Step 5 - Greenhouses
What Size Greenhouse Do I Need?
Work out the size from what you intend to grow. Then go one size up, because almost nobody regrets that direction.
Janet H. - the hands-on owner behind Janet Media Group.
Published - last updated
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For seed starting and hardening off alone, 6 x 6 ft works. For seed starting plus growing tomatoes and cucumbers to maturity, 8 x 10 ft is a realistic minimum. The most common regret in this category is buying too small, and it is worth planning against.
Capacity by size
| Size | Floor area | Seed trays with staging one side | Mature crops |
|---|---|---|---|
| 6 x 6 ft | 36 sq ft | About 18 to 24 | 2 to 3 plants, or none if full of trays |
| 6 x 8 ft | 48 sq ft | About 24 to 32 | 3 to 4 plants plus staging |
| 8 x 10 ft | 80 sq ft | 40 or more | 6 to 8 plants plus staging and a bench |
| 8 x 12 ft | 96 sq ft | 50 or more | 8 to 12 plants, staging, and room to work |
Those numbers assume three-tier staging down one side. Without staging, halve or third the tray figures.
Size from the plan, not the space
- Count the seed trays you expect to run at peak -- usually March. A standard 1020 tray is roughly 10 by 20 inches.
- Count the crops you want to grow to maturity under cover. Tomatoes need about 18 to 24 inches each and cannot share space with staging above them.
- Add a working area. A potting bench and somewhere to put a bag of compost is genuinely useful and always forgotten.
- Add a path. You have to get in, and reach both sides.
- Then go one size up if the budget and footprint allow.
Why bigger is more forgiving
A larger air volume swings less. Small greenhouses heat and cool faster, which makes ventilation more urgent and overnight lows harsher. A larger structure is more stable, easier to work in, and more tolerant of the day you forget to open the vent.
The costs are real -- price, footprint, and more glazing to clean -- but the growing behavior genuinely improves with size.
The length decision
Most ranges fix the width and vary the length, and many kits can be extended later with an additional bay. If that is an option, check it before buying: starting at 6 x 8 with the ability to extend to 6 x 12 is a good hedge against underestimating.
For the small end of the range, see small greenhouse kits. For what will actually go in it, what to grow in a greenhouse.
What to buy
The products that fit the advice above, chosen by the process on our how we choose page. We have not hands-on tested them; specifications are the manufacturers’ own.
Quick picks
| # | Product | Best for | Score | Price |
|---|---|---|---|---|
| 1 |  Top pickPalram Canopia Mythos GreenhouseAluminum frame, 4 mm twin-wall polycarbonate | A first real greenhouse | 9.0 | Check price on Amazon |
| 2 |  Palram Canopia Glory Greenhouse10 mm twin-wall polycarbonate throughout | Cold climates and winter growing | 8.9 | Check price on Amazon |
| 3 |  Greenhouse Staging and Shelving UnitTiered staging sized for standard trays | Doubling usable space | 8.2 | Check price on Amazon |
#ad We may earn a commission - prices are shown by Amazon, not by us
Scores are our own editorial judgment, formed from published specifications and independent sources. They are not aggregated customer ratings, and they are not the result of hands-on testing, which we do not currently do. See how we choose.
Common questions
What size greenhouse is best for a home garden?
6 x 8 ft handles seed starting, hardening off and a few crops grown to maturity. If you want to raise seedlings and also grow tomatoes, cucumbers and peppers under cover, 8 x 10 ft is a more realistic minimum. Almost nobody regrets going one size up.
Is a 6x6 greenhouse big enough?
For seed starting and hardening off, yes -- with three-tier staging it holds around 18 to 24 standard trays. It is not big enough to do that and grow tall crops to maturity at the same time, because staging and mature plants compete for the same light and space.
How many tomato plants fit in a greenhouse?
Allow 18 to 24 inches per plant in floor space, and remember they cannot sit under staging. A 6 x 8 greenhouse holds three or four alongside staging on one side; an 8 x 10 holds six to eight with room to work.
Do bigger greenhouses hold temperature better?
Yes. A larger air volume swings less between day and night, which makes overheating less sudden and overnight lows less harsh. Small structures heat and cool fast, which is why ventilation is more urgent in them, not less.
Sources
Factual claims on this page are backed by the published sources below, and product specifications are the manufacturers’ own. Where a figure is not published, we say so rather than estimating it.