Aquarium Lighting PAR Calculator

PAR (Photosynthetically Active Radiation) measures the light intensity in the 400–700 nm wavelength range that plants actually use for photosynthesis. It is expressed in µmol/m²/s (micromoles of photons per square meter per second). Unlike lumens or lux, PAR tells you exactly how much usable light your aquatic plants receive.

PAR Estimation Formula

Light intensity decreases with water depth following an exponential decay model:

PAR at Depth = Surface PAR × e^(−k × d)

Where:

• Surface PAR = PAR measured directly under the light at the water surface (provided by manufacturer or measured with a PAR meter)
• k = the attenuation coefficient (typically 0.02–0.08 per cm for clear aquarium water; tannin-stained or green water can be 0.10+)
• d = water depth in centimeters from the surface to the substrate
• e = Euler’s number (≈ 2.71828)

Worked Example

A light rated at 120 µmol/m²/s at the surface, over a 45 cm deep tank with clear water (k = 0.03):

PAR at substrate = 120 × e^(−0.03 × 45) = 120 × e^(−1.35) = 120 × 0.2592 ≈ 31.1 µmol/m²/s

This falls in the medium-light range, suitable for crypts, anubias, and java fern.

About that k value, because the number surprises people

Pure water barely absorbs light in the PAR band. Its real attenuation coefficient is on the order of 0.001 per cm, which over a 45 cm tank costs you about 4% and nothing more.
The k values on this page are twenty to eighty times larger than that, and the difference is not absorption. It is geometry. A fixture sitting above the tank spreads its light over a widening area as it goes down, and that inverse-square falloff dwarfs anything the water does. So treat k here as an empirical fit to what PAR meters actually read in aquariums, not as a property of water. It is why the same fixture on a taller tank loses so much more than a physics textbook would suggest.

PAR Level Reference Table

These are the same band edges the plant fertilizer and CO2 injection calculators use, so a PAR figure carries across all three.

Factors Affecting PAR

Water clarity is the biggest variable. Activated carbon and UV sterilizers keep water clear and k low. Tannins from driftwood, suspended algae, or detritus increase k significantly. Measuring PAR with a Seneye or Apogee meter is the gold standard, but this calculator gives a reliable estimate when you know your fixture’s output.

High-PAR setups (above 60) almost always require CO2 injection and a complete fertilization regimen to prevent algae outbreaks. Without CO2, excess light energy drives algae growth rather than plant growth.