The aquarium PAR light calculator estimates the photosynthetically active radiation (PAR) reaching your substrate based on fixture PAR output, mounting height above water, and tank depth. PAR (µmol/m²/s) is the primary measure of light available for plant photosynthesis — knowing your substrate PAR prevents both algae from too much light and poor growth from too little.
Light & Tank Settings
Reference distance for preset values: 12 inches (30 cm)
Suitable Plants
How to Use the Aquarium PAR Light Calculator
PAR at substrate is the light that actually reaches your plants' leaves and roots — not just the fixture output. Every inch of distance and water attenuates the light reaching the bottom of your aquarium.
Step 1: Find Your Light's PAR Output
Quality LED fixtures publish PAR data in their documentation. Look for PAR at a specific distance (usually 12 inches). If you can't find PAR data, use a PAR meter or estimate based on watt equivalent — quality LEDs at 12" typically produce 30–80 µmol per watt.
Step 2: Measure Mounting Height
Measure from the light source (LED chips) to the water surface. Hanging fixtures reduce this distance vs. clip-on lights. Raising the light reduces PAR but gives more even coverage across the substrate.
Step 3: Measure Water Depth
Measure from the water surface to the substrate surface. Deeper tanks (18"+) attenuate light significantly. The calculator applies a water attenuation factor of approximately 5% per 10 cm of water depth.
Step 4: Match Plants to PAR Level
Low-light plants (Anubias, Java fern, Bucephalandra) do well at 15–30 µmol. Medium plants (Cryptocoryne, most stem plants) need 30–60 µmol. High-light plants (HC Baby Tears, red Rotala, carpeting plants) need 60–150+ µmol and require CO2 injection to prevent algae outbreaks.
FAQ
What is PAR in aquariums?
PAR stands for Photosynthetically Active Radiation, measured in µmol/m²/s (micromoles per square meter per second). It measures the light energy actually usable by aquatic plants for photosynthesis. Most PAR meters measure light between 400–700 nm wavelengths.
What PAR level do planted tanks need?
Low-light plants (Anubias, Java fern, moss) thrive at 15–30 µmol/m²/s. Medium-light plants (Cryptocoryne, most stem plants) need 30–60 µmol/m²/s. High-light plants (carpeting plants, red stems) need 60–150+ µmol/m²/s. CO2 injection is typically needed above 50 µmol/m²/s.
How does mounting height affect PAR?
PAR decreases with the inverse square law — doubling the distance from light to substrate roughly quarters the PAR. Moving a light from 6 inches to 12 inches above water reduces PAR by approximately 75%. Water depth further attenuates light, especially in turbid or tannin-stained water.
Do I need a PAR meter?
A PAR meter is the most accurate way to measure light at substrate level. Budget options like the Seneye or Apogee SQ-500 work well for aquariums. As an alternative, this calculator provides estimates based on known fixture PAR values and distance adjustments.
Is this calculator free?
Yes, completely free with no signup required.
Does the calculator support metric and imperial?
Yes. Toggle between inches and centimeters for all distance inputs.