# Daylight Factor: How Much Daylight Reaches Into a Floor

> Daylight factor is the ratio of indoor daylight illuminance at a point to the outdoor illuminance under an overcast sky, expressed as a percentage. It describes how much of the available daylight reaches a given position, so it falls sharply as you move away from a window and is a useful way to judge how deep a floor plate can usefully be daylit.

## Why it matters on a project

Daylight is the thing occupants respond to most strongly and the thing fit-out
decisions most often destroy without anyone noticing.

Daylight factor is useful because it makes the loss measurable. It shows how far
into the plate daylight usefully reaches, which in turn tells you where enclosed
rooms can sit without cost, and where they will darken the whole floor behind
them.

## What raises and lowers it

| Raises daylight factor | Lowers it |
| --- | --- |
| Larger glazed area | Deep floor plate |
| Higher window head height | Low window heads |
| Higher glass light transmission | Heavily tinted or reflective glass |
| Light internal surfaces | Dark ceilings and walls |
| Clear sky view | External obstructions, adjacent buildings |
| Open layout near the facade | Cabins along the window line |

## The head-height point is underused

Two windows of the same area do not daylight a room equally. **Light entering
high travels deeper.** A tall window daylights a deeper band than a wide, low
one of the same glazed area.

That has a direct fit-out consequence: dropping the ceiling near the facade,
or running a deep bulkhead along the window line, cuts the top of the window
and removes the part doing the most work. It is a common casualty of services
coordination, and it is worth defending.

## What it does not tell you

Daylight factor is defined against an **overcast sky**. That makes it
comparable and stable, but it says nothing about:

- Direct sun and glare at particular hours and orientations
- Solar heat gain, which is **SHGC**
- Seasonal and daily variation
- Whether blinds will end up permanently closed

So it answers "how deep does daylight reach", not "is this a comfortable place
to sit". Both questions matter.

Interior illumination practice is set out in **IS 3646**, with lighting and
daylighting control provisions in **ECBC 2017** and building services in
**NBC 2016, Part 8**.

## Common mistakes

- **Lining the facade with enclosed rooms.** The most damaging daylight decision
  available at fit-out stage.
- **Cutting the top of the window with a bulkhead or dropped ceiling.** Removes
  the most effective part of the aperture.
- **Optimising glass for solar control alone.** A dark facade moves energy from
  cooling to lighting.
- **Daylight with no lighting control.** No daylight strategy saves energy if the
  lights do not respond.
- **Treating daylight factor as a comfort measure.** It does not describe glare
  or direct sun.

## Standards referenced

Interior illumination practice in **IS 3646**; lighting, control and daylighting
provisions in **ECBC 2017**; building services provisions in
**NBC 2016, Part 8**. Daylight analysis and compliance for a specific building
must be established by the project's lighting or energy consultant.

## Frequently asked questions

### Why does daylight fall off so quickly from a window?

Because the window is a limited aperture and light spreads. A position twice as far from the glass sees a much smaller apparent area of sky, so the daylight reaching it drops far faster than distance alone suggests. This is why a deep floor plate has a bright perimeter band and a middle that depends on artificial light regardless of how good the glazing is.

### What affects it most?

Window area and head height, first. A window's head height matters disproportionately — light entering high up travels further into the room than light entering low down, which is why a tall window daylights a deeper zone than a wide one of the same area. Then glass transmission, external obstructions, internal surface reflectance, and anything blocking the path such as cabins along the facade.

### Does higher daylight factor always mean a better space?

No. Daylight factor says nothing about glare, direct sun or heat. A perimeter with a high figure and unshaded east or west sun can be unusable at certain hours, and the blinds get closed permanently — at which point the daylight is gone anyway. It has to be read alongside orientation, shading and solar control.

### How does it relate to energy?

Daylight only saves energy if the lighting responds to it. A well-daylit perimeter with lights on a single switch for the whole floor saves nothing. Daylight-linked control — dimming or switching the perimeter row as daylight rises — is what converts daylight into reduced consumption, and ECBC 2017 addresses lighting and control provisions for commercial buildings.

### What does the layout have to do with it?

A great deal, and it is the part most within a fit-out's control. Enclosed rooms along the window line stop daylight at that line and leave the open floor behind them on artificial light all day. Placing enclosed rooms toward the core, or glazing them so light passes through, is usually the single most effective daylight decision available at fit-out stage.

### Is the overcast sky assumption a limitation?

Yes, and it is worth knowing. Daylight factor is defined against an overcast sky, which makes it a stable, comparable metric but means it does not describe sunny conditions, orientation or seasonal variation. It is a good tool for comparing how deep daylight reaches; it is not a full description of what a space will feel like through the year.

## Sources

- [IS 3646](https://standards.bis.gov.in/website/published-standards/department-wise) — Code of practice for interior illumination
- [ECBC 2017](https://beeindia.gov.in/) — Lighting and controls — daylighting
- [NBC 2016, Part 8](https://www.bis.gov.in/standards/national-building-code/?lang=en) — Building services — lighting and ventilation

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Source: https://hagerstone.com/glossary/daylight-factor