Light calculators: lumens, distance and running cost
How many lumens a layer needs for a room's size and surfaces, what moving a lamp does to the light on the page, and what a bulb costs to run at your rate. Pre-filled from your setup.
These are planning numbers. The lumen calculator rests on published targets that differ from each other by several times and on a coefficient of utilization we assume rather than look up, so its answer is a starting range for a trial, not a purchase list. The move-the-lamp estimate holds for a small source. Your own before-and-after reading in the same spot, with the same device, settles each of them (Survey and test your rooms).
How many lumens a layer needs
Enter the room's floor size, pick the room or task, and say how light the walls and ceiling are. The result shows each published target beside the lumens it implies, so the disagreement between them is in front of you. Layers, fixtures and glare explains the method.
Assumptions behind the lumen calculator
- The lumen method. Lumens needed = target illuminance in footcandles × floor area in square feet ÷ (coefficient of utilization × light loss factor). An-Najah National University's lighting design lecture gives the formula and says the method assumes an empty room, perfectly diffuse surfaces and estimated factors. Physical principle
- Coefficient of utilization (CU) is the share of the bulbs' lumens that reaches the work plane. It depends on the fixture and on how much light the walls, ceiling and floor reflect. We found no free published CU for small residential rooms, so the calculator uses bands that are our assumption: light surfaces 0.40 to 0.50, medium 0.30 to 0.40, dark 0.20 to 0.30. They bracket the 0.48 in Cornell's zonal cavity worked example (an office of 30 by 20 ft with 80, 50 and 20 percent reflectances), set lower because small rooms lose more light to their walls. Convention
- Light loss factor (LLF) allows for bulbs dimming with age and dirt. The calculator uses 0.8; Cornell's example works out to 0.78.
- Defense Department targets come from section 3-9 of the Unified Facilities Criteria for interior lighting (UFC 3-530-01, 2023), written for military housing: living room and hallway 30 lux at the floor, bedroom 50 lux at 2 ft, kitchen 50 lux at the floor and 300 lux at the cooking surface, dining 50 lux at the table, bathroom 300 lux at 3 ft, laundry 200 lux at 3 ft, and stairs at least 100 lux on the treads (section 3-3.2). They are averages of maintained light. Convention
- Commonly cited ranges come from Penn State's EGEE 102 course notes, which attribute them to the Illuminating Engineering Society without naming an edition; they match older IES category tables. The current IES residential practice is sold, not free, and sets levels by age group, so the page never says "IES recommends". Convention
- Older adults: the Lighting Research Center's guidance (archived copy of a page the Lighting Research Center no longer hosts) gives at least 300 lux of ambient light and at least 1,000 lux on tasks. The calculator applies 1,000 to the kitchen task and home office choices and 300 to the rest. Convention
- Units. 1 footcandle = 10.764 lux, from NIST Special Publication 811, Appendix B. One square foot = 0.0929 m².
- Rounding. Lumens are rounded to the nearest 10 and bulb counts up to the next whole bulb. The default 800 lumens is the replacement for an old 60-watt bulb in ENERGY STAR's bulb purchasing checklist (a pre-2025 document; ENERGY STAR no longer certifies bulbs, but the lumen table still holds).
- Task choices give the light to bring the whole room to a task level. A lamp or under-cabinet light aimed at the work reaches the level on the work with far fewer lumens; measure it there.
Move the lamp
For a small source, the light on a surface falls with the square of the distance: twice as far gives a quarter of the light, half as far gives four times as much. Enter a reading taken now and the two distances. How light works in a room explains the law and where it stops holding.
Assumptions behind the distance estimate
- Estimated reading = reading now × (distance now ÷ new distance)². HyperPhysics at Georgia State University states the law for a point source: whatever reaches a surface at 1 meter is one fourth as much at 2 meters. Physical principle
- It assumes the light comes from one small source aimed at the surface, with little light bounced from walls. A large shade, a long strip or light bounced off the ceiling falls off more slowly, and no source we fetched gives a rule for when a fixture counts as small. The reading after the move is the answer. Test it
Running cost and efficacy
What one bulb costs to run in a year, at your rate and at the rate printed on the label, and how many lumens it gives per watt. Bulbs, labels and color explains the label and the federal rules.
Assumptions behind the cost estimate
- kWh a year = watts × hours a day × 365 ÷ 1,000; cost = kWh × rate. The Lighting Facts label's yearly cost uses 3 hours a day and 11 cents per kWh, under the FTC's labeling rule (16 CFR 305.23). In its 2024 proposal the FTC kept that figure, saying the label is for comparing bulbs, not predicting bills. Convention
- Efficacy = lumens ÷ watts. Since July 25, 2022, the sale of general service lamps below 45 lumens per watt is prohibited (10 CFR 430.32(dd)). Product-specific
- The 2028 line uses the equation in 10 CFR 430.32(dd) for integrated omnidirectional short lamps without standby mode, the class that covers a common A-shape LED bulb, for lumens from 310 to 3,300. It applies to lamps made or imported from July 25, 2028; DOE's 2024 final rule gives 124.6 lumens per watt at 810 lumens as its example. Smart bulbs with standby mode have a lower line. These rules are current as of 2026-10-14; this manual re-reads them each year.
- The calculator does not count the cost of the bulb, how long it lasts in your fixture, or heat. Your bill depends on how many bulbs, their hours and your rate.
See it with a fictional room
Marisol rents a one-bedroom apartment. Her living room is 12 by 14 ft (168 sq ft) with medium gray walls, so the calculator uses the medium band. Her lights add up to 2,050 lumens: two 800-lumen bulbs in the ceiling fixture and a 450-lumen bulb in the floor lamp. The Defense Department line for a living room (30 lux at the floor) gives 1,460 to 1,950 lumens; the commonly cited range (10 to 20 fc) gives 5,250 to 14,000. Under these assumptions her room sits just above the first and far below the second. Neither settles whether the room is dark for her: her phone readings before and after a change do (Survey and test your rooms, Your lighting plan).
Harold reads under a table lamp 36 in from the page, and his meter reads 140 lux there. Moving the lamp to 24 in gives an estimated 315 lux; to 18 in, 560 lux. The Lighting Research Center's floor for older adults' tasks is 1,000 lux, so a closer lamp helps and a brighter bulb is the next trial. His own reading, and whether the page is comfortable, decide.
Marisol's 800-lumen bulbs use 9 watts. At 3 hours a day each costs $1.08 a year at the label's 11 cents and $1.68 at the 17 cents on her bill, at 89 lumens per watt. An old 60-watt incandescent bulb of the same brightness used 65.7 kWh a year, $7.23 at 11 cents, at 13 lumens per watt. All figures are invented for the example.
The plan sheet's lumens column (layers) takes the range from the first calculator; the tests sheet (tests) holds the readings that overrule it.