calculatemygarden
Units

Onion Planting Calculator

How far apart to plant onions, and how many fit your bed, row or area.

Harvest basket with tomatoes, peppers, carrots, garlic and a zucchini on a garden table
Units
In the row
2 to 4 in
Between rows
12 to 18 in
In a bed, each way
4 to 6 in
Square foot method
9 per square
Planting depth
0.3 to 0.5 in
in

Published for Onion: 4 to 6 in.

Where you plant
Pattern
ft
ft
More optionsEdge margin
Space at the edge

Half a spacing keeps each plant inside the bed, so neighbours in the next bed are still a full spacing away. Plant on the edge when the plants may spread past it, such as the first and last post of a hedge line.

Onions that fit

171plants

About 5.76 per sq ft of ground.

Triangular (staggered) spacing would fit 185 here.

Top view, 8 × 4 ft. Each circle is the space one plant gets.
Plants per sq ft
5.76
Plants per acre
250,905
Plants per hectare
620,001
Ground per plant
0.17sq ft
Ground covered
32sq ft

Common raised beds at this spacing

BedAt 5 in, squareSquare foot method
4 x 4 ft81144
4 x 8 ft171288
3 x 6 ft98162
4 x 12 ft252432

How these numbers are worked out

In a square pattern the rows are one spacing apart, so each plant takes spacing × spacing of ground.

5 in × 5 in = 0.1736 sq ft per plant

43,560 sq ft per acre ÷ 0.1736 sq ft = 250,906 plants per acre

Per-area figures ignore the edges. The count for a bed, row or circle places each one, so it is usually a little lower. One acre is 43,560 sq ft; one hectare is 10,000 m².

Growing in pots? .

How far apart to plant onions

Virginia Cooperative Extension and Colorado State University Extension set onions 2 to 4 inches apart in the row, with rows 12 to 18 inches apart. For full-size bulbs, space them 3 to 4 inches apart at the final spacing, whether you start from seed, sets or transplants. In a raised bed, 4 to 6 inches each way works, and University of Wisconsin Extension's square foot guide fits 9 onions per square foot. The calculator starts on 5 inches each way.

How many onions fit a raised bed

Bed5 in each wayRows, 3 × 15 inSquare foot method (9 per sq ft)
4 × 4 ft8148144
4 × 8 ft17196288
3 × 6 ft9848162
4 × 12 ft252144432

The first two columns keep half a spacing clear at the bed edges, the way the calculator counts. The square foot count is 4 inches apart packed to the edges, so it is the most bulbs a bed will hold. In an in-ground row, 10 feet takes 40 onions at 3 inches, 25 feet takes 100.

Plant close, then thin for green onions

You can plant closer than the final spacing on purpose. Sow or set onions at 2 inches, then pull every other one as they grow and use the pulls as green onions. The plants left behind stand at 4 inches and go on to form bulbs.

Choose the right day length

Bulbing is triggered by day length, not by plant size. Utah State University Extension and Johnny's Selected Seeds sort varieties into three groups:

  • Short-day onions suit the South.
  • Intermediate-day onions suit the middle latitudes.
  • Long-day onions suit the North and other high latitudes.

Buy the group that matches where you live; the spacing is the same for all three.

Depth and timing

  • Seed: 1/4 to 1/2 inch deep, started indoors 6 to 8 weeks before the last frost.
  • Transplants: go out 2 to 4 weeks before your last spring frost. Onions are hardy, so they go in well before tender crops.

Colorado State counts about 110 days from seed to harvest. The onion seed starting dates give the exact windows for your ZIP code or zone.

Using the calculator

The calculator opens on a 4 × 8 ft bed at 5 inches, square pattern. Switch to Rows for in-ground planting with separate row spacing, to Container for a pot, or enter an area. To mix onions with other crops in one bed, use the square foot gardening calculator or the raised bed planner.