Aquarium light calculator
This aquarium light calculator estimates the lumens and rough LED wattage your planted tank needs for a low, medium or high light setup.
How the aquarium light calculator works
Plants are driven by the amount of light that reaches them, which is easier to compare in lumens than in watts. The calculator converts your volume to liters and multiplies by a target rate for your chosen level: about 15 lumens per liter for low light plants, 30 for medium and 50 or more for high light. It then reports the total lumens, the lumens per gallon, and a rough LED wattage using a typical efficiency of around 100 lumens per watt.
Treat the wattage as a guide only, because LED fixtures vary a great deal in how much light they produce per watt. Volume is a simple stand in for tank size here, but light fades quickly with depth, so a tall tank needs more output to push the same light down to the substrate. Where a manufacturer lists PAR at depth, that is an even better number to compare than lumens.
As an example, a 40 gallon tank set to medium light comes out near 4,500 lumens, or about 45 watts of typical LED. Drop to low light and it falls by roughly half, while high light nearly doubles it, which is why the plants you choose set the fixture you need.
Low, medium and high light plants
Low light plants are the easy, forgiving group: anubias, java fern, java moss, cryptocoryne and many stem plants grow well on modest lighting and little fuss, usually without added carbon dioxide. Medium light opens up more color and faster growth in plants like amazon swords, ludwigia and some rotala, often with root tabs and a liquid fertilizer.
High light plants, including carpeting species and bright red stems, need strong light plus added carbon dioxide and regular dosing to thrive. They reward the effort with dense, colorful growth, but the same light on an under fertilized tank simply grows algae. Match the light level to how much maintenance you want to take on.
Lumens, PAR and watts
Watts measure power use, not light output, and modern LEDs vary so much in efficiency that watts alone tell you little. Lumens measure the light a fixture puts out as the human eye sees it, which is a better comparison and what this calculator uses. The catch is that lumens weight the colors people see, not exactly the colors plants use.
PAR, or photosynthetically active radiation, measures the light in the range plants actually use for photosynthesis, taken at the substrate. It is the best single number for planted tanks because it accounts for depth and spectrum. If a fixture lists PAR at a given depth, use that to compare; otherwise lumens per liter is a solid guide.
Getting good growth without algae
More light is not automatically better. Bright light speeds up plant demand for carbon dioxide and nutrients, and when those run short the extra light feeds algae instead. Match your light level to your plants and your willingness to dose fertilizers or add carbon dioxide. Low light plants like anubias, java fern and cryptocoryne thrive on modest lighting and little fuss, while carpeting plants and red stems usually need high light plus extra support.
If algae starts to build, treat light as the first dial to turn down. Shorten the photoperiod or lower the intensity, get carbon dioxide and fertilizer in balance, and only then consider adding light. A slightly under lit healthy tank beats a bright one fighting algae.
Color temperature and spectrum
Color temperature, measured in Kelvin, sets how warm or cool the light looks. A daylight range of about 6500 to 7500 Kelvin looks natural and grows plants well, which is why it is the common choice. Warmer light below 5000 Kelvin looks yellow, while cooler light above 8000 Kelvin looks blue and is more of a marine look.
Spectrum matters as much as color temperature. Full spectrum lights that include some red and blue alongside white tend to give the best growth and make greens and reds pop. Many planted fixtures let you tune the channels, so you can favor a natural look while keeping enough of the colors plants use.
Photoperiod and timers
Run the light on a timer so the tank gets a steady day length, which plants and fish both prefer. Start at about six to eight hours a day. Longer is not better: a long, bright day without matching carbon dioxide and nutrients feeds algae, and it also stresses fish that expect a regular night.
Some keepers use a siesta, splitting the light into two shorter blocks with a dark break in the middle, which can help control algae by letting carbon dioxide recover between blocks. It is optional, and a single steady period works well for most tanks, but it is a simple lever to try if algae is a problem.
Light, depth and carbon dioxide
Two things trip up new planted tanks. The first is depth: light weakens fast as it travels down through water, so a tall tank needs a stronger fixture to reach the same intensity at the substrate as a shallow one. If you keep carpeting plants in a deep tank, plan for a light rated well above the medium figure.
The second is balance. Light, carbon dioxide and nutrients work as a set, and the slowest of the three limits growth. High light without added carbon dioxide is the classic recipe for algae, so raise light and carbon dioxide together, or keep both modest for an easy low tech tank.
Frequently asked questions
How many lumens does a planted aquarium need?
As a guide, low light plants want about 15 lumens per liter, medium light around 30, and high light plants 50 or more. This tool multiplies your volume by the rate for your chosen level.
How many watts of LED for a planted tank?
It varies with the fixture, but as a rough guide a modern LED at about 100 lumens per watt needs roughly 0.4 to 0.5 watts per liter for medium light. The calculator gives an estimate, though lumens and PAR are more reliable than watts.
Is watts per gallon a good way to size a light?
Not anymore. Watts per gallon was built around old fluorescent tubes. Modern LEDs vary widely in efficiency, so lumens and, where available, PAR are far better guides than raw wattage.
What is PAR and why does it matter?
PAR is the amount of light in the range plants actually use for photosynthesis, measured at the substrate. It is the best single number for planted tanks because it accounts for depth and spectrum, which lumens alone do not.
What color temperature is best for plants?
A daylight range of about 6500 to 7500 Kelvin looks natural and supports plant growth. Full spectrum lights that include some red and blue tend to give the best results.
How long should the light be on?
Start with a photoperiod of about 6 to 8 hours a day on a timer. Longer periods or very bright light without enough carbon dioxide and nutrients tend to feed algae rather than plants.
Do I need CO2 for a planted tank?
Not for low and many medium light plants, which grow well without it. High light plants and carpets usually need added carbon dioxide, because bright light without enough CO2 and nutrients feeds algae instead of plants.
Why is my planted tank getting algae?
Usually too much light for the available CO2 and nutrients, or too long a photoperiod. Shorten the light to 6 to 8 hours, lower the intensity, and balance CO2 and fertilizer before adding more light.
Does a taller tank need a stronger light?
Yes. Light fades quickly with depth, so a tall tank needs more output to push the same light down to the substrate. Compare PAR at depth if the maker lists it, and favor a stronger fixture on deep tanks.
Can I use a regular room light for plants?
Some low light plants get by near a bright window or a daylight room bulb, but growth is weak and uneven. A proper full spectrum aquarium light gives even, controllable light and much better results.
Should I run a siesta or split photoperiod?
A siesta, splitting the light into two shorter blocks with a dark break, can help control algae in some tanks by letting CO2 recover. It is optional; a single steady 6 to 8 hour period works well for most setups.