Indoor Lighting Level Calculations: Zonal Cavity Method

How Many Light Fixtures Do I Need?
Indoor Lighting Level Calculations using Zonal Cavity Method

Step 1. Determine cavity ratios.

Cavity Ratio = (2.5 x height of cavity x cavity perimeter)  / area of cavity base

Step 2. Determine effective cavity reflectances

Use the Effective Cavity Reflectance Chart below.

Step 3. Select coefficient of utilization

Use the manufacturer's tables with effective cavity reflections from Step 2 for specific fixture type.

Step 4. Find the Number of Fixtures Needed

Use formula below to determine appropriate number of fixtures.

Number of Fixtures = (room area in square feet x desired foot  candles) / (lumens  per fixture x coefficient of utilization)

Find recommended foot candles for indoor and outdoor locations.

Note: This method is used for estimating purposes only and other factors may contribute to resulting illuminance level. Refer to the IESNA Lighting Handbook for further details.

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In This Section:

Effective Cavity Reflectance Chart

Ceiling to Floor Reflectance 90%
Wall Reflectance 90% 70% 50% 30%
Cavity Ratio
0.2 89 88 86 85
0.4 88 86 84 81
0.6 87 84 80 77
0.8 87 82 77 73
1.0 86 80 75 69
1.2 85 78 72 66
1.4 85 77 69 62
1.6 84 75 67 59
1.8 83 73 64 56
2.0 83 72 62 53
2.2 82 70 59 50
2.4 82 69 58 48
2.6 81 67 56 46
2.8 81 66 54 44
3.0 80 64 52 42
3.2 79 63 50 40
3.4 79 62 48 38
3.6 78 61 47 36
3.8 78 60 45 35
4.0 77 58 44 33
4.2 77 57 43 32
4.4 76 56 42 31
4.6 76 55 40 30
4.8 75 54 39 28
5.0 75 53 38 28
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Effective Cavity Reflectance Chart

Ceiling to Floor Reflectance 50%
Wall Reflectance 70% 50% 30% 10%
Cavity Ratio
0.2 49 48 47 30
0.4 48 47 45 30
0.6 47 45 43 30
0.8 47 44 40 30
1.0 46 43 38 30
1.2 45 41 36 30
1.4 45 40 35 30
1.6 44 39 33 29
1.8 43 38 31 29
2.0 43 37 30 29
2.2 42 36 29 29
2.4 42 35 27 29
2.6 41 34 26 29
2.8 41 33 25 29
3.0 40 32 24 29
3.2 39 31 23 29
3.4 39 30 22 29
3.6 39 29 21 29
3.8 38 29 21 28
4.0 38 28 20 28
4.2 37 28 20 28
4.4 37 27 19 28
4.6 36 26 18 28
4.8 36 26 18 28
5.0 35 25 17 28
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Effective Cavity Reflectance Chart

Ceiling to Floor Reflectance 10%
Wall Reflectance 50% 30% 10%
Cavity Ratio
0.2 10 10 09
0.4 11 10 09
0.6 11 10 08
0.8 11 10 08
1.0 12 10 08
1.2 12 10 07
1.4 12 10 07
1.6 12 09 07
1.8 13 09 06
2.0 13 09 06
2.2 13 09 06
2.4 13 09 06
2.6 13 09 06
2.8 13 09 05
3.0 13 09 05
3.2 13 09 05
3.4 13 09 05
3.6 13 09 04
3.8 14 09 04
4.0 14 09 04
4.2 14 09 04
4.4 14 08 04
4.6 14 08 04
4.8 14 08 04
5.0 14 08 04
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Warning: When using this information to perform electrical work, call a licensed electrician and consult the NEC® for safety. All licensed electricians have passed examinations covering the National Electric Code®, know state and local building codes, and may carry insurance to cover damages.