Lighting Layout & Foot-Candle Calculator
Enter the room size, ceiling height and fixture to get a fixture count, grid spacing, estimated foot-candles and watts per square foot. Built for contractors, energy auditors and facility teams sizing commercial spaces before a retrofit or new install.
Lighting layout calculator
Enter the space type, room length and width, ceiling height and the fixture's lumens and watts. The calculator returns the number of fixtures, rows and columns, spacing, estimated average foot-candles, total watts and watts per square foot, with a top-view layout. Advanced settings: target foot-candles, work plane height, light loss factor, coefficient of utilization, spacing criterion, and feet or meters with foot-candles or lux. Results are estimates, not a photometric layout.
Foot-candle and lux converter
1 foot-candle equals 10.764 lux, and 1 foot-candle is 1 lumen per square foot. Enter foot-candles to get lux, lux to get foot-candles, or foot-candles and an area to get the lumens needed.
Recommended foot-candles by space type
Use these as starting points when you pick a target. They are typical design targets based on common IES guidance, not code minimums. Verify for your project, especially where the work involves fine detail or safety risk.
| Space | Typical target (fc) | Typical range (fc) |
|---|---|---|
| Open office | 30 | 30 to 50 |
| Private office | 40 | 30 to 50 |
| Conference room | 30 | 30 to 40 |
| Classroom | 40 | 30 to 50 |
| Corridor | 10 | 5 to 15 |
| Lobby | 15 | 10 to 20 |
| Restroom | 15 | 10 to 20 |
| Retail sales floor | 50 | 30 to 75 |
| Warehouse, bulk storage | 15 | 10 to 20 |
| Warehouse, active or picking | 30 | 20 to 35 |
| Manufacturing, general | 50 | 30 to 75 |
| Fine assembly | 100 | 75 to 150 |
| Gymnasium | 50 | 30 to 75 |
| Loading dock | 20 | 15 to 25 |
| Parking garage | 5 | 2 to 10 |
Typical design targets based on common IES guidance. Verify for your project.
How the lighting layout calculator works
It uses the lumen method, the same approach lighting designers use for a first pass on a room. The formula is:
Fixtures = (target foot-candles × area) ÷ (lumens per fixture × CU × LLF)
- Target foot-candles is the average light you want on the work plane, usually desk height (2.5 ft).
- Lumens per fixture comes from the spec sheet. Use initial lumens, not wattage.
- CU (coefficient of utilization) is the share of a fixture's lumens that reaches the work plane. It drops in narrow or tall rooms, because more of the light hits the walls first. The calculator works out the room cavity ratio, RCR = 5 × height above the work plane × (length + width) ÷ (length × width), and picks a CU for a typical fixture in a room with 80% ceiling, 50% wall and 20% floor reflectance.
- LLF (light loss factor) is the allowance for lumen depreciation and dirt over time. 0.80 is a common planning value for LED.
| Room cavity ratio | Default CU |
|---|---|
| Up to 2 (large, low rooms) | 0.75 |
| 2 to 4 | 0.65 |
| 4 to 6 | 0.55 |
| 6 to 8 | 0.48 |
| Over 8 (small, tall or narrow rooms) | 0.42 |
Then it lays the fixtures out in an even grid and checks spacing. If fixtures would sit more than 5% farther apart than the spacing criterion times the mounting height above the work plane, it adds a row or column and tells you why. Up to 5% over, it keeps the grid and shows a note. The default spacing criterion depends on the fixture type: 1.5 for troffers, 1.3 for strips, 1.2 for downlights and high bays. Fixture counts are always rounded up, never down.
Why it's an estimate: the real CU depends on the fixture's photometry and the actual room surfaces, and the lumen method gives an average. It doesn't show dark spots between fixtures or at the walls. For a final design, or anything that has to meet a specification, run the manufacturer's IES files in AGi32 or DIALux.
How to calculate foot-candles by hand
Here is the same math the calculator does, for a 40 × 30 ft office with a 9 ft ceiling, a 30 fc target and 2x4 LED troffers rated 4,000 lumens.
- Area: 40 × 30 = 1,200 sq ft.
- Height above the desk: 9 − 2.5 = 6.5 ft.
- Room cavity ratio: 5 × 6.5 × (40 + 30) ÷ (40 × 30) = 1.9, so the CU is about 0.75.
- Lumens needed on the desk: 30 fc × 1,200 sq ft = 36,000 lumens.
- Fixtures: 36,000 ÷ (4,000 × 0.75 × 0.80) = 15 troffers.
- Layout: 3 rows of 5 gives 8 ft spacing along the length and 10 ft across the width.
- Check the average: 15 × 4,000 × 0.75 × 0.80 ÷ 1,200 = 30 fc.
Last, check spacing against the troffer's spacing criterion. 10 ft ÷ 6.5 ft is 1.54, about 3% over a spacing criterion of 1.5. That's within normal tolerance for troffers, so the 8 × 10 grid stands, and the calculator keeps 15 with a note. If the rows were more than 5% over, you'd add a row: 4 rows of 5 is 20 troffers and about 40 fc.
To go the other way and find the foot-candles an existing layout gives you: fc = (number of fixtures × lumens × CU × LLF) ÷ area.
What is a foot-candle?
A foot-candle (fc) measures how much light lands on a surface: one lumen spread over one square foot. Lux is the metric version, one lumen per square meter, so 1 fc equals 10.764 lux. Lumens describe what a fixture puts out. Foot-candles describe what actually reaches the desk or floor. To measure it, hold a light meter flat at work-plane height, usually 30 inches, and take readings under fixtures and between them.
Common layout mistakes on retrofits
Swapping one for one without checking light levels
Old fluorescent and HID fixtures have usually lost a good share of their output by the time you audit them. A new LED at the "equivalent" wattage can put noticeably more light on the floor than the space had. Run the numbers before you assume the same count and wattage. A lower-lumen kit or dimming often meets the target for less energy.
Ignoring a change in ceiling height
When a drop ceiling comes out, or pendants become surface mounts, the mounting height changes, and so do the CU and the maximum spacing. Recalculate with the new height instead of reusing the old layout.
Mixing color temperatures
3500K fixtures next to 5000K replacements look like a mistake to the client, even when the light level is right. Pick one color temperature per space, write it on the order, and check it on delivery.
Spacing too wide at the walls
The outer row should sit no more than about half the fixture spacing from the wall. Farther than that, walls and shelving go dim and the room feels darker than the meter says. The calculator places the outer rows this way.
Designing to initial lumens
A layout that hits the target on day one falls short a few years later, faster in dusty spaces. Keep a light loss factor in the math, 0.80 for clean spaces with LED and lower for dirty ones.
From layout to proposal
A layout is step one. LumaQuote turns it into an area-based audit with LED options, rebates and a client-ready proposal.
The fixture count is only part of the job. A full lighting audit adds operating hours, controls and utility rates, and the proposal has to show savings and payback the client can check. Here's how to build a lighting retrofit proposal that wins the job.
Start freeFree plan: 3 complete proposals, no credit card.
Lighting calculator FAQ
How many foot-candles do I need for an office?
Most open offices are designed for about 30 fc on the desk, and private offices for 30 to 50 fc. Conference rooms are usually 30 to 40 fc, often with dimming for presentations. If the work involves fine detail, or the people doing it are older, design toward the top of the range.
How do I convert lux to foot-candles?
Divide lux by 10.764. For example, 500 lux ÷ 10.764 = 46 fc. To go from foot-candles to lux, multiply by 10.764. The converter on this page does both.
How many lumens equal one foot-candle?
One foot-candle is one lumen per square foot. So 30 fc on a 1,200 sq ft floor means 36,000 lumens reaching the work plane. Fixtures have to put out more than that, because not all of their light gets there. Divide by CU × LLF, about 0.45 to 0.6 in a typical room, to get the fixture lumens to install.
How far apart should light fixtures be spaced?
Keep center-to-center spacing at or below the fixture's spacing criterion times the mounting height above the work plane. For troffers in a 9 ft ceiling, that usually means 8 to 10 ft. Fixtures next to a wall should sit about half that distance from it. The spacing criterion is listed with the photometric data on the spec sheet.
Is this calculator accurate enough for a final design?
No. It's an estimate for scoping, budgeting and proposals. It assumes a typical CU and gives an average light level, not uniformity or point-by-point values. For a final layout, use the manufacturer's IES files in AGi32 or DIALux, especially where codes, specifications or safety are involved.
Does it work for LED retrofits of existing fixtures?
Yes. Enter the new LED fixture's lumens and watts and the room as it is, then compare the suggested count with what's installed. If the space has more fixtures than it needs, a lower-lumen kit or dimming can save energy. If it has fewer, the space may already be under-lit. For the full retrofit with savings, rebates and a proposal, LumaQuote handles it area by area.