How Many Onions Should You Plant?
The honest starting point is how many onions your kitchen actually uses. An average household cooks through one to two onions a week, so a year-round supply is roughly 60 to 120 bulbs. Because onions store well for months in a cool, dry spot, planting enough to last past the growing season is realistic in a way it is not for tomatoes or lettuce. Once you know your target harvest, you simply work backward to the number of sets or transplants to buy, padding for the few that will not make it.
That padding matters more than most charts admit. Onion sets and bare-root transplants do not all survive, especially in cold, wet, or pest-heavy springs. A survival rate of 80 to 90 percent is normal, so to harvest 100 onions you should plant closer to 115 to 120. This calculator builds that buffer in automatically using the survival rate you enter.
The Spacing Math Behind the Numbers
Onions are planted in rows, with two spacing decisions that drive everything else: how far apart the bulbs sit within a row, and how far apart the rows themselves are. In-row spacing controls final bulb size. Crowd them at 3 inches and you get small bulbs or scallions; give them 5 to 6 inches and each one can swell into a large or jumbo onion.
Plants to buy = ceil( target bulbs / survival rate )
Row feet = plants / (12 / in-row spacing in)
Bed space (sq ft) = plants x (in-row in x row spacing in) / 144
Why Bed Space Surprises People
A modest goal of 120 medium onions at 4-inch spacing in 12-inch rows needs only about 47 row feet but roughly 47 square feet of bed once you account for the gaps between rows. Tightening rows to a 10-inch gap in a raised bed shrinks that footprint noticeably, which is why intensive growers favor blocks of four or five close rows over long single ones.
Frequently Asked Questions
How many onions can I grow in a 4x8 raised bed?
A 4x8 bed is 32 square feet. At 4-inch in-row spacing with rows 6 inches apart you can fit roughly 190 to 230 onions, since each plant occupies about 0.17 square feet. Pushing to 6-inch spacing for jumbo bulbs drops that to around 130. Onions tolerate tight planting better than most crops, which makes raised beds especially efficient for them.
Should I plant onion sets, transplants, or seeds?
Sets (small dormant bulbs) are the easiest and most forgiving, sprouting quickly and shrugging off cool soil, though they can bolt more readily. Bare-root transplants give the largest, most uniform bulbs and are the choice of serious growers. Seeds are the cheapest per plant but need an 8 to 10 week indoor head start. This calculator works for any of the three since it counts finished plants.
How far apart should onions be planted?
Space bulbs 3 to 6 inches apart in the row depending on the size you want, with 12 to 18 inches between rows for easy weeding, or as little as 6 inches in intensive raised beds. Closer spacing yields more, smaller onions; wider spacing yields fewer, larger ones. Four inches is the classic compromise for everyday cooking onions.
How much does each onion weigh at harvest?
A medium cooking onion runs about 6 to 7 ounces, or roughly 0.4 pounds. Small bulbs are nearer 4 ounces, large storage onions reach 9 to 10 ounces, and jumbo sweet varieties like Walla Walla can top 14 ounces. Multiply your bulb count by the expected weight to estimate total harvest, which is handy for planning storage and pantry space.
Practical Guide for Onion Planting Calculator
Match your variety to your latitude before you buy a single set. Onions bulb in response to day length, so long-day types suit the northern US and Canada, short-day types suit the South, and day-neutral varieties work almost anywhere. Planting the wrong type for your region is the single most common reason gardeners harvest disappointing golf-ball onions no matter how perfect their spacing was, so confirm the day-length category first.
Get the planting depth and timing right and the spacing math pays off. Sets and transplants go in shallow, with the bulb tip just at or barely below the soil surface; planting too deep restricts how wide a bulb can grow. Onions are cold-hardy, so plant as early as the soil can be worked in spring, or in fall for overwintering types in mild climates. The longer the cool growing season before bulbing, the bigger the leafy top, and bigger tops mean bigger bulbs.
Feed and weed relentlessly through the leafy stage. Each onion leaf becomes a ring, so every leaf grown before bulbing translates to bulb size, and that growth demands steady nitrogen and zero competition. Onions have sparse, shallow roots and lose badly to weeds, so keep the rows clean and side-dress with nitrogen every few weeks until the bulbs start to swell, then ease off so they can cure and store well.
Quick Checklist
- Choose long-day, short-day, or day-neutral varieties to match your latitude.
- Plant sets and transplants shallow, with the bulb tip near the soil surface.
- Keep rows weed-free; onions lose hard to competition for water and light.
- Stop watering and feeding once tops flop, then cure bulbs 2-3 weeks before storing.