Fixture count
N = E x A / (Phi_fixture x UF x MF)
Target illuminance times floor area, divided by delivered lumens, utilization factor, and maintenance factor, rounded up.
Guide
Warehouse High Bay Lighting: Fixture Count, Mounting Height, and Rack Shadows
Why warehouse lighting is a geometry problem first and a fixture problem second, and how to size high bay counts that survive a real rack layout.
Choose the target for what happens on the floor
Storage and rack access generally needs 150 to 300 lux, order picking and staging about 300, and shipping desks or inspection activity 300 to 500. The target belongs to the task at the floor or at the rack face, and it is worth writing down, because a warehouse lit to the storage recommendation is not lit well enough for the picking team.
Mounting height decides almost everything downstream
Mounting above the rack top keeps light out of the rack faces and puts the fixture where it is hard to reach, while mounting below the rack top puts light on the floor but risks trestle shadows. Most designs land 1 to 1.5 metres above the rack top, then let the spacing rule set the grid. Height also drives maintenance: relamping a twelve metre ceiling is a different budget from relamping at eight.
Size the count from the illuminance, then draw the grid
Divide the target illuminance times floor area by delivered lumens, utilization factor, and maintenance factor. Warehouses lose more light than offices, so utilization factors of 0.6 to 0.8 and a maintenance factor of 0.7 are realistic defaults. Then arrange the result into a rectangular grid with rows parallel to the aisles: a count with an awkward aspect ratio usually means one more row and fewer columns, not more fixtures.
Line the rows up with the aisle pitch
Rack aisles create repeating bright and dark bands. Placing fixture rows on the aisle centre lines puts the light where the picker stands; placing them over the rack tops puts it where the racking is. Check the perpendicular spacing against the distribution: high bay photometry typically carries a spacing-to-height ratio of 0.9 to 1.3, and wide distributions let the grid stretch further.
Controls in a warehouse pay for themselves in the off shift
Aisles are occupied in a fraction of the shifts. Aisle-level occupancy sensing and dimming, stacked with daylight capture near dock doors, is usually the largest single saving available, and it changes the fixture arrangement: fixtures that can be dimmed in groups need circuiting that matches the sensing zones.
Formulas used
Grid from an aspect ratio
Columns = ceiling(sqrt(N x (L / W))), Rows = ceiling(N / Columns)
A fast way to turn a required count into rows and columns that match the room shape before real photometry is run.
Illuminance with the final grid
E = N_grid x Phi_fixture x UF x MF / A
Round the grid up, then check the light level the rounded grid actually delivers, because rounding always adds light rather than removing it.
Worked examples
A 100 by 60 foot racked warehouse
Inputs: 200 lux target, 20,000 delivered lumens per high bay, utilization factor 0.7, maintenance factor 0.7, 150 input watts.
The area is 557 m2, so the requirement is 200 x 557 = 111,400 lumens on the floor and 111,400 / 0.49 = 227,351 lumens at the source. At 20,000 lumens per fixture that is 11.4, so twelve high bays. Arranged on a 100 by 60 foot plan the grid lands near 4 by 3, which gives 80,000 square feet of coverage per fixture at a spacing of about 25 feet.
The same building lit to picking level
Inputs: The same shell, but 300 lux at the floor for order picking.
300 x 557 = 167,100 lumens on the floor, 341,020 at the source, or eighteen fixtures at 20,000 lumens. The extra six fixtures are the cost of picking accuracy, and they should be circuited by aisle so the off shift can dim them.
Questions readers ask
Should high bay fixtures be mounted above or below the rack top?
Above, in most racked buildings, because light aimed down the aisle at the picker is more useful than light striking the rack fronts. Mounting below the rack top is common in open storage and manufacturing halls where there is no racking to shadow the floor.
Does rack geometry change the fixture count enough to matter?
Yes. Racking absorbs light and narrows the effective room cavity, which lowers the utilization factor. A layout screened from an empty-floor assumption can be ten to twenty percent short once the racks are installed, which is why the count should be confirmed with photometry for the real rack layout.
Is LED always the right answer for a high bay retrofit?
Usually, on energy and maintenance, but check the light distribution rather than only the wattage. A high output LED with a narrow distribution can leave the floor dimmer than the metal halide it replaced, because the optical efficiency of the old high bay was lower but its beam was wide.
Size the high bay count for your building
Sources
- Standards and recommended practice Illuminating Engineering Society · 2026-10-15
- Illumination, construction industry standard 1926.56 U.S. Occupational Safety and Health Administration · 2026-10-15
- Qualified Products List for commercial lighting DesignLights Consortium · 2026-10-15