Square Foot Garden Calculator

How the square foot method divides a bed

Square foot gardening replaces rows with a grid. You build a raised bed, mark it into one-foot squares, and plant a set number of plants in each square instead of thinning along a row. The bed is usually 4 feet wide, because that is the widest most people can reach across from either side, and any length you have room for.

A 4 by 8 bed has 32 squares. Each square gets its own crop, so a single bed can hold tomatoes in one corner, lettuce in another and carrots along an edge, all at spacing that suits each plant. The grid also makes weeding and harvesting quick, because you always know what is planted where.

The method came from Mel Bartholomew, who wrote it up in 1981 and kept refining it. Two rules matter: the grid and the soil. The grid sets the spacing, and the soil, a blend he called Mel's Mix, gives plants a loose, rich bed that roots push through without much resistance. You fill the bed once and top it off each season rather than digging or tilling.

Plants per square, by crop

Spacing is the whole game. The number of plants per square comes from the mature size of the plant, and the table below covers most of what a home gardener grows.

The pattern matters as much as the count. Four plants per square go in a 2 by 2 arrangement, nine go in a 3 by 3 grid, and sixteen go in a 4 by 4 grid. A single plant per square, like a tomato, sits in the middle. Getting the pattern right is what keeps a square from crowding itself.

Worked example: planting a 4 by 8 bed

A 4 by 8 bed gives you 32 squares. Here is one plan that uses all of them and feeds a household through the summer.

• Tomatoes: 8 squares at 1 per square = 8 plants
• Lettuce: 8 squares at 4 per square = 32 plants
• Carrots: 16 squares at 16 per square = 256 carrots
• Total: 8 + 8 + 16 = 32 squares, the whole bed

That mix works because the crops have different habits. Tomatoes go tall and take a square each, but they produce for months. Lettuce is fast, ready in 45 to 55 days, so those 8 squares can be replanted with something else by midsummer. Carrots fill the low edge and hold in the ground until you pull them.

If you would rather grow fewer kinds, a simpler plan is 16 squares of tomatoes and peppers, 8 squares of beans, and 8 squares of salad greens. The point of the calculator is to let you test a layout before you buy seed, because running out of squares in June is a common and avoidable mistake.

Mel's Mix: what fills the bed

A raised bed needs soil that drains and holds moisture at the same time, and Mel's Mix is the standard for square foot beds. It is equal parts by volume of three things: compost, peat moss, and coarse vermiculite.

The volume math is easy. A 4 by 8 bed at 6 inches deep needs 4 x 8 x 0.5 = 16 cubic feet of mix. Split three ways, that is about 5.3 cubic feet each of compost, peat and vermiculite. At 8 inches deep the same bed needs about 21 cubic feet, and at 12 inches, about 32 cubic feet.

Compost is the part that feeds the plants, and using several kinds, such as mushroom compost, leaf compost and worm castings, gives a wider range of nutrients than a single bag. Peat moss holds water and keeps the mix light. Vermiculite holds water and air, and it is the ingredient people try to skip, usually by substituting perlite. Perlite drains better but holds less water, so a bed with perlite dries out faster in summer. Buy coarse vermiculite if you can, because the fine grade compacts.

Do not fill a raised bed with garden soil. It compacts, drains poorly in a box, and brings weed seeds. The USDA has general guidance on soil and home gardening, and state extension services like University of Minnesota Extension publish free, region-specific planting calendars worth checking before you buy seed.

Tall and vining crops: spacing that breaks the grid

Not every plant respects a one-foot square. Tomatoes, cucumbers, melons, squash and corn all grow beyond their footprint, and if you ignore that, the bed shades itself and yields drop.

Two fixes cover most cases. The first is vertical growing. A trellis on the north side of the bed lets cucumbers, pole beans and small melons climb instead of sprawl, which keeps their fruit off the ground and their leaves out of the neighboring squares. A single cucumber on a trellis can be grown at 2 per square instead of taking several squares of floor space.

The second is to give the big plants squares at the edges, where they lean out over the path rather than into the bed. A tomato in the middle of a 4 foot bed shades the squares behind it, while the same tomato on the south edge leans into the sun and leaves the bed clear.

Corn is the exception that does not fit the method well. It is wind pollinated, so a single square of corn gets poor pollination. If you want corn, plant a block of at least 4 squares together and accept that it takes space other crops could use.

Succession planting for two crops per square

The grid makes succession planting simple, because you can harvest one square and replant it the same day without disturbing the rest of the bed. That is how a 32 square bed produces more than 32 plants worth of food in a season.

The trick is to pair a fast crop with a slow one. Radishes are ready in about 25 days, lettuce in 45 to 55, and spinach in about 40. Plant those in early spring, and by the time the tomatoes need their space, the early squares are free again. A square that grows radishes in April can grow bush beans in June and garlic in the fall.

Keep a simple record. A sheet of paper with the bed drawn as a grid, labeled with what is planted and when, saves you from replanting a square with a crop that will not have time to finish. Note the days to maturity for each variety, count back from your first frost date, and only plant a square if the crop can finish before the cold arrives. The USDA plant hardiness zone map and your local extension office planting calendar give you the frost dates to count from.