Lighting Calculators and Tools

Recessed lighting layout calculator

The rule most electricians start from is simple: divide the ceiling height by two, and that is your spacing in feet. An eight-foot ceiling wants cans about four feet apart, a ten-foot ceiling about five. Hold the first row roughly half that distance off the wall so the light washes the wall evenly instead of scalloping the baseboard.

That gets you a grid that will not look wrong. It does not get you a lighting design. Beam angle changes everything, and a 25-degree spot at ten feet covers a fraction of what a 60-degree flood covers. Dark walls, dark floors and tall furniture all eat light that the arithmetic assumes you keep. And a room lit only by a downlight grid tends to look flat and unflattering, which is why good residential schemes layer in sconces, pendants or lamps rather than simply adding more cans.

The footcandle targets behind the lumen figure are typical published values for each room type. The Illuminating Engineering Society publishes the authoritative recommendations, and commercial projects should be designed to those rather than to a rule of thumb.

Incandescent, halogen, CFL and LED: what replaces what

Wattage never measured brightness. It measured how much power a filament drew. Because every incandescent bulb was about as inefficient as every other, watts became a usable shorthand for light output, and two generations of buyers learned to shop that way. LEDs broke the shorthand completely. A nine-watt LED and a sixty-watt incandescent put out roughly the same light, so the only number that still means brightness is lumens.

Lumens is the figure to shop on, and it is printed on the Lighting Facts label on every bulb sold in the US. The equivalent wattages above are typical values rather than guarantees, because efficiency varies by manufacturer, by colour temperature and by colour rendering. A 90+ CRI lamp usually draws a little more power for the same lumens, and that is a trade worth making anywhere finishes, food or faces are on display.

Three other things matter as much as brightness when you swap. Check the bulb is marked dimmable and appears on your dimmer manufacturer's compatibility list. Check it is rated for enclosed fixtures if it is going into a sealed globe, because trapped heat shortens driver life. And match the location rating for damp or wet spaces the same way you would match the fixture.

Energy cost and payback calculator

The arithmetic is straightforward: fixtures multiplied by watts, multiplied by hours per day, multiplied by 365, divided by 1,000 gives kilowatt hours a year. Multiply by your rate per kilowatt hour and you have the annual cost. Your actual rate is on your utility bill and is usually a little higher than the headline number once delivery charges are included.

What the calculator leaves out generally works in your favour. It ignores labour, which matters on a large retrofit. It ignores the reduction in air-conditioning load that comes from fixtures running cooler, which is not trivial in South Florida. It ignores lamp replacement cost and the labour of getting to a fixture on a high ceiling. And it ignores utility rebates, which for commercial projects can cover a meaningful share of the changeover.

For commercial work the honest answer is that a payback under two years is usually easy to justify, and most lighting retrofits in older buildings land well inside that once controls are included.

Colour temperature visualiser

Colour temperature describes whether light reads warm and yellow or cool and blue. It is measured in Kelvin, and the scale runs backwards from what instinct suggests: lower numbers are warmer, higher numbers are cooler. 2700K is the familiar incandescent glow, 4000K is the neutral white of an office, 5000K and above starts to read like daylight or a hospital corridor.

The single most common mistake is mixing temperatures in one space. A 3000K downlight next to a 2700K pendant looks like a mistake even to people who could not name what is wrong, because the eye is very good at comparing two whites side by side and very bad at judging one in isolation. Pick a temperature for the room, and keep it consistent across any sightline where two sources are visible at once.

Colour rendering matters just as much and gets far less attention. CRI measures how faithfully a light source shows colour compared with daylight, on a scale to 100. An 80 CRI lamp is acceptable in a garage and disappointing over a dining table. Anywhere finishes, artwork, food or skin tones are on show, specify 90 CRI or better and accept the small efficiency penalty.

Beam angle and spread calculator

Beam angle decides whether a fixture washes a wall or puts a tight pool on a coffee table. The geometry is simple: the beam diameter is twice the distance multiplied by the tangent of half the angle. A 40 degree fixture eight feet up throws a circle roughly six feet across. The same fixture on a twenty foot ceiling throws nearly fifteen feet, which is why lighting a tall space with the same trims you used downstairs rarely works.

One subtlety worth knowing: beam angle is conventionally measured to the point where intensity has fallen to half its centre value. There is usable light beyond that cone, called the field angle, so a beam does not stop dead at the number on the spec sheet. Treat the calculated diameter as the bright core rather than the whole footprint.

If you have the centre beam candlepower from the manufacturer's data, the calculator will also give you the light level directly under the fixture. That figure obeys the inverse square law, which is the least intuitive thing about lighting: doubling the distance does not halve the light, it quarters it. It is the reason high ceilings need so much more output than people expect.

Chandelier and pendant sizing

The oldest rule in the trade still works: add the room's length and width in feet, and the answer in inches is roughly the right fixture diameter. A twelve by fourteen room takes something around twenty-six inches across. For height, allow two and a half to three inches of fixture for every foot of ceiling, so a nine foot ceiling suits a fixture roughly twenty-two to twenty-seven inches tall.

Over a dining table the room dimensions stop mattering and the table takes over. Aim for between half and two thirds of the table's width, and never go wider than the table minus twelve inches, or people at the ends will knock their heads reaching across. Hang it thirty to thirty-six inches above the table top at an eight foot ceiling, and add roughly three inches for every additional foot of ceiling height.

These are proportion rules, not physics, and a good eye beats them. An airy fixture of thin metal and glass can go noticeably larger than the arithmetic suggests without dominating a room, while a solid drum shade at the same diameter will feel oppressive. Long tables and two-storey foyers usually want a linear fixture or a matched pair rather than one enormous chandelier.

Low-voltage landscape voltage drop

Voltage drop is what quietly ruins landscape lighting installations. Twelve volt systems have almost no margin, so resistance in a long cable run arrives at the far fixture as visibly dimmer, warmer light. The calculation is standard: two times the run length, times the current, times the wire's resistance per thousand feet, divided by a thousand.

The figure above is deliberately worst case, because it assumes every watt sits at the far end of the run. Real installations rarely look like that. Running a hub layout, where several short spurs radiate from a central point, or feeding from the middle of a run rather than one end, cuts the drop dramatically for the same amount of copper.

How much drop you can live with depends on the lamp. Halogen is unforgiving and shifts noticeably warm and dim below about 10.8 volts. LED landscape fixtures are far more tolerant, and most run happily anywhere between nine and fifteen volts, which is why multi-tap transformers exist. Check the fixture's stated input range rather than assuming, and remember that going up one wire size is almost always cheaper than a service call.

LED tape run length and driver sizing

Two things go wrong with LED tape more than anything else: undersized drivers and over-long runs. The driver sizing is arithmetic. Multiply watts per foot by the length, then add headroom. Twenty per cent is the usual minimum, because running a driver at its absolute rated limit shortens its life and leaves nothing for the inrush when the circuit energises.

Run length is a physical limit rather than a preference. Voltage drops along the tape itself, so the far end of a long run is dimmer than the near end, and beyond a certain length it becomes visible. Twenty-four volt tape typically tolerates roughly twice the run of twelve volt for the same drop, which is why most quality architectural tape is now twenty-four volt. The manufacturer publishes the real maximum for their product and it should always win over any rule of thumb.

If the run is too long, the fix is usually feeding it from both ends or splitting it into sections with their own home runs, rather than accepting the fade. And if the tape is dimmable, the driver must be dimmable too and must match the control protocol, whether that is 0-10V, forward phase, reverse phase or DALI. A dimmable tape on a non-dimming driver is a surprisingly common and entirely avoidable mistake.

Wall wash, graze and accent spacing

Washing, grazing and accenting are three different jobs and they want the fixture in three different places. Washing sets it back from the wall so the beam covers the whole plane evenly. Grazing pulls it tight against the wall so light rakes across the surface. Accenting aims it at one object at roughly thirty degrees, which is the angle that lights a picture without bouncing the reflection back into the viewer's eyes.

The thirty degree convention is worth internalising because it solves two problems at once. Shallower than that and you get a hot reflection off glass or a glossy finish, straight into the face of anyone standing in front of the piece. Steeper and the object is lit flat from above, which flattens texture and drops a shadow off the frame onto the wall below.

One warning about grazing: it is brilliant on stone, brick, board-formed concrete or heavy plaster, and merciless on drywall. Raking light across a flat painted surface reveals every taping seam, screw pop and roller mark. If the wall is not meant to be looked at that closely, wash it instead.

Dimming compatibility checker

This one has its own page too — dimmer compatibility checker — and the full explanation of forward phase, ELV, 0-10V, DALI and PWM is in the lighting controls guide.

Most dimming complaints — flicker, buzz, a lamp that will not go below half, a bank that drops out all at once — trace back to a mismatch between the load and the control. The pairings are not interchangeable, and some of them are actively damaging rather than merely disappointing.

The two that cause real harm are worth memorising. A magnetic low-voltage transformer must be dimmed on forward phase; reverse phase can introduce a DC component and cook it. An electronic low-voltage transformer is the opposite, and forward phase on one causes buzzing and early failure. Getting these backwards is a warranty call rather than an adjustment.

The other thing that catches people is minimum load. Dimmers were designed for incandescent circuits drawing hundreds of watts, and many still specify a minimum. Replace six 60-watt lamps with six 9-watt LEDs and the circuit that drew 360 watts now draws 54, which can fall below what the dimmer needs to operate. The symptom looks like a faulty lamp and is actually a control problem.

Fixture schedule builder

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Build the schedule line by line, then send it straight to us for pricing. Quantity and fixture type are all that is required — brand, catalogue number, room and notes are there if you have them, and the more you fill in the tighter the quote comes back. Nothing is stored on our servers while you work; the schedule lives in your browser until you choose to send it.

Pressing send carries the schedule into our quote form and drops it into the message, so you only fill in your contact details once. You can also copy it to the clipboard if you would rather paste it into an email, a spreadsheet or your own bid document.

This is the fastest way to get a real number out of us. A schedule with quantities and catalogue numbers can usually be priced the same day. A photograph and a rough description takes longer, because someone here has to work out what you meant before anyone can price it.