Home › Blog › High-Bay LED Fixture Layout
Published May 22, 2026 · By Industrial Lighting GR Editorial · ~13 min read
The number of high-bay LED fixtures a warehouse needs comes from the lumen method: target foot-candles times floor area, divided by lumens per fixture times the coefficient of utilization times the light loss factor. A 40,000 square foot West Michigan warehouse at 30 foot-candles, lit with 200-watt UFO high-bays, lands around 75 to 80 fixtures. Spacing then follows the fixture's spacing-to-mounting-height ratio. Get the count right and the spacing wrong and the building still has dark aisles, so both numbers matter.
Every warehouse lighting layout answers two separate questions. First, how many fixtures does the building need to hit its foot-candle target. Second, where do those fixtures go so the light is even across the floor. Facility managers usually ask the first question and assume the second takes care of itself. It does not. A building can have exactly the right fixture count and still leave dark gaps between aisles if the spacing ignores the optics. Both numbers have to be solved together.
This guide walks the math behind fixture count, the rule that governs spacing, a worked example for a typical West Michigan warehouse, and the way racking forces the layout off a clean grid. It is the design step that sits between picking a target light level and ordering the fixtures, and on most projects it is where the money is either spent well or wasted.
Four inputs drive how many high-bay fixtures a space needs. Change any one and the count moves.
Target foot-candles. This is the light level you are designing to, measured at the work plane. General pallet storage runs 20 to 30 foot-candles. Active picking and packing run 30 to 50. Inspection benches need 50 or more. Setting this target by zone, rather than lighting the whole building to the highest number, is covered in our warehouse foot-candle requirements guide.
Floor area. The square footage being lit. Simple enough, but the area that matters is the working floor, not the building footprint, so mezzanines, offices, and dock areas usually get pulled out and designed separately.
Lumens per fixture. The total light output of the fixture you are speccing. A 150-watt UFO high-bay produces roughly 21,000 lumens. A 200-watt unit produces roughly 28,000. A 240-watt unit pushes past 33,000. Higher output per fixture means fewer fixtures, but it also concentrates the light, which interacts with spacing.
Loss factors. Two numbers knock the theoretical output down to what actually reaches the floor. The coefficient of utilization (CU) accounts for light that never makes it to the work plane because of room shape, ceiling height, and surface reflectance. The light loss factor (LLF) accounts for dirt on the fixture, lumen depreciation as the LED ages, and ambient temperature. Together they are why you cannot just divide lumens by area and call it done.
The lumen method is the standard calculation for general lighting in a large open space. The formula:
Number of fixtures = (target foot-candles × area in square feet) ÷ (lumens per fixture × CU × LLF)
The target foot-candles and area are set by the job. Lumens per fixture comes from the product photometric file. The CU depends on the room: a tall, narrow warehouse with dark walls might land at 0.55, a lower, wider building with reflective surfaces at 0.75. For most West Michigan warehouse high-bay work the CU lands around 0.65. The LLF for an LED high-bay in a reasonably clean warehouse runs about 0.85, accounting for dirt and lumen depreciation over the maintenance cycle.
The method gives you an average foot-candle level across the floor. It does not tell you whether that average is evenly delivered, which is the job of the spacing rule and, ultimately, the photometric model. But it is the right starting point for fixture count.
Fixture count tells you how many lights to buy. The spacing-to-mounting-height ratio tells you how far apart to hang them.
Every high-bay fixture has a published spacing criterion, also called the spacing-to-mounting-height ratio, often written as SMH or simply the spacing ratio. It comes straight off the photometric file. The number describes how wide a beam the optic throws. A ratio of 1.0 means fixtures can sit one mounting height apart. A ratio of 1.5 means they can sit one and a half mounting heights apart and still overlap enough to keep the floor even.
The mounting height that matters is the height above the work plane, not the height above the floor. The work plane in a warehouse is usually taken at 30 inches, roughly the height of a pallet or a workbench. So a fixture hung at 25 feet sits 22.5 feet above the work plane. With a fixture rated at a 1.0 spacing ratio, the maximum spacing is 22.5 feet. Hang the fixtures wider than the ratio allows and the floor develops scalloped dark bands between rows, no matter how many fixtures the lumen method called for.
This is the trap behind a fixture-count rule of thumb. The count can be correct while the spacing quietly fails. Round (UFO) high-bays and linear high-bays carry different spacing ratios and behave differently over racking, which is the core of our high-bay vs linear high-bay comparison.
Walk a real layout. The building is 160 feet by 250 feet, 40,000 square feet of open storage floor. The ceiling is 28 feet, fixtures will mount at 25 feet. The operation is general pallet storage and order picking, so the target is 30 foot-candles. The spec is a 200-watt UFO high-bay producing 28,000 lumens with a spacing ratio of 1.0.
Fixture count. Run the lumen method with a CU of 0.65 and an LLF of 0.85:
(30 × 40,000) ÷ (28,000 × 0.65 × 0.85) = 1,200,000 ÷ 15,470 = about 78 fixtures.
Spacing. Mounting height above the 30-inch work plane is 22.5 feet. With a spacing ratio of 1.0, maximum spacing is 22.5 feet. Lay 78 fixtures into a grid: 6 rows of 13 works cleanly. Along the 250-foot length, 13 fixtures sit about 22.7 feet apart center to center after the end margins. Across the 160-foot width, 6 rows sit about 22.9 feet apart. Both are inside the 22.5-foot maximum once the half-spacing wall margins are accounted for, and the resulting spacing-to-mounting-height ratio is right at 1.0. The grid and the optics agree.
That is the design working as intended: the lumen method set the count at roughly 78, the spacing rule confirmed a 6-by-13 grid holds the light even, and nothing needs to be forced. If the math had called for 78 fixtures but the only grid that fit put them 30 feet apart, the answer would be a higher-output fixture, a tighter optic, or a different grid, not 78 fixtures hung too wide.
The worked example assumed an open floor. Most West Michigan warehouses are not open floors. They are rows of pallet racking 16 to 30 feet tall, and racking breaks the clean grid completely.
Pallet racking blocks and shadows light. A fixture centered over a rack row dumps most of its output onto the rack tops, where nobody works, and leaves the aisle below in shadow. The fix is to align the fixture rows with the aisle centerlines, not the building grid. The light has to drop down into the aisle, between the racks, onto the floor and the lower shelves where picking happens.
That aisle-aligned layout usually changes the fixture choice too. Linear high-bays and narrow-beam aisle optics throw a tighter, longer beam pattern that fits an aisle better than a wide UFO flood. Fixture count for a racked warehouse is driven less by total floor area and more by the number of aisles and their length. A building that would take 78 UFO high-bays as an open floor might take a different count entirely once the racking plan is laid in. This is why the rack layout has to be on the table before the lighting layout is finalized. Designing the lighting first and discovering the racking second is one of the most expensive sequencing mistakes in a warehouse retrofit.
Spacing fixtures by the count, ignoring the optics. The lumen method says 78 fixtures, so the installer drops 78 fixtures on whatever grid is convenient. If that grid exceeds the spacing ratio, the floor has dark bands. The count was right and the building still looks underlit.
One foot-candle target for the whole building. Lighting open storage to the same level as a packing bench wastes energy and money. Zones get different targets.
Ignoring lumen depreciation. Designing to the fixture's day-one output, with no LLF, means the building is correctly lit for about a year and progressively underlit after that. The LLF exists to design for the maintained light level, not the brand-new one.
Lighting the building, not the racking. A uniform grid over a racked warehouse lights the rack tops and shadows the aisles. The layout has to follow the aisles.
Skipping the photometric model. A rule of thumb gives an average. It cannot show a dark corner near a dock door, a hot spot under a cluster of fixtures, or the shadow a mezzanine throws. Only the model shows those.
The lumen method and the spacing rule get a layout 90 percent of the way. The last 10 percent, and the verification that the first 90 is actually correct, is the photometric model.
We run every West Michigan high-bay project through AGi32, the industry-standard photometric software. The model imports the real photometric file for the speccd fixture, the building dimensions, the ceiling height, the surface reflectances, and the racking plan. It then calculates the light level at a grid of points across the entire floor and produces a rendering that shows exactly where the building is bright, where it is dim, and how uniform the result is. The model reports the average foot-candle level, the minimum, and the uniformity ratio, which is the number that tells you whether the light is genuinely even or just averages out to the target while hiding dark spots.
The Illuminating Engineering Society publishes the task-based light level targets the model is checked against. The IES standards for industrial and warehouse spaces define both the target foot-candles and the acceptable uniformity for each task. A layout that hits the average but fails the uniformity ratio is not a passing design, and the model is the only tool that catches it before fixtures are ordered.
Every Industrial Lighting GR layout starts with a site walk. We measure the building, record the ceiling height and mounting height, document the racking plan and aisle layout, and note the surface reflectances and any obstructions such as HVAC duct, mezzanines, or crane rails. We set the foot-candle target by zone, not by building.
From there the lumen method gives a first-pass fixture count and the spacing ratio confirms a workable grid or aisle layout. We then build the AGi32 model and verify the average, minimum, and uniformity against the IES target before anything is ordered. Fixtures are speccd from the DesignLights Consortium qualified products list so the layout also clears the path for Consumers Energy and DTE rebates. The final layout drawing shows the exact fixture positions, mounting heights, and circuiting, and it is what the install crew works to.
The full retrofit scope is on our warehouse LED lighting page, and the LED retrofit page covers the move from fluorescent or HID into a modern high-bay layout. Fixture count and spacing are one decision in that process, but they are the decision that determines whether the building is actually lit the way the foot-candle target promised.
Fixture count comes from the lumen method: target foot-candles times floor area, divided by the lumens per fixture times the coefficient of utilization times the light loss factor. A 40,000 square foot West Michigan warehouse at 30 foot-candles, lit with 200-watt UFO high-bays around 28,000 lumens, typically needs roughly 75 to 80 fixtures. Ceiling height, target light level, and fixture output all move that number.
High-bay spacing is governed by the spacing-to-mounting-height ratio. Most warehouse high-bay optics carry a published ratio between 1.0 and 1.5. Multiply the mounting height above the work plane by that ratio to get the maximum spacing. At a 22.5-foot mounting height above a 30-inch work plane, a ratio of 1.0 puts fixtures about 22 feet apart. Wider than the published ratio allows produces dark gaps between fixtures.
Yes, in two ways. Higher ceilings spread each fixture's light over a larger floor area, which lowers the foot-candles reaching the floor and usually calls for higher-output fixtures or tighter optics. Higher ceilings also allow wider spacing because the spacing-to-mounting-height ratio scales with mounting height. A 35-foot ceiling and a 22-foot ceiling holding the same foot-candle target will land on very different fixture counts and layouts.
Racking changes everything. An open-floor layout can use a uniform grid, but a racked warehouse needs fixtures aligned over the aisles, not the rack tops. Rows of pallet racking block and shadow light, so the fixture rows have to follow the aisle centerlines. Aisle-aligned layouts often use linear high-bay or narrow-beam optics so the light drops down into the aisle instead of spilling onto rack tops where nobody works.
General storage and pallet handling usually target 20 to 30 foot-candles. Active picking, packing, and shipping zones run 30 to 50. Detailed inspection benches need 50 or more. The Illuminating Engineering Society publishes task-based targets in RP-7, and OSHA sets a separate minimum floor for general industry. Lighting a whole building to inspection levels wastes energy, so the foot-candle target should be set zone by zone.
A rule of thumb gives an average foot-candle figure but says nothing about uniformity, glare, or how light behaves around racking and obstructions. A photometric model in software such as AGi32 maps the actual light level at every point on the floor, flags dark spots and hot spots, and verifies the layout against the foot-candle target before any fixture is ordered. On a real warehouse, that prevents an expensive layout mistake.
Industrial Lighting GR's editorial is led by senior lighting designers with 15+ years of West Michigan industrial and commercial experience. We run AGi32 photometric models on every retrofit, set foot-candle targets and fixture layouts by working zone, verify uniformity against IES standards, and carry Consumers Energy and DTE rebate paperwork through pre-approval, install, and final payment. We service Grand Rapids, Wyoming, Kentwood, Walker, Holland, Muskegon, Kalamazoo, and surrounding West Michigan facilities.