PAR estimator
Roughly how much light is reaching a coral where it actually sits — from a published figure or your own meter reading, scaled by how far the light has to travel. An estimate with its uncertainty shown, not a substitute for a meter.
Your light
Start from a published figure, or — better — from a reading you have taken yourself. Any PAR number works as long as you know how far from the fixture it was measured.
Why is the fixture list so short?Because almost no reef LED manufacturer publishes a PAR figure tied to a stated distance, and a PAR number without the distance it was measured at cannot be scaled to your tank. Checked while building this: EcoTech's Radion pages quote beam angle and “even PAR” but no values, Kessil and Bulk Reef Supply give coverage areas and mounting heights without a figure, and AI's own Hydra and Blade pages publish no PAR at all. What circulates instead is forum meter readings that vary two- to three-fold for the same fixturedepending on spectrum mix, and usually without the mounting geometry that would make them comparable. Rather than pad the list with numbers we can't cite, this tool takes your own readingas the primary input — your meter, a borrowed or rented one, or a figure from your fixture's manual.
100 PAR — peak PAR, quoted by the manufacturer at a depth of 24 inches, 24" x 24" spread (published spec).
Scaled straight down from the figure above, which assumes that figure was taken at full power. Channel mixes complicate this — a fixture at 50% white and 100% blue is not “75%” of anything.
Where the coral sits
Measure down to the coral itself, not to the sand — a frag on top of the rockwork can sit in twice the light of one at the base of the same rock.
That lands in the softies and mushrooms band. Zoanthids, mushrooms, leathers, Xenia. The most forgiving band in the tank, and where most beginners' corals actually want to be.
- Low light0–75 PARShaded overhangs and the sand under rockwork. Fine for non-photosynthetic corals and most macroalgae; too dim for most of what people buy.
- Softies & mushrooms75–150 PARZoanthids, mushrooms, leathers, Xenia. The most forgiving band in the tank, and where most beginners' corals actually want to be.
- LPS150–250 PARHammers, torches, frogspawn, Acans. Bright, but not the top of the rockwork — several LPS will retract if you put them there.
- SPS250–350 PARAcropora, Montipora and friends, up in the light. Colour tends to come from this band; so does bleaching if you arrive at it too quickly.
- Very high light350+ PARAbove what most tanks run. Some Acropora and Stylophora take it once fully acclimated; almost nothing takes it as a sudden change.
How this number is worked out (and why it's a range)
The starting figure is scaled by distance and intensity: PAR = starting PAR × (starting distance ÷ your distance)n × intensity. Your distance is the air gap above the water plus the water above the coral, added together.
The range is the honest part. Everything turns on that exponent n, and nobody can tell you what it is for your fixture. At n = 2 you have the inverse-square law, which is the physics of a single point of light — but a reef LED is an array of emitters behind optics, so it holds its intensity better than that. Hobbyist measurements often describe the falloff as closer to linear over short distances, nearer n = 1. This tool computes both n = 1.4 and n = 2 and shows you the span, because a single confident number here would be invented.
Not modelled at all: spectrum, how much light your rockwork shades, surface ripple, reflection off the glass, and whether the fixture is actually pointed at the coral. Nor is water absorption — over a tank's depth it is small next to the disagreement about n, and there is no citable coefficient for a given tank's clarity.
Published fixture figures are usually a peak reading directly under the centre, and manufacturers are not always explicit about whether the quoted distance is measured from the lens or from the water's surface. Treat the result as an order of magnitude for deciding where on the rockwork something goes. For anything that matters — a new SPS, a coral that keeps bleaching, a light you are deciding whether to buy — borrow, rent or buy a real PAR meter. The bands below come from Bulk Reef Supply's PAR-meter guide and Reefco's per-genus table, which agree closely; they are targets for an acclimated coral, not hard limits.