Most vegetables need a pot at least as deep as their roots and at least one third the height of the mature plant. Herbs and salad greens do fine in 1 to 2 gallons at 6 to 8 inches deep. Carrots, kale and chard want 8 to 12 inches. Peppers, cucumbers and cherry tomatoes need 5 gallons; a full-size tomato wants 10. When in doubt on a fruiting crop, size up.
A pot is not a decoration that happens to hold soil. It is the entire soil volume, water reservoir and root run your plant will ever have. Everything else you do — the mix, the feeding, the watering routine — is downstream of how many gallons you gave it. Get the size right and an average gardener grows good food. Get it wrong and a skilled gardener spends the summer compensating.
The one rule that covers most plants
Match the pot volume to the mature root system, not to the seedling in your hand. A tomato start is six inches tall in May and four feet tall in August, with a root mass to match, and the pot you choose in May is the one it lives in.
The quick visual check: the pot should be at least one third the height of the mature plant. A 4-foot tomato wants something in the region of 16 inches tall and wide. A 10-inch basil plant is fine in an 8-inch pot. This rule is crude, but it catches the worst mistakes at the garden centre, where a 6-inch nursery pot always looks like enough.
When you are choosing between two sizes for a fruiting crop — tomato, pepper, cucumber, squash, eggplant — take the larger one. The cost difference is a few dollars. The performance difference across a whole summer is roughly double the harvest, because the bigger pot never runs dry between waterings.
Leafy and root crops are less forgiving in the other direction. They rarely need more volume than the numbers below, and giving them a much bigger pot mostly means more mix to keep wet. Spend your extra gallons on the fruiting plants.
How are pot sizes actually measured?
Nursery gallons are not liquid gallons. A container sold as a "1 gallon" or "#1" pot holds about three quarters of a liquid gallon of soil, and the gap widens as sizes go up. The number is a trade size, not a measurement, so two 5-gallon pots from different makers can differ by a couple of quarts. Treat the table below as the working range rather than a spec sheet.
| Nominal size | Approx. diameter | Approx. depth | Soil volume (dry qt) | Wet weight |
|---|---|---|---|---|
| 1 gallon | 6–7 in | 6–7 in | 3 qt | 8–10 lb |
| 2 gallon | 8–9 in | 8 in | 6 qt | 16–20 lb |
| 3 gallon | 10 in | 9–10 in | 9 qt | 24–30 lb |
| 5 gallon | 11–12 in | 11–12 in | 15 qt | 40–50 lb |
| 7 gallon | 14 in | 12–13 in | 21 qt | 56–70 lb |
| 10 gallon | 16 in | 14 in | 30 qt | 80–100 lb |
| 15 gallon | 18 in | 16 in | 45 qt | 120–150 lb |
| 20 gallon | 21 in | 17 in | 60 qt | 160–200 lb |
Wet weight is soil only, at roughly 8–10 lb per nominal gallon. Add the pot: a fabric bag adds under a pound, a glazed ceramic of the same size can add 30.
Fabric grow bags run a little smaller than their label, usually by 10 to 20 percent, because the fabric collapses inward as the mix settles. A 10-gallon fabric bag behaves more like an 8-gallon rigid pot. That is not a reason to avoid them — they are the best large container on a weight-limited balcony — but it is a reason to buy one size up when a crop is at the edge of its minimum.
What size pot for each crop?
Find your crop, read the minimum, and buy at or above it. The depth column matters more than the gallon column for anything grown for its root, and the plants-per-pot column is where most first seasons go wrong. These are minimums that produce a real harvest, not survival floors.
| Crop | Min. pot (gal) | Min. depth | Plants per pot | Notes |
|---|---|---|---|---|
| Basil | 2 | 6–8 in | 1 per 2 gal | Pinch the growing tip early and often |
| Parsley | 2 | 8–10 in | 1–2 | Taproot; the extra depth is not optional |
| Cilantro | 1 | 6–8 in | 3–4 | Bolts in heat; resow every week |
| Chives | 1 | 6–8 in | 1 clump, 8 in apart | Perennial; divide every second spring |
| Thyme, oregano, sage | 2 | 6–8 in | 1 | Prefer to dry out; use a small pot on purpose |
| Rosemary | 3–5 | 10–12 in | 1 | Woody and long-lived; size up in year two |
| Mint | 2 | 8 in | 1, alone | Its own pot, always — it takes over any shared one |
| Lettuce and salad mix | 1–2 | 6–8 in | 4–6 baby, or 1 head per 6 in | Cut-and-come-again; resow every 7 days |
| Spinach | 1–2 | 6–8 in | 4–6 | Cool weather only; bolts above 75°F |
| Mustard greens | 2 | 8 in | 2–3 full size | Baby leaf can be sown thickly |
| Bok choy | 2 | 6–8 in | 2–3 | Fast; resow every 10 days |
| Swiss chard | 3 | 8–12 in | 1–2 | Cuts all season from one plant |
| Kale | 3–5 | 8–12 in | 1 | Sweeter after the first frost |
| Radishes | 1–2 | 6 in | 10–12 per gal, 2 in apart | The fastest thing you can grow in a pot |
| Beets | 3 | 8–10 in | 4–6, 4 in apart | Thin ruthlessly; the tops are edible |
| Carrots | 3–5 | 10–12 in | 3 in apart after thinning | Match depth to variety; mix must be stone-free |
| Celery | 3 | 10–12 in | 1 | Thirstiest thing on this list |
| Scallions | 1–2 | 6–8 in | Clumps 1 in apart | Regrows from the base after cutting |
| Garlic | 3 | 8–10 in | 3–4, 6 in apart | Plant in autumn, harvest mid-summer |
| Shallots | 3 | 8–10 in | 3–4, 6–8 in apart | One set becomes a cluster |
| Leeks | 5 | 10–12 in | 3–4, 6 in apart | Deep pot lets you blanch the stems |
| Brussels sprouts | 5 per plant | 12–18 in | 1 | Tall and top-heavy; needs a sheltered spot |
| Bush beans | 3 | 8–10 in | 3–4 | No trellis needed; resow every 10 days |
| Peas | 3–5 | 8–10 in | 6–8 with a trellis | Cool season; the trellis goes in at sowing |
| Peppers | 3–5 | 10–12 in | 1 | 5 gal for full-size bells, 3 for chillies |
| Cherry tomato | 5 | 12 in | 1 | Stake or cage at planting time |
| Full-size tomato | 10 (5 minimum) | 12–18 in | 1 | 10 gal is the difference between fruit and foliage |
| Cucumber | 5 | 12 in | 1–2 with a trellis | Grow up, not out, in a small space |
| Strawberries | 1–2 per plant | 8 in | 1 per gal | Replace the plants every third year |
Where a range is given, the smaller number is the honest minimum and the larger is what the crop would choose. For a fuller picture of what earns its place in a small space, see the crop guide.
Why depth beats width for root crops
For anything you eat the root of, depth is the hard constraint and width only sets how many you get. A carrot cannot grow sideways when it hits the bottom of an 8-inch planter. It stubs, forks, or stops, and no amount of feeding fixes it.
Size carrots to the variety rather than to the word "carrot". A short Nantes or Chantenay type finishes at 4 to 5 inches and will do well in an 8-inch pot. A standard main-crop carrot wants 10 to 12 inches. A long Imperator wants more than most balcony pots offer, which is why the round and stump-rooted varieties are the sensible container choice. Whatever the depth, the mix has to stay loose — a heavy or lumpy mix forks roots as reliably as a shallow pot does.
Garlic and shallots need 8 to 10 inches, which surprises people, because the bulb itself sits in the top three. The depth is for the root plate underneath and for the overwintering the crop does before it bulks up. Leeks want 10 to 12 inches for a different reason: the extra depth is what lets you mound mix around the stems to blanch them white.
A shallow 6-inch planter is not useless. It grows radishes, salad leaves, spinach, bok choy, scallions and most herbs perfectly well, and it is the right choice for a rail planter where weight is tight. What it cannot do is grow carrots, beets, leeks, celery or anything fruiting. Buy shallow planters for what they are good at and stop asking them to do more.
What happens when the pot is too small?
An undersized pot runs out of water before the day does, and every other symptom follows from that. In a 2-gallon pot, a mature tomato can drain the entire reservoir between breakfast and mid-afternoon. The plant wilts, recovers overnight, and does it again tomorrow.
The visible signs, roughly in the order they arrive:
- Daily afternoon wilting even though you watered in the morning
- Growth stalls — the plant holds the same size for weeks
- Greens turn bitter or bolt, because heat and drought stress trigger flowering
- Blossom end rot on tomatoes and peppers, from moisture swinging between soaked and bone dry
- Roots circling the wall in a tight mat, visible the moment you tip the plant out
- Water runs straight through in seconds because there is more root than soil left
None of these are watering problems, though they look like watering problems. You cannot water your way out of a pot that holds four hours of supply. If several of these are showing up at once, the diagnosis walk-through in why container plants die will help you separate a size problem from a light, feeding or drainage problem.
What happens when the pot is too big?
An oversized pot stays wet, because roots are what dry soil out and there are not enough of them. A small seedling dropped into a 10-gallon container occupies maybe a fifth of the volume. The rest of the mix holds water it has no way to lose except slowly, from the surface.
Three things follow. The mix goes sour and anaerobic in the zone the roots have not reached. It stays cold well into spring, because a large wet mass warms slowly, and cold roots do not grow. And the seedling's own small root ball sits permanently damp, which is how young stems rot at the soil line.
The fix is to pot up in stages rather than jumping to the final container. Move a plant into roughly double its current volume each time: a seed cell to a 3-inch pot, a 3-inch to a quart, a quart to a gallon, a gallon to the final 5 or 10. Each step gives the roots room they can actually fill before the next one. Use an up-potting mix — 2 parts potting soil to 1 part seed-starting mix, plus perlite — so the new soil drains as freely as the old.
One exception worth knowing: a strong, well-rooted transplant going out into warm soil in early summer handles an oversized pot without much trouble. It is spring, cold mix and small seedlings that make big pots dangerous.
Does pot material change the size you need?
Yes — the faster a material dries, the more volume you want for the same plant. A tomato in a 10-gallon fabric bag and a tomato in a 10-gallon glazed pot are not having the same experience. The fabric bag may need water twice a day in August while the glazed pot coasts for two.
| Material | Empty weight | Drying speed | Winter tolerance | Rough cost | Best use |
|---|---|---|---|---|---|
| Fabric grow bag | Under 1 lb | Fastest | Empty and fold away; nothing to crack | $5–$15 | Anything large on a weight-limited balcony |
| Unglazed terracotta | Heavy | Fast | Cracks if left full through a freeze | $10–$40 | Rosemary, thyme, sage — plants that like to dry |
| Glazed ceramic | Heaviest | Slow | Freeze-thaw cracks it; move it inside | $25–$90 | One or two feature pots, close to the wall |
| Plastic | Light | Slow | Turns brittle after a few seasons of UV | $5–$25 | Nursery stages, back rows, cheap volume |
| Resin or fiberglass | Light to medium | Slow | Good; the most weatherproof of the lot | $30–$120 | Large planters where weight and looks both matter |
| Wood | Medium to heavy | Moderate | Good in cedar; softwood rots in 3–5 years | $40–$150 | Rail troughs and long raised planters |
| Metal | Medium | Moderate, but heats fast | Fine structurally; roots cook in full sun | $20–$80 | Cool or partly shaded positions only |
Rule of adjustment: add one to two gallons over the crop minimum for fabric and terracotta, stay at the minimum for glazed, plastic and resin.
The practical consequence is that material is a watering decision as much as an aesthetic one. If you are away two days a week, glazed and resin buy you slack. If you tend to overwater, or your balcony is windless and humid, terracotta and fabric save you from yourself.
Weight, and what a balcony can actually hold
On a balcony, pot size is a structural decision. Wet mix runs roughly 8 to 10 pounds per gallon of container volume, and residential balconies are commonly rated near 40 pounds per square foot (about 195 kg per square metre) as a distributed load. Both of those numbers are easy to blow past without noticing.
Work an example. A 20-gallon planter holds 160 to 200 pounds of wet mix. If it is a 21-inch round pot, its footprint is about 2.4 square feet, so it puts roughly 70 to 85 pounds per square foot on the slab — double the common rating, concentrated in one spot. Spread the same soil across four 5-gallon bags standing on 4 square feet of floor and you are at about 45 pounds per square foot. Line them along a wall on an 8-square-foot strip and you are near 25.
Distribution is the whole game. The practical rules:
- Heavy pots go against the building wall, where the slab is supported, never on the outer edge of a cantilever
- Anything over 10 gallons goes in fabric or resin, so the container itself adds pounds rather than tens of pounds
- Spread the load with a plank, a pallet or a wheeled dolly under large planters rather than resting them on four small feet
- Count the water — a freshly watered pot is at its heaviest, and that is the number that matters
- Read the lease before you commit; many buildings cap balcony loads or ban them outright
If you are laying out a whole balcony rather than choosing one pot, the weight map should come before the plant list. Patio Designing 101 covers how to sequence those decisions — sun, wind, water and load first, planting last.
Drainage, saucers, and lifting the pot off the floor
Every container needs drainage holes, and most decorative pots do not have enough. Aim for at least one half-inch hole per gallon of volume, spread across the base rather than clustered in the centre. If a pot you love has a single small hole, drill more. If it has none, use it as a cachepot with a plain nursery pot inside it, lifted clear of the bottom.
Lift the pot off the floor. Pot feet, a couple of tiles, or a slatted riser give the holes somewhere to drain to and stop the base sitting in a puddle after rain. On a wooden deck this also prevents the dark, permanently damp ring that rots boards.
A saucer left standing full of water is a slow way to kill a plant. The bottom two inches of mix stay saturated, roots suffocate there, and the plant declines over weeks with no obvious cause. Empty saucers within an hour of watering, or skip them outdoors entirely. A pot with no drainage hole at all is not a planter — it is a bucket.
Do not put a gravel layer in the bottom. It is the most persistent piece of bad advice in container gardening. Water will not move from fine potting mix into coarse gravel until the mix directly above is fully saturated, so a gravel layer lifts the waterlogged zone higher up the pot instead of draining it away. You lose root volume and gain a soggy band. Perlite mixed through the soil — one part in four, as in the standard container mix — does the job gravel is supposed to do.
When should you size up?
Size up when the roots have filled the pot, which you can read from the plant before you ever tip it out. Water running straight through in a few seconds, daily wilting despite a moist surface, growth that has flatlined in good conditions, roots pushing out of the drainage holes or coiling visibly at the surface: any two of those together mean the plant has outgrown its container.
Tip a candidate out and look. A healthy root ball holds its shape with white root tips visible at the edges and mix still crumbling away between them. A root-bound one is a solid felted mass, tan or brown, with soil you can barely see. Tease or slice the outer roots before repotting, or they keep circling in the new pot as though the old wall were still there.
A workable cadence for the first year:
- Seed cell to 3-inch pot when the first true leaves are fully open
- 3-inch to 1 quart roughly 3 to 4 weeks later, once roots reach the sides
- 1 quart to 1 gallon for anything waiting on warm weather to go outside
- 1 gallon to final container at transplant, into 5 or 10 gallons for fruiting crops
- Perennial herbs — repot or divide every second spring, one size up each time
Annual vegetables should not need a mid-season repot if you sized the final pot correctly, and that is the point of getting it right in spring. If you are still working out what your space can carry, the free planning tool will size the pots against your actual sun, wind and load before you buy anything. Start there, then come back to the crop table above.
Working out what your balcony can hold?
ecoshop's free planning tool asks six questions about your space and returns an architect-reviewed blueprint — sun, wind, water, weight, and what will actually grow there.
Start your blueprintFrequently asked questions
What size pot do tomatoes need?
How deep does a container need to be for carrots?
How many plants can I put in one pot?
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Do I need to put gravel in the bottom of a pot for drainage?
What size pot is best for herbs?
Sources
Pot sizes and minimum depths by crop follow published extension recommendations. Load figures are general guidance only. Weight limits vary by building, and a structural engineer is the authority on your balcony, not this table.
- Gardening in Containers (Circular 787) University of Georgia Extension
- Growing Vegetables in Containers University of Wisconsin–Madison Division of Extension
- Container Vegetable Gardening: Four Keys to Success Penn State Extension