Square Foot Gardening Math


For square foot gardening, there are multiple resources online, and in print that will tell you how many carrots to plant per square foot. But picture this…

You are in your favorite place…the garden. You carried out a pack of carrot seeds to get started, and you discover to your dismay that your husband loaned your square foot gardening bible to a neighbor who isn’t home, and that while the package tells you how deep to sow the seed, when, and how far to space them apart, but it doesn’t mention anything at all about how many carrots to sow per square foot.

First, don’t panic. Even if you sucked at math in high school and you have avoided it entirely since then, you can figure out how many carrots to plant in one square foot of soil. It’s not hard, and it makes a lot more sense than Algebra..or Chicken Math (which is a thing).


Recommended Spacing Still Matters



Before you throw that seed package away, it contains everything you need to know to figure out how many of any vegetable, herb or flower seed you should plant in one square foot.

This special mathematic formula starts with the recommended spacing on the back of that seed package.

The Secret Formula for Square Foot Gardening


Take 12 inches and divide it by the plant’s spacing in inches.

Then square that number if you want the total for one square foot.

Formula:

Plants per square foot = (12 ÷ spacing)^2

Let’s say your plant needs 12 inches of space.
12 ÷ 12 = 1

and then we square it by 1 x 1 = 1

the answer is 1 plant per square foot.


Now, obviously, that was easy math…I don’t care how much it sucked in high school, or how much you sucked at it.

Let me explain all this jibberish in plain East Texas speak.

12 divided by 12 is 1 (because 12 can only go into 12 once). So, the answer to that part of the equation is 1..but now we need to square it.

Before you start quivering in your little math boots….to square a number simply means that you multiply a number by itself. So, if the number is 1 it’s 1 x 1, which equals 1. If the number is 3, then to square it, you do 3 x 3, which equals 9…and in square foot math, that’s where the equation ends, and your answer would be 9 plants per square foot.

Come on, now! This is FUN math…the kind that gets more food on the table! Here are some more examples:

If a plant needs 6 inches of space:

  • 12 ÷ 6 = 2
  • 2 × 2 = 4 plants per square foot

If a plant needs 4 inches of space:

  • 12 ÷ 4 = 3
  • 3 × 3 = 9 plants per square foot

If a plant needs 3 inches of space:

  • 12 ÷ 3 = 4
  • 4 × 4 = 16 plants per square foot

If a plant needs 2 inches of space:

  • 12 ÷ 2 = 6
  • 6 × 6 = 36 plants per square foot

The easy way to think about it


You are figuring out:

  • how many plants fit across
  • and how many plants fit down

Then multiply those two numbers together.

So if plants are spaced 4 inches apart:

  • 3 fit across a 12-inch square
  • 3 fit down
  • 3 × 3 = 9

Quick reference pattern

  • 12-inch spacing = 1
  • 6-inch spacing = 4
  • 4-inch spacing = 9
  • 3-inch spacing = 16
  • 2-inch spacing = 36

Important real-life note


This works best for:

  • carrots
  • radishes
  • onions
  • lettuce
  • basil
  • bush beans
  • beets
  • spinach


For bigger plants like:

  • tomatoes
  • peppers
  • broccoli
  • cabbage

You’ll usually use 1 seed per square, sometimes even more than 1 square per plant if the plant gets wide.

For sprawling plants like:

  • squash
  • melons
  • pumpkins

Those often need trellising or extra squares.

The formula gives you a number, but the growth habit matters more than the math. This means you may not need strong math skills as much as you need strong common sense for square foot gardening – especially if you know to look at the suggested spacing on the package and go from there.