through a window opening. Where it does, the beam illuminance on the horizontal plane is added and the point is marked as in direct sun, which is what feeds the sunlit-area percentage.
Location and time set the sun. A city preset fills latitude, longitude and time zone; the building rotation from true north then turns the room relative to that sun, which is usually the fastest way to see whether an orientation works.
The sky condition changes the balance between beam and diffuse. Clear sky gives the strongest direct component and the sharpest sun patch. Overcast removes the beam entirely and is the condition to use when you want a daylight factor that does not depend on the hour.
Room length, width and height set the geometry, and the workplane height sets the plane the results are reported on, conventionally 0.76 m for a desk. The three reflectances, ceiling, wall and floor, drive the internally reflected component. Typical values are high for a white ceiling, moderate for walls and low for a floor; the tool flags a value outside a sensible range rather than accepting it silently.
Windows are added one at a time, each with a wall, a width, a height, a sill height, an offset along the wall and a visible transmittance. Transmittance is the single most under-appreciated input here: a high-performance solar control glass can pass barely half the visible light of clear double glazing, and that difference shows up directly in the result. The tool validates that a window fits within its wall and that the sill and head fit within the room height.
Photopic lux describes how bright a space looks. It does not describe how the light affects the body clock, because the retinal photoreceptor responsible for that has a different spectral sensitivity, peaking in the blue rather than the green.
CIE S 026 defines the quantity that does: melanopic equivalent daylight illuminance. The tool computes it by estimating the correlated colour temperature at each grid point, blending a warmer value for direct sun with a cooler value for sky-dominated light, and applying the melanopic daylight efficacy ratio for that colour temperature. The result is reported alongside the photopic average.
This matters because daylight is the only practical way to reach the melanopic levels the WELL Building Standard and current circadian guidance ask for during the day. A space can be comfortably lit at 300 lux from electric light and still be nowhere near the circadian target, while a window wall reaches it easily. Seeing both numbers together is what makes the trade-off visible.
Load the example and the tool fills an 8 by 6 metre room, 3.2 m high, in Jakarta on the September equinox at 10:30, with a north-facing window and a smaller east-facing one, moderate reflectances and clear glazing.
Generate the result and you get the average workplane illuminance with a verdict, the minimum and maximum, the uniformity ratio, the average and minimum daylight factor, the melanopic average, the sunlit percentage, the sun altitude and azimuth, sunrise and sunset, the direct normal and diffuse horizontal illuminance, and an annual proxy across four representative dates.
The plan view is where the design conversation happens. A high average with poor uniformity means one bright band near the glass and a dim rear half, which is a rooflight or a light-shelf problem rather than a bigger-window problem.
This is a single-room split-flux model with no exterior obstruction. It does not know about the building across the street, an overhang, a fin, a balcony above, or an internal partition. Any of those can dominate the real result, and none of them are inputs.
It is a point-in-time calculation with a four-date annual proxy, not a climate-based annual simulation. Spatial daylight autonomy and annual sunlight exposure as LEED and WELL define them require an hourly run against a real weather file, and the figures reported here are indicative of those metrics rather than equal to them.
It does not assess glare. Daylight glare probability depends on luminance in the field of view from a specific viewpoint and direction, which a workplane illuminance grid cannot tell you. A large window that produces excellent illuminance figures can still be unusable without shading.
Treat the output as early-design guidance for orientation, window sizing and glazing selection, and as the basis for deciding whether a full simulation is warranted.
This calculator sits alongside the other lighting and envelope tools in the design calculators set. The photometric calculator covers the electric-light counterpart, the illuminance one fixture puts on one point. The OTTV calculator shows the cost side of glazing, the solar heat it admits along with the light.
For the reference documents behind the numbers, see the standards library, and for the vocabulary, the terminology reference.
Daylight and circadian work is part of ALTA Integra's passive design and sustainability practice, alongside lighting design. If you need climate-based annual simulation, glare analysis, or a WELL or LEED daylight credit submission, talk to the team.
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