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Electrical

Lighting Calculator

How many fixtures a room needs for a target light level.

Your measurementsEvery assumption editable01

Foot-candles — lumens per square foot at the working surface.

Printed on the box. A common LED downlight is 800–1,100 lm.

How much fixture output reaches the working plane. 0.5–0.7 is typical.

Dirt, lamp ageing and voltage variation. 0.8 is a common allowance.

FixturesCalculated live02

Result

20fixtures

20 × 900 lm gives about 30.9 fc

Room area
280 sq ft
Target level
30 fc
Raw lumens neededbefore losses
8400 lm
With utilization and lossesCU 0.6, LLF 0.8
17500 lm
Fixtures required
20 × 900 lm
Delivered light levelafter rounding up to whole fixtures
30.86 fc

Two separate derates apply and both matter. Coefficient of utilization is how much of a fixture's output actually reaches the working plane — room shape, ceiling height and surface colors all affect it. Light loss factor covers dirt accumulation, lamp depreciation and voltage variation over the installation's life.

Foot-candles measure light arriving at a surface, not light leaving a bulb. That is why lumens alone tell you very little about whether a room will be bright enough.

Even distribution matters as much as total output. The same lumens from four fixtures light a room far better than from one, and this calculates quantity rather than layout.

Lumens leave the fixture; foot-candles arrive at the work surface. The gap between them is utilization and losses, and both are exposed here rather than assumed — which is why a room lit on raw lumens comes out dim.

Why use this tool?What it does differently03

Why use this tool?

Two derates, both exposed

Utilization and light loss are different things and both matter.

Task-based presets

From corridor at 10 foot-candles to fine detail work at 100.

Delivered level shown

What you actually get after rounding up to whole fixtures.

Distribution flagged

The same lumens from four fixtures light a room far better than from one.

How this worksThe method04

How this lighting calculator works

Raw lumens needed is the target foot-candle level times the floor area, since a foot-candle is one lumen per square foot. That is then divided by the coefficient of utilization and the light loss factor to get the lumens fixtures must actually produce.

Coefficient of utilization is how much of a fixture’s output reaches the working plane — room proportions, ceiling height and surface reflectance all affect it. Light loss factor covers dirt, lamp depreciation and voltage variation over the life of the installation.

Skipping either derate underestimates badly. A room designed on raw lumens typically delivers about half the intended level once both are accounted for.

How to use itStep by step05

How to use it

  1. Step 1: Measure the room

    Length and width. Ceiling height affects utilization rather than area.

  2. Step 2: Pick a target level

    By task — general living, kitchen work, or fine detail.

  3. Step 3: Read the fixture lumens

    Off the box. Lumens, not watts.

  4. Step 4: Spread them out

    This gives quantity, not layout. Even distribution matters as much as total output.

Example usageWorked figures06

Example usage

A kitchen
20 by 14 ft at 30 foot-candles with 900 lumen downlights, 0.6 utilization and 0.8 loss factor: 17,500 lumens required, so 20 fixtures delivering about 30.9 foot-candles.
Ignoring the derates
The raw figure of 8,400 lumens would suggest 10 fixtures — half what the room actually needs.
A living room
The same room at 20 foot-candles needs 13 fixtures. Task level drives the count more than room size does.
Frequently asked questionsCommon questions07

Frequently asked questions

How many recessed lights do I need?

For a 280 square foot kitchen at 30 foot-candles with 900 lumen fixtures, around 20 once utilization and loss factors are accounted for. Living spaces at 20 foot-candles need noticeably fewer.

What is a foot-candle?

One lumen per square foot arriving at a surface. It measures light delivered, which is why lumens alone tell you little about whether a room will be bright enough.

What is coefficient of utilization?

The share of a fixture’s output that reaches the working plane. Room proportions, ceiling height and how reflective the surfaces are all affect it — 0.5 to 0.7 is typical.

Why include a light loss factor?

Because fixtures get dirty and lamps dim with age. Designing to initial output means the room falls below target within a year or two.

Does spacing matter?

Considerably. This calculates how many fixtures, not where to put them — the same total from four fixtures lights a room far more evenly than from one.

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