What Are Lumens? How Many Your Cannabis Plants Need, and How to Measure Them

Lumens tell you how bright a lamp is: they measure a bulb's light output. For growing cannabis, a common rule of thumb is about 2,000-5,000 lumens per square foot in veg and 5,000-10,000 in flower.

Put simply, lumens (lm) measure the total amount of visible light (what the human eye can see) coming from a lamp or light source. The higher the lumen number, the brighter the bulb.

Light is hugely important for plant growth, whether you run LED or HPS, so measuring lumens and keeping them under control can help you get a better yield.

How much light plants need changes with each stage of the grow. Seedlings can't handle much light, but once plants hit the vegetative stage, they pretty much want all the light they can get. That lets them grow strong and reach their full potential. Same goes for flowering. Plants do better under a different light spectrum in bloom than in veg, but they still need plenty of light to make the buds as big as possible.

For indoor cannabis grows, the usual options are LED grow lights or high-intensity discharge (HID) lights like metal halide (MH) or high-pressure sodium (HPS). How many lumens you need depends on the size of your grow space, how many plants you're running and what stage they're in.

As a general guideline, in veg you want around 2,000 to 5,000 lumens per square foot (roughly 21,500 to 53,800 lumens per square meter), while in flower you want around 5,000 to 10,000 lumens per square foot (roughly 53,800 to 107,600 lumens per square meter).

Keep in mind that lumens aren't the only thing to look at when picking a grow light for cannabis. Spectrum quality, energy efficiency, durability and other factors matter too.

How much light do cannabis plants need?

To measure light intensity at your plants, a lux meter is the way to go. How many you need depends on the size of your grow space and how many plants you're growing.

For a single plant in a small grow space, one lux meter may be all you need. In a bigger space with several plants, you might want several meters to get accurate readings at different spots across the canopy.

In general, measure light intensity at several points in your grow space to make sure every plant gets enough light. You want enough intensity for optimal growth, but not so much that it burns or damages the leaves.

As for recommended lux levels for cannabis, aim for roughly 15,000 to 50,000 lux in veg and about 45,000 to 65,000 lux in flower. These numbers can shift depending on the strain and the specific conditions in your grow space.

A lux meter is a pretty simple device that measures how much light (lux = lumens/m²) is hitting your grow room. The best part, compared to the PAR meters or quantum sensors pros use, is that it's way cheaper.

A lux meter can also tell you when it's time to swap out your bulbs if you're running HPS, since their output drops over time. It's a cheap way to make sure your plants are getting the right amount of light.

How many lumens does a cannabis plant need?

During the vegetative stage, cannabis plants need moderate light intensity to encourage healthy growth. Aim for at least 2,000 to 5,000 lumens per square foot of canopy. You can get there with LED grow lights, fluorescents or lower-wattage HID lamps, like 250 to 400 W HPS bulbs.

In flower, cannabis plants need more light intensity to push bud production. Aim for at least 5,000 to 10,000 lumens per square foot at this stage. For that, you can use higher-wattage HID lamps, like 600 to 1,000 W HPS, or LED systems designed specifically for flowering.

How many lumens is good for growing?

How many lumens count as "good" for growing plants, cannabis included, depends on several things: the growth stage, the type of light and what your particular plants need. Here are the lumen ranges generally considered right for each stage:

1. Vegetative stage:

In veg, aim for 2,000 to 5,000 lumens per square foot. You can hit that with LED grow lights, fluorescents or lower-wattage HID lamps, like 250 to 400 W HPS bulbs.

2. Flowering stage:

In flower, cannabis plants need more light intensity to drive bud production. Aim for 5,000 to 10,000 lumens per square foot. To get there, use higher-wattage HID lamps, like 600 to 1,000 W HPS, or LED systems built for flowering.

These ranges are just general guidelines and can change with things like genetics, temperature, humidity and how high you hang your lights.

Adjust your light intensity and watch how your plants respond to dial in the right number of lumens for your grow.