Lighting Calculator
The lumen method — room × footcandles ÷ lumens per fixture — plus the spacing rule that places them: height ÷ 2, offset half that.
Lighting Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Uniform-layout assumption — no ray-tracing.
- No dimmer/layer split beyond the note.
- fc targets are residential IES guidance, not code.
In short: A 12 × 14 living room at 20 fc: 168 ft² × 20 = 3,360 lumens → 4.2 → 5 fixtures at 800 lm each. Spacing: 8-ft ceiling ÷ 2 = 4 ft on center, wall offset 2 ft. The 14 × 18 case at 9 ft: 6.3 → 7 fixtures, 4.5-ft spacing.
Formula
lumens = area × footcandles
fixtures = ⌈lumens ÷ lumens per fixture⌉
spacing = ceiling height ÷ 2 (o.c.)
wall offset = spacing ÷ 2
[('1 fc', '1 lumen per ft² — the target unit'), ('10/20/40/75', 'hall / living / kitchen / task fc'), ('÷ 2', 'spacing = height over two'), ('800 lm', 'the 60W-equivalent LED standard')]
Worked Example
- Measure the room; multiply for area.
- Pick the fc target off the ladder.
- Lumens = area × fc; divide by fixture lumens.
- Round up — dimmers eat the surplus.
- Space at height ÷ 2; offset rows half that.
12 × 14 at 20 fc: 3,360 lm → 5 × 800-lm fixtures, 4-ft spacing, 2-ft offset. 14 × 18 at 9 ft: 6.3 → 7 fixtures at 4.5-ft spacing.
Strengths & Limits Of This Model
Where this engine is strong
- Count AND placement from two honest rules
- The fc ladder carries the room-type judgment
- Lumens-first, the only LED-era unit that matters
Where it stops
- Assumes even ceiling height
- No daylight contribution
Practical Use Cases
Recessed layouts
Count and place the cans.
Kitchen plans
The 40-fc counter band.
Home offices
Task-fc upgrades.
LED swaps
Lumens, not watts, forever.
Aging eyes
The double-fc rule.
Methodology & Editorial Standards
Lumen method: lumens = (l × w) × fc target (free-entry 5-100 with the IES ladder as chips); fixtures = ceil(lumens ÷ fixture lumens). Placement per the height-halving rule: spacing = h ÷ 2 on center, wall offset = spacing ÷ 2. Zero/negative dimensions refused; fc outside 5-100 and fixture lumens outside 200-3,200 refused.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Lighting Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
How many lumens does a living room need?
Area × target: a 168-ft² living room at 20 fc wants 3,360 lumens — about five 60W-equivalent LED bulbs' worth, split across layers rather than one blinding center fixture.
How far apart should recessed lights be?
Ceiling height ÷ 2, on center: 8-ft ceilings space cans 4 ft apart, with the nearest row half that — 2 ft — off the wall. That overlap makes even wash instead of hot spots.
What is a footcandle?
One lumen falling on one square foot — the working unit of IES lighting design. Kitchens work at 40, hallways manage at 10, reading wants 75+.
How many recessed lights for a 12x14 room?
Five 800-lm fixtures covers the 20-fc target — 3,360 lumens ÷ 800 = 4.2, rounded up. Lay them roughly 4 ft apart with two rows offset 2 ft from the long walls.
Are 60 watts and 800 lumens the same?
The wattage is the PAST — incandescent draw. The lumen is the light: a 60W-equivalent LED makes ~800 lm on 12 W. Buy lumens, ignore watts.
Does ceiling height change the count?
Not the count — the fc law is area-based. Height changes PLACEMENT: taller ceilings push spacing wider (height ÷ 2) but dim the floor; very tall rooms add fixtures or drop the pendants.
Does wall color change the lumen count?
Dark walls absorb and bright walls bounce — professional layouts adjust the target a few fc for room finishes. The area × fc law is the starting budget; the paint is the trim adjustment.
Why round fixtures up?
4.2 fixtures means the budget is 5% short — the room reads dim. Dimmers turn surplus into comfort; shortfall is a re-wire.