What size greenhouse do I need? 6x8, 8x12 or bigger

Picture a typical week in April. Trays of seedlings on a bench, a few pots of herbs, maybe the first tomatoes going in. Then July: tomato vines up strings, cucumbers, peppers, and the bench now full of cuttings. A greenhouse fills up twice a year, and the summer fill is the one that catches people out.

So list what you want to grow, then add a third. That is the honest size.

A 6x8 gives you about 49 sq ft: a bench on each side and a path, or one bench and some big pots. It is perfect for seed starting, overwintering a few tender plants and a handful of tomato plants. On our list, the Palram Mythos ($889) and Hybrid ($899) are the quality picks, and the Outsunny 6x8 ($381.99) is the budget one.

The catch with small greenhouses is the door. The Palram 6x8 doors are 22.2 in wide and 64.2 in high: you duck in and you carry compost rather than wheel it.

At about 96 sq ft, an 8x12 holds two full benches and a path, or a bench plus a soil bed of tomatoes. It is the classic hobby size, and the one we would steer most gardeners toward. It also tends to come with double doors: the Palram Essence has doors 47 in wide with a ramp, and the Snap & Grow 45.7 in. On our list, 8x12-class kits cost from $1,899 to $6,999; see the 8x12 ranking.

If you want to feed a family through summer, look at the tunnels. The YourGreenhouses Polycarbonate covers 128 sq ft for $2,765, the Bloomcabin High Tunnel 129 sq ft, and the Outsunny 8x16 131 sq ft for $595.99. The best part: the two steel tunnels are modular. YourGreenhouses extends in 6.5 ft sections; Bloomcabin in 6 ft 6¾ in modules up to 388 sq ft. Start at 13 ft and grow later. Our large greenhouse ranking compares them.

• Palram Mythos and Hybrid: 6 ft by 8 ft, 49.1 sq ft.
• Outsunny 6x8: 6.2 ft by 8.3 ft, 51.5 sq ft.
• Palram Essence, Glory and Balance: about 8 ft by 12 ft, 95.4 to 96 sq ft.
• Scandiglas: 7 ft 9 in by 12 ft 7 in, 97 sq ft.
• Palram Snap & Grow: 8.2 ft by 12.25 ft, 100 sq ft.
• YourGreenhouses Arrow: 8.2 ft by 13 ft, 106.6 sq ft.
• Rion Grand Gardener: 8 ft 9 in by 12 ft 8 in, 110.8 sq ft.
• YourGreenhouses Polycarbonate: 9.84 ft by 13 ft, 128 sq ft.
• Bloomcabin High Tunnel: 9 ft 11 in by 13 ft 2 in, 129 sq ft.
• Outsunny 8x16: 8 ft by 16.4 ft, 131.2 sq ft.

Use our comparison tool to put any three side by side with prices, panels and headroom.

Floor area is only half the story. A round tunnel can have plenty of floor but low sides, so you only stand straight down the middle: the YourGreenhouses Polycarbonate peaks at 6 ft 7 in. A straight-walled greenhouse with high eaves lets you work at the benches standing up: the Palram Glory has 75 in walls and an 8 ft 10 in peak. If you are tall or want to grow vines and hang baskets, check both numbers. Our walk-in ranking weights headroom at 25%.

Two practical details decide how a greenhouse feels to use. First, the door: a narrow single door is fine for carrying trays, but if you will move bags of compost, big pots or a wheelbarrow, look for double doors and a low threshold, like the Essence (47 in wide with a ramp) or the Rion Grand Gardener (47.5 in). Second, the path: plan at least 2 ft of clear path down the middle, more if you use a wheelbarrow. In a 6 ft wide greenhouse, that leaves two benches of about 2 ft each. In an 8 ft wide one, the benches can be 3 ft deep, which is where an 8x12 starts to feel generous.

Buying for this spring only. The first year you start a few trays; by the third you are rooting cuttings, overwintering a lemon tree and growing tomatoes in beds. Size for year three. Counting floor area, forgetting the path. A 6x8 with two benches leaves a path barely wide enough to turn around in. Sketch benches and path on paper with real widths before you order. Ignoring headroom. Low eaves mean you only stand up in the middle, and tall crops like tomatoes need height along the sides. Choosing a narrow door. A door narrower than about 2 ft means no wheelbarrow, which you will feel every time you move compost. Forgetting heating costs. A bigger greenhouse costs more to keep frost-free; if you plan to heat in winter, a well-insulated 8x12 beats a thin-walled 10x16.

If you are still torn, an 8x12 like the Palram Essence is the size we suggest most often; our kits under $2,000 ranking lists the best-value options.

Measure the spot, then add room around it: about two feet on each side to build it, clean the panels and get at the gutters. Put the long side facing south if you can for winter light, keep it away from overhanging trees (falling branches and shade), and close to a water tap. Check local rules too: some cities and HOAs limit the size or placement of garden structures. And remember the base: our foundation guide covers what each kit needs.