Reef Lighting PAR Estimate Calculator
Estimate the PAR reaching your corals at depth from the fixture output at the surface and the water column, and see which coral types suit that light level.
How it works
PAR at depth = surface PAR × e^(−k × depth) · k = attenuation per inch (water clarity)
Light falls off exponentially as it passes through water, following the Beer–Lambert law, so a coral near the sand can sit at a fraction of the PAR the same fixture delivers just below the surface. Clearer, well-skimmed water lets more light through (a smaller attenuation coefficient), while tannins and suspended particles absorb it faster. This estimate assumes a straight column and ignores reflectors, beam angle and the shading of rock and neighbouring corals, so it is a planning aid, not a substitute for a PAR meter — place light-hungry SPS high and near the fixture, and keep low-light softies shaded or deeper.
Worked example
A fixture delivering 400 µmol at the surface, over a typical reef tank (k = 0.012 per inch) with the coral 12 inches down, reaches 400 × e^(−0.012 × 12) ≈ 346 µmol — solidly in the high-light SPS range.
Frequently asked questions
What PAR do different corals need?
As a rough guide: soft corals and mushrooms are happy around 50–150 µmol, LPS around 100–200, and SPS such as acropora want roughly 200–400 µmol. These are ranges, not exact thresholds, and corals also acclimate slowly to changes.
Why do I still need a PAR meter?
This estimate assumes a clean vertical column and uniform light. Real tanks have hot spots under the fixture, dark corners, and shading from rock and corals, so measured PAR can differ by hundreds of µmol across the same tank. A meter (or a borrowed/rented one) is the only way to place corals confidently.
Can too much PAR harm corals?
Yes — excess light bleaches corals by driving out their symbiotic algae, and moving a coral straight into high light without acclimation is a common cause of loss. Raise intensity gradually over weeks and watch for paling.
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This calculator is for educational purposes. Double-check important results before acting on them.