Daylight Factor (DF%) Calculator

Introduction: How to calculate daylight factor (DF%)

The daylight factor (DF) is a simple, long‑standing metric used in architecture and building design to describe how much natural light reaches a point indoors compared with the available daylight outdoors. It is expressed as a percentage and is traditionally defined under a CIE Standard Overcast Sky (or similar fully overcast conditions) to reduce the effects of direct sun and fast‑changing sky brightness.

What this calculator does

This calculator computes DF% using simultaneous illuminance measurements:

• Outdoor illuminance (E_o) in lux, measured outside in an unshaded location.
• Indoor illuminance (E_i) in lux, measured at the specific point indoors (e.g., desk height).

Because daylight factor is a ratio, it is best used for comparing design options (window size, glazing, surface reflectance, room layout) rather than predicting exact indoor lux in all weather.

The formula (with variable definitions)

Daylight factor is calculated as:

• DF = daylight factor (%)
• E_i = indoor illuminance at the point of interest (lux)
• E_o = outdoor horizontal illuminance at the same time (lux)

How to take measurements (so the result is meaningful)

1) Measure outdoor illuminance correctly

• Measure outdoors on a horizontal plane (sensor facing up), away from buildings/trees that shade the sky.
• Avoid placing the meter near bright walls or reflective surfaces that can artificially increase the reading.
• Record E_o and E_i as close in time as possible (ideally within seconds).

2) Measure indoor illuminance at a defined point

• Choose a specific point (e.g., the center of a desk, the middle of a room, or along a workplane grid).
• Use a consistent height (commonly 0.8 m above the floor for a desk/workplane).
• Try not to cast a shadow on the sensor; keep your body and the meter positioned consistently.

3) Prefer overcast/neutral conditions

DF is most comparable under overcast conditions. On sunny days, direct sun patches and fast‑moving clouds can cause large swings in indoor and outdoor lux, producing DF% values that are not representative.

Interpreting your daylight factor result

“Good” depends on building type, climate, glare risk, and design goals. Still, these broad ranges are commonly used for quick interpretation at a point:

Important: DF is point‑specific. A single DF% at one location does not describe the entire room. Designers often take multiple readings on a grid to evaluate distribution and uniformity.

A reading from a real desk, step by step

Picture an open-plan office on a grey afternoon. You stand a friend outside on the roof terrace with a lux meter facing the sky while you crouch at a desk two rows back from the window, both of you calling out numbers on a three-count so the readings line up:

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

Then:

DF% = (360 ÷ 12,000) × 100 = 3.0%

Three percent lands squarely in the range most designers are happy to see on a workplane: enough diffuse light that people can read and work without reaching for the switch on a dull day, without being so bright that the desk beside the glass turns into a glare trap. Before you sign off on it, 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.

Daylight factor vs. other daylight metrics

DF is easy to measure and compare, but it is not the only way to evaluate daylighting:

• Daylight Autonomy (DA) / sDA: time‑based metrics that estimate how often a space meets a target lux level across a year (often from simulation).
• UDI (Useful Daylight Illuminance): focuses on whether daylight levels are within a useful range (not too dim, not too bright).
• ASE (Annual Sunlight Exposure): highlights potential over‑exposure to direct sun (glare/overheating risk).

If you need year‑round performance, DF is a starting point; annual metrics are typically more informative.

Assumptions and limitations (read before using)

• Overcast‑sky assumption: The classic definition assumes an overcast sky distribution. Sunny conditions can yield misleading DF values.
• Simultaneous readings required: If outdoor lux changes between measurements, the ratio can be wrong even if both numbers are “reasonable.”
• Point measurement: DF applies to the exact point where E_i is measured. Rooms often vary significantly from window to back wall.
• Meter accuracy and cosine response: Low‑cost sensors (including many phone apps) may have poor cosine correction and calibration, affecting accuracy.
• Indoor lighting must be off (or accounted for): Electric lights will increase E_i and inflate DF unless you measure with lights off.
• Exterior obstructions matter: Nearby buildings, balconies, deep reveals, and trees can reduce sky visibility and lower DF compared with unobstructed conditions.
• Not a glare/comfort guarantee: A “high” DF can still mean glare; a “low” DF can still be comfortable with good electric lighting design.

Practical tips

• Take multiple indoor readings across a simple grid (e.g., every 1–2 m) to understand distribution.
• Record date/time, sky condition, and whether blinds/shades were open—so results are comparable later.
• If you’re testing design changes (paint color, shading, glazing), keep measurement locations and procedure identical.

Questions that come up on site

What is a “good” daylight factor?

Many designers consider 2%–5% at a workplane point to be a practical range under overcast conditions, but targets vary by use, climate, glare risk, and standards.

Why must indoor and outdoor readings be taken at the same time?

Outdoor daylight can change quickly with clouds. Because DF is a ratio, even small timing differences can distort the percentage.

Can I use this calculator on a sunny day?

You can compute the ratio, but it may not represent the traditional daylight factor definition. Direct sun indoors or partial cloud can make the value unstable and not comparable.

Do I measure outdoor lux in the sun or shade?

Measure outdoors in an unshaded location with a clear view of the sky and the sensor on a horizontal plane. Avoid shade from buildings/trees.

Should electric lights be on or off?

For DF, measure with electric lighting off (or otherwise ensure your indoor reading reflects daylight only).

What units should I use?

Use lux for both indoor and outdoor measurements. The units cancel, producing a percentage.

How is daylight factor different from daylight autonomy?

DF is a single‑moment ratio under specific sky assumptions; daylight autonomy is a time‑based metric (often annual) that describes how frequently daylight meets a target level.

References (for deeper reading)

• CIE daylighting guidance and sky models (e.g., CIE Standard Overcast Sky).
• Lighting handbooks and building daylighting guidance that describe DF and measurement practice.

Working the calculator itself

The tool only asks for the two numbers off your meter, but the order you collect them in matters more than the arithmetic. Grab 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. Type the outdoor illuminance in lux into the first field—the roof, terrace, or open-ground reading with nothing shading the sensor.
2. Type the indoor illuminance in lux at your chosen workplane point, taken with the electric lights off so you are measuring daylight alone.
3. Hit Compute daylight factor. The result reports DF% plus a quick low/moderate/high band; re-run it at two or three points across the room and compare, since one reading never speaks for the whole space.