The problem is not always caused by poor-quality lighting products. In many cases, it comes from an inaccurate lighting estimation at the beginning of the project.

Many projects are still calculated based on experience:

“How many square meters, how many lights?”

This method is fast and easy for quotation. However, it often ignores key technical factors that directly affect the actual brightness after installation.

With 28 years of experience in the lighting industry, Paragon LED has found that 3 factors are commonly overlooked when calculating the number of lights:

UF – Utilization Factor
MF – Maintenance Factor
E – Required Illuminance

These are also the 3 main reasons why a project may have “enough lights” based on experience, but still appear underlit in actual use.

WHY IS THE “m² PER LIGHT” METHOD EASY TO GET WRONG?

A common method is to divide the room area by an estimated number.

For example: installing one 12W or 18W LED light for every 4–5 m².

This method can be used for a very quick estimation, but it is not accurate enough for projects that require stable brightness, clear acceptance standards, or long-term performance.

With the same area, the required number of lights can vary greatly depending on:

How many Lux does the space require?
What is the actual lumen output of each light?
Are the ceiling, walls, and floor light-colored or dark-colored?
Is the environment clean or dusty?
Will the lights be cleaned and maintained regularly?

For a more accurate calculation, contractors should refer to the lumen method:

N = (E × A) ÷ (F × UF × MF)

Where:

N = Number of lights required
E = Required illuminance, measured in Lux
A = Lighting area, measured in m²
F = Luminous flux of each light, measured in lumens
UF = Utilization Factor
MF = Maintenance Factor

The key point is this: many people only look at room area and lamp wattage, while overlooking UF, MF, and E.

When these 3 factors are estimated incorrectly, the final lighting calculation can differ significantly from the actual brightness on site.

TRAP 1: IGNORING THE UTILIZATION FACTOR UF

UF indicates how much of the light output from a fixture actually reaches the working plane.

Not 100% of the lumens from a light fixture will reach the table, floor, or target area.

Part of the light will be absorbed or reflected by the ceiling, walls, floor, and surrounding objects.

UF depends on several factors:

Color of the ceiling, walls, and floor
Mounting height of the luminaires
Room size and room proportions
Beam angle of the light fixture
Lighting layout within the space

Some reference UF values:

If the ceiling is white, the walls are light-colored, and the space is open:
UF is around 0.70–0.80
If the ceiling is white and the walls are cream or neutral-colored:
UF is around 0.55–0.65
If it is a warehouse or factory with dark walls or floors:
UF is around 0.40–0.50
A common mistake is assuming UF = 1 or using UF = 0.8 for every project.

In reality, if the walls are dark, the ceiling is high, the floor is dark, or the lighting layout is not optimized, UF may only be around 0.45–0.55.

This means a large portion of the light does not effectively reach the required working area.

As a result, the project may look sufficient on paper, but appear much darker after installation.

TRAP 2: NOT CALCULATING THE MAINTENANCE FACTOR MF

MF accounts for the reduction of light output during operation.

Newly installed LED lights are usually brighter than they are after 1–2 years of use.

This reduction can be caused by:

Lumen depreciation of the LED chips over time
Dust accumulation on the lens, reflector, or diffuser
Moisture, oil vapor, chemicals, or industrial dust in the environment
Cleaning and maintenance frequency of the lighting system

Some reference MF values:

Clean office with regular maintenance:
MF is around 0.85–0.90
Normal factory environment:
MF is around 0.75–0.80
Dusty or harsh industrial environment:
MF is around 0.70 or requires a suitable maintenance plan
A common mistake is not including MF in the calculation, which means MF is implicitly assumed to be 1.
In that case, the project may meet the required brightness when the lights are newly installed. However, after 18–24 months, the actual illuminance may drop significantly due to dust accumulation and lumen depreciation.

This is why many customers say:

“The lights were bright at first, but they seem to get dimmer over time.”

This does not always mean the lights are defective. In many cases, the original lighting design did not compensate for light loss over the product’s lifetime.

TRAP 3: USING THE WRONG REQUIRED ILLUMINANCE E

E refers to the required illuminance, measured in Lux.
This is one of the most important factors in lighting calculation.
Different spaces require different lighting levels. A single value, such as 300 Lux, cannot be used for every type of project.
A corridor does not need the same brightness as an office.
A warehouse does not need the same brightness as a precision assembly area.
Reading rooms, classrooms, and quality inspection areas require higher lighting levels than general circulation areas.

Some reference illuminance levels:

Corridor and circulation area: Around 100 Lux
Staircase: Around 150 Lux
General storage area: Around 100 Lux
Warehouse with frequent work activity: Around 200 Lux
Auxiliary office area, storage, photocopy area: Around 300 Lux
Office work area, reading, writing, data processing: Around 500 Lux
Regular classroom: Around 300 Lux
Reading room, board area, laboratory, lecture room: Around 500 Lux
Precision assembly or detailed inspection area: Around 500–750 Lux or higher depending on the task requirement
A common mistake is using the same 300 Lux level for every type of project.
For some spaces, 300 Lux may be sufficient. However, for continuous office work, reading areas, detailed work zones, or precision assembly areas, this level may not meet the actual usage requirement.

Therefore, before calculating the number of lights, contractors need to clearly identify:

What is this space used for?
How detailed is the visual task?
Does the project need to meet a specific lighting standard for acceptance?

Only when E is correctly defined can the lighting calculation be accurate.

PRACTICAL CALCULATION EXAMPLE: 60 m² OFFICE

Assume we need to calculate the number of lights for a 60 m² office.

Given information:

Area: 60 m²
Light fixture: 18W LED panel
Luminous flux per fixture: 1,800 lumens
White ceiling, cream-colored walls
Clean office environment with regular maintenance

Incorrect experience-based calculation:

60 m² ÷ 5 m²/light = 12 lights

At first glance, this may seem reasonable. However, this method does not consider the required illuminance, utilization factor, or maintenance factor.

A more accurate calculation using the lumen method:

In this example, we temporarily calculate based on 300 Lux.

Input values:

E = 300 Lux
A = 60 m²
F = 1,800 lumens/light
UF = 0.65
MF = 0.85

Apply the formula:

N = (300 × 60) ÷ (1,800 × 0.65 × 0.85)
N = 18,000 ÷ 994.5
N ≈ 18.1 lights

In lighting calculation, the result should always be rounded up, not rounded down.

Therefore:

18.1 lights → round up to 19 lights

If only 12 lights are installed based on experience, the actual illuminance will be around 200 Lux, much lower than the target of 300 Lux.

If this area is used as a regular office workspace requiring around 500 Lux, the required number of lights will be even higher.

This explains why the same 60 m² area may require very different numbers of lights depending on the actual purpose of the space.

LIGHTING ESTIMATION CHECKLIST FOR CONTRACTORS

Before quoting the number of lights for any project, contractors should confirm these 5 factors:

Required illuminance E

Do not use one Lux value for every space.
Clearly identify whether the area is a corridor, warehouse, office, classroom, factory, or quality inspection zone.
Each space has a different lighting requirement.

Actual luminous flux F

Do not look only at wattage.
Two 18W lights can have very different lumen outputs depending on LED chips, driver quality, optical design, and overall product quality.
For calculation, lumen output should be taken from the official catalogue or specification sheet.

Utilization Factor UF

Evaluate the ceiling, wall, and floor colors, mounting height, and light distribution of the fixture.
Do not assume UF = 1.
For projects requiring higher accuracy, photometric data or lighting simulation software should be used.

Maintenance Factor MF

Consider dust, lumen depreciation, and cleaning frequency.
The more dust, oil vapor, moisture, or chemicals in the environment, the more carefully MF must be considered.

Always round up

If the calculation result is 18.1 lights, choose 19 lights, not 18.
Rounding down can cause the project to be underlit during acceptance or after a period of operation.

CONCLUSION: DO NOT LET A PROJECT BECOME UNDERLIT BECAUSE OF WRONG CALCULATION

Calculating the number of lights based only on area is fast, but it is not safe enough for projects that require stable brightness and long-term performance.

For a more accurate lighting estimation, contractors need to calculate based on:

E – Required illuminance
F – Luminous flux
UF – Utilization Factor
MF – Maintenance Factor

If just one of these factors is ignored, the required number of lights may be underestimated by 30–50%.

A good lighting system is not only about installing enough fixtures.

More importantly, it must deliver the right brightness at the working area, maintain performance over time, and satisfy the end user after installation.

PARAGON LED PROVIDES FREE LIGHTING CALCULATION SUPPORT

With 28 years of experience in the lighting industry, Paragon LED is ready to support contractors, dealers, and installation partners in calculating the right number of lights for each specific project.

Simply provide:

Area to be illuminated
Ceiling height
Project type
Ceiling, wall, and floor colors
Purpose of the space
Clean or dusty environment

Paragon LED’s technical team will support you with a suitable lighting solution, helping the project achieve better brightness, reduce the risk of underlighting, and optimize investment cost.

Contact Paragon LED for free lighting calculation support for your project.

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