Light reaching aquarium plants falls off with depth, so hitting a target at the substrate means driving the surface harder than the target itself. Enter the PAR you are aiming for at the plants, the depth from the surface down to them and an attenuation share per 4 in, and this tool returns the surface PAR you need plus a depth-by-depth table.
The math is surface PAR = target PAR / (1 - attenuation)^(depth / 4), an exponential decay, and the formula used is printed with the results. There is no officially published PAR table by plant species, and no standard figure for how fast water attenuates light. Sources disagree for the same plant, and attenuation shifts with turbidity, tannins and CO2 use, so this tool supplies no figures and asks for yours (as of October 2026).
This tool does not suggest a correct PAR for any plant, and it does not convert fixture watts or lumens into PAR. It assumes the attenuation share is identical in every depth band, so it will differ from meter readings. This calculator is for plant care reference and does not replace veterinary care. Contact your veterinarian or an aquatic animal hospital right away if animals in the same tank show symptoms.
Frequently Asked Questions
Use the figure from your own reference or an aquatic plant retailer. There is no official table, so this tool does not supply one.
With a PAR meter you can read the surface and a deeper point and work out the ratio. Entering 0 returns the target unchanged.
No. Watts and lumens do not convert to PAR, and this tool does not support that conversion. Use the PAR printed by the maker or a meter reading.