What Size Grow Tent Do You Actually Need?

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Grow tents come in every size, from a 60 cm cube to rooms you can walk into. Which one you need isn't a guess: it is a sum. Work out the height from the floor up, then choose only as much floor as you can light well, because harvest follows light, not floor space. And sometimes the right answer is no tent at all.

The short answer ▶ 0:25

Work out the height first: pot, plant, stretch and the light. Then choose a floor you can light well, because harvest follows light, not floor space. The number of plants matters less than you'd think. And a dark, well-aired room can do the same job.

What a tent actually does ▶ 0:43

A tent does three jobs. Its shiny walls bounce stray light back onto the plants. It gives them a properly dark night. And with a fan, it lets you control the air.

That dark night matters more than most people think. Light as dim as twilight was enough to delay flowering in some hemp varieties, as the dark-period article explains. A good tent keeps a landing light or a street lamp out.

Source · Zhang et al., Front. Plant Sci. (2021)

The air matters because plants breathe out almost all the water you give them. Well over 90% of the water a plant takes up leaves again through its leaves. In a closed tent, that water and the light's heat build up fast, so a fan has to carry them out.

Source · Taiz et al., Plant Physiology and Development (textbook)

How tall? ▶ 1:22

Do the sum from the floor up. The pot: say 30 cm. The plant at the flip: say 60 cm. Then the stretch. In one study, White Widow plants grew about 40% taller after the flip, so 60 becomes about 85. Then the gap your light needs above the tops, plus the light and its hangers: often 50 cm or more. That is about 165 cm, so a tent around 1.8 m fits.

Source · Hesami et al., Plants (2023)

But stretch varies a lot. In another study, a month into long nights, one variety stood 52 cm tall and another 106 cm. Late-flowering varieties tend to need the most height. Autos, and plants you flip early, need the least.

Source · Spitzer-Rimon et al., Front. Plant Sci. (2019)

If you are short on height, flip earlier, choose shorter varieties, or train the plants flat. What you can't do is let the tops grow into the light.

The floor (and the light) ▶ 2:19

Common tents run from 60 × 60 cm, about 2 ft square: a third of a square metre. 80 × 80 cm is nearly two thirds. A metre square is one. And 120 × 120 cm, about 4 × 4 ft, is nearly one and a half: four times the floor of the smallest.

But floor space alone doesn't grow anything. In a 2021 Canadian trial, the harvest kept rising in a straight line as the light got brighter, all the way to the strongest level tested.

Source · Rodriguez-Morrison et al., Front. Plant Sci. (2021)

So a bigger tent only pays off if you can light all of it. Lighting four times the floor at the same brightness takes about four times the light. A big tent with a small light just gives you dim corners.

And the number of plants? A 2019 review that pooled cannabis yield studies found that plants per square metre was not an effective predictor of yield per square metre. A few big plants or many small ones can fill the same floor.

Source · Backer et al., Front. Plant Sci. (2019)

Tent or room? ▶ 3:17

You don't always need one. A cupboard or a spare room does the same job if you can make it truly dark, move air through it, and keep light on the plants with white walls. Outdoors, the sun does it all for free. A tent just makes those three jobs much easier in a room you also live in.

So: measure the height first, leaving room for the stretch and the light. Choose a floor you can light properly. Check the inside measurements, because pots, fans and the light all take up space. And keep it dark at night, with air moving.

Height is pot, plant, stretch and light; floor is whatever you can light. Measured your space? Compare it with the tent's inside size, not just the size on the box.