Lighting Design Calculator

About the Lighting Design Calculator

Calculates how many light fixtures a room needs to reach a target illuminance (lux), using the lumen method with coefficient of utilization and light loss factor.

How to use this calculator

1. Enter the target illuminance in \(lux\), which is the light level you want on the surface.
2. Enter the area in square meters, then enter the lumens output of one fixture.
3. Add the coefficient of utilization \(CU\) and the light loss factor \(LLF\), which adjust for real-world performance.
4. Click calculate to estimate the number of fixtures needed, then round up to the next whole fixture.

The formula explained

The formula \(fixtures = \frac{lux \times area}{lumens \times CU \times LLF}\) computes the number of fixtures needed to deliver a target light level over a given area. It adjusts the raw light output for how efficiently the light reaches the work surface and for losses over time.

• fixtures = the estimated number of light fixtures needed
• lux = the target illuminance, or light level, in \(lx\)
• area = the space size in square meters, \(m^2\)
• lumens = the lumen output of one fixture
• CU = coefficient of utilization, the fraction of light that reaches the useful area
• LLF = light loss factor, the fraction of light still available after losses

Step by step method

1. Find the target light level \(lux\) and the room area \(area\).
2. Multiply \(lux\) by \(area\) to get the total light needed on the space.
3. Divide by \(lumens \times CU \times LLF\) to account for fixture output and losses.
4. Round up the result, because you cannot install part of a fixture.

Worked example

Problem. A classroom needs \(500\) \(lx\) over an area of \(60\) \(m^2\). Each fixture produces \(4{,}000\) lumens, with \(CU = 0.65\) and \(LLF = 0.80\). How many fixtures are needed?

1. Use the formula, \(fixtures = \frac{500 \times 60}{4{,}000 \times 0.65 \times 0.80}\).
2. Compute the denominator, \(4{,}000 \times 0.65 \times 0.80 = 2{,}080\). Then \(500 \times 60 = 30{,}000\).
3. Divide, \(\frac{30{,}000}{2{,}080} \approx 14.42\), so round up to \(15\) fixtures.

Tips and common mistakes

• Always round up, because a fraction of a fixture is not practical and rounding down can leave the space underlit.
• Make sure the area is in \(m^2\) and the fixture output is in lumens, because mixed units will give the wrong result.

Frequently asked questions