Planted Aquarium Light Calculator
Use this planted aquarium light calculator to find out how much light your tank really needs. Type in your tank size and pick your plant type. You’ll get a PAR range, a watt range, and how many hours to run your light — all in one go.
- 15–30 PAR for easy plants
- 30–60 PAR for most planted tanks
- 60–100 PAR for carpets & red plants
- 6–10 hrs a day, on a timer
Fill in your tank below. The planted aquarium light calculator does the rest.
Measure water height from the substrate up to the waterline, not the outside glass.
Low: Anubias, Java fern, moss, Crypts · Medium: swords, most stems · High: carpets, red stems
Not sure? Leave it as is. Higher fixtures need a bit more power to reach the same PAR.
Your light target
30–60 PAR at substrateWhat’s on this page
How this planted aquarium light calculator works
You tell it your tank size and what you’re growing. That’s it.
Think of it as a planted aquarium lighting calculator that also checks depth, water clarity, and mounting height, not just tank size. It shows you how to calculate light for planted aquarium tanks without needing a PAR meter on day one.
It then works out your PAR target, a safe watt range, and how many lumens you should be aiming for.
Most other tools online only ask for tank size. This one goes a bit further, so the number you get is closer to what you’ll really need to buy.
Working out your tank size first? Check the Goldfish Tank Size Calculator, then come back here for the light.
How much light does a planted tank need?
It depends on what you’re growing, not how many gallons you have. This aquarium plant light calculator sticks to freshwater planted tanks only — reef and coral lighting is a different topic entirely.
Plants use PAR — light in the 400–700 nm range — and PAR at the bottom of your tank is what actually counts, not PAR at the surface.
Here’s the chart the planted aquarium light calculator above is built on:
| Light level | PAR at substrate | Lumens per cm² | Typical plants | CO2 | Hours a day |
|---|---|---|---|---|---|
| Low | 15–30 µmol/m²/s | ~0.7 | Anubias, Java fern, moss, Crypts | Not needed | 6–8 |
| Medium | 30–60 µmol/m²/s | ~1.2 | Swords, most stem plants | Helps a lot | 7–9 |
| High | 60–100 µmol/m²/s | ~2.0 | Carpets, red stems | You need it | 8–10 |
This is for freshwater planted tanks only. Reef tanks need a different spectrum and different numbers, so don’t use this for corals.
How many hours should the light stay on?
Six to ten hours a day works for almost every planted tank. Low light tanks can sit at the shorter end, high light tanks need the longer end.
Put your light on a cheap timer. Don’t jump straight to 10 hours on day one — start at 6 or 7 and add an hour every week or two, so algae doesn’t get a head start.
Which light should you buy?
Pick a full-spectrum LED made for plant growth, not one made just so your fish look nice. Check if the seller shows a real PAR number at some depth — that tells you far more than the watt number on the box.
Use this as your aquarium led light calculator when you’re comparing two LED bars side by side. If you landed here searching for a plain light calculator for aquarium tanks in general, just know this one is built for freshwater plants, not reef.
A wide, flat LED bar spreads light evenly across the tank. A single pendant lights the middle well but leaves the corners dim, so plan for that if you go with one.
Watts, lumens, or PAR — what actually matters?
Planted aquarium lighting isn’t just about watts, even though that’s the number every store puts on the box. Watts-per-gallon made sense years ago, when almost every tank ran the same kind of fluorescent tube.
LEDs broke that rule — two lights can pull the same watts and put very different amounts of usable light on your plants.
So treat watts as a rough starting point only, never as the real answer. If you only remember one thing from this planted aquarium light calculator, remember your PAR number, not your watt number.
Lumens are one step better than watts, but they’re built around what the human eye sees, not what a plant uses. PAR (also called PPFD) is the number that actually lines up with plant growth (source: Michigan State University Extension — canr.msu.edu).
Why depth changes your numbers
Light fades fast in water — faster than most people expect. A tall tank needs a stronger light than a short tank to hit the same PAR at the bottom.
Tea-colored or cloudy water blocks light too. Raising your fixture higher above the tank does the same thing.
This is exactly why the planted aquarium light calculator above asks about water height, clarity, and mounting height — not just tank size. Skip those and you’re just guessing with extra steps, which isn’t really a light calculator for planted aquarium tanks at all, just a size calculator wearing a costume.
A mistake I made with my own tank
A classic mistake I made early on was trying to save on electricity by placing my tank near a window for “free” light. I didn’t account for the brutal Indian summer heat waves and intense ambient glare. Within weeks, the water temperature spiked past 32°C, triggering a catastrophic Green Water and Black Beard Algae explosion. I learned the hard way that relying on unpredictable natural light in our climate is a recipe for disaster; I had to move the tank to a dark corner and switch to a controllable Sobo LED fixture on a strict 7-hour timer to regain control.
Planted Aquarium Light Calculator FAQs
Quick answers to what people usually search for before buying a light.
How much light is required for a planted aquarium?
Most planted tanks need 15–30 PAR for easy plants, 30–60 PAR for a normal mixed tank, and 60–100 PAR for carpets and red plants. Check this at the substrate, not the surface — that’s the number that matters.
How many hours of light for my planted aquarium?
Six to ten hours a day. Start on the shorter side and add time slowly over a couple of weeks instead of running a full day of light right away.
What is the best light for a planted aquarium?
A full-spectrum LED built for plants, with a real PAR number from the maker, beats picking by watts or by looks. Even spread across the tank matters just as much as brightness.
How many watts for aquarium plant light?
As a rough guide with a modern LED: about 0.2–0.35 W per liter for low light, 0.35–0.6 W per liter for medium light, and 0.6–1.0+ W per liter for high light. This is the same range the aquarium light wattage calculator on this page uses — treat it as a starting point, then check the real PAR data before you buy.
How many watts for a planted aquarium, by tank size?
How many watts for planted aquarium tanks comes down to volume and plant demand, not a flat per-gallon rule. Multiply your water volume in liters by the watt-per-liter range above.
A 40-gallon tank (about 150 liters) at medium light lands around 53–90 W of modern LED. At high light, the same tank is closer to 90–150 W. Run your own numbers through the planted aquarium light watt calculator above instead of guessing.
What is PAR in aquarium lighting?
PAR is the part of light (400–700 nm) that plants can actually use to grow. It’s measured as PPFD, in µmol/m²/s, using a sensor called a quantum sensor (source: Vernier Science Education — vernier.com).
Are aquarium LED lights better than old T5 or T8 tubes?
Usually yes — LEDs run cooler, last longer, and the good ones put out more usable light per watt. A cheap LED bar can still lose to a good T5 tube though, so don’t assume “LED” alone means better. Check the PAR data.
How many lumens does a planted aquarium need?
As a rough planning number: about 0.7 lumens per cm² of tank footprint for low light, 1.2 for medium, and 2.0 for high light. It’s a decent shortcut for comparing lights while shopping, but PAR still wins once you’ve narrowed your choices down.
How accurate is this planted aquarium light calculator?
It’s built to be a solid starting point, not a lab result. It uses real PAR ranges and light-loss math for depth, clarity, and mounting height — but your exact fixture, its age, and its spectrum still matter. A cheap PAR meter is the only way to know your actual numbers for sure.
What to do next
References
- Michigan State University Extension, “Light terminology for plants” — PAR defined as the 400–700 nm waveband.
- Vernier Science Education, PAR/PPFD measurement guide — how PPFD is measured with a quantum sensor.
- University of Florida IFAS, Center for Aquatic and Invasive Plants — freshwater aquatic plant research and resources.