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 = attenuation coefficient of the water (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.

PAR Level Reference Table

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.