Grow Garden Calculator: Real Garden Planner

What this garden calculator works out

Most garden planning comes down to a handful of sums. How many plants fit in the space. How much soil to fill a bed. How much water to put on each week. How much fertilizer to spread. Each one is simple on its own, but they all depend on the same thing — the size and shape of your bed — and it is easy to use one measurement in one sum and a slightly different one in the next.

So this page asks for the bed once. Enter a rectangle, a square, a round bed or a triangular corner, and say how many identical beds you have. Every tab then works from that same area: plants and seeds, soil and mulch, water, fertilizer, harvest, and a final plan with the costs you chose to enter.

It is built for home gardens in the United States first, so it opens in feet, inches, cubic yards and gallons. One switch turns everything to metres, litres and kilograms, and the numbers you already typed are converted rather than lost.

Using it in five minutes

Measure the growing area, not the outside of the frame. A raised bed built from 2 x 6 boards loses a few inches on each side to the wood, and that matters more than you would think when you are counting plants or buying soil.

Then pick a tab. In Plants & seeds, choose a crop to start from its published spacing, or type your own. In Soil & mulch, set the depth of each layer and the bag size your store sells. Water needs a weekly target and last week's rain. Fertilizer needs the three numbers on the bag. Harvest uses the plants you already counted. Plan & cost puts it all on one list you can copy or print.

Every result shows the working underneath it, in the "How this was worked out" box. If a number looks odd, that is where to check which figure went in.

Spacing: how many plants really fit

Spacing is the distance from the centre of one plant to the centre of the next. Seed packets usually give two numbers: the gap between plants in a row, and the gap between rows. Row spacing is often wider because traditional gardens leave room to walk and hoe. In a raised bed you never walk on, many gardeners use the in-row figure in both directions instead, which is the square grid option.

The calculator does not divide the area by the space per plant. That shortcut overcounts, because it pretends plants can sit right on the edge of the bed and that fractions of plants exist. Instead it places every plant on a grid, keeps each one at least half a spacing from the edge, and counts them. A 4 x 8 ft bed at 12 inches holds exactly 32 plants this way: eight along the bed and four across.

The offset pattern shifts every other row by half a spacing, so the plants form triangles. The rows can then sit closer together — 0.866 of the spacing — while every plant stays the same distance from its neighbours. In a big bed that fits more plants. In a narrow one it can fit fewer, because the shifted rows lose a plant at the end. The same 4 x 8 bed at 12 inches holds only 30 offset plants, but at 6 inches it holds 140 offset against 128 square. The calculator counts both and tells you which wins in your bed.

Round and triangular beds are counted the same way, row by row across the real shape. A 10 ft circle is not treated as a 10 ft square, which would overstate its area by more than a quarter. Where the grid sits matters too: lined up with one edge, a grid can straddle the middle of a round bed and waste space, so the calculator also tries a grid centred on the bed and keeps whichever fits more. A 10 ft round bed at 12 inches holds 69 plants that way.

Soil, compost and mulch by volume

Volume is area times depth, with every measurement in the same unit. A 4 x 8 ft bed filled 12 inches deep is 4 × 8 × 1 = 32 cubic feet. Bulk soil is sold by the cubic yard, which is 27 cubic feet, so that bed takes about 1.19 cubic yards. Bagged soil is sold by the cubic foot, so it takes sixteen 2 cubic foot bags. The calculator always rounds bags up, because half a bag still has to be bought.

Depth is the choice that moves the answer most. The University of Maryland Extension suggests raised beds at least 8 inches deep for greens, beans and cucumbers, and 12 to 24 inches for tomatoes, peppers and squash. For compost it suggests working 2 to 4 inches into the top 4 inches of soil. Utah State University Extension suggests 1 to 2 inches of mulch around vegetable plants.

Fresh, fluffy mixes settle once they are watered. If you want the bed full to the rim after the first few weeks, add a settling margin in percent; the calculator applies it to the bed soil only.

How much water a garden needs

Garden watering advice is usually given in inches per week, the same way rain is measured. Utah State University Extension puts most vegetables at 1 to 2 inches a week and sweet corn at 1.5 to 2.5. Starting at 1 inch and watching your plants is a sound approach; sandy soil, heat and wind all push the need up.

Turning inches into gallons is exact arithmetic. One inch of water over one square foot is 144 cubic inches, and a US gallon is 231, so it is 0.623 gallons. An inch a week over 100 square feet is about 62 gallons. Rain counts: subtract whatever fell, and if the rain covered the target the calculator says so instead of showing zero as if it were a mistake.

Not all the water you run reaches the roots. Colorado State University Extension puts drip irrigation above 90 percent efficient and sprinklers at 50 to 70 percent, because spray evaporates and drifts. The calculator divides by the efficiency you choose, so a sprinkler on a hot, windy day asks for nearly twice what drip does. Add your hose's flow rate and it also tells you how many minutes each watering takes.

Reading N-P-K and working out fertilizer

The three numbers on a fertilizer bag are percentages by weight: nitrogen, then phosphate, then potash. A 50 lb bag of 10-10-10 holds 5 lb of nitrogen. Everything else is carrier. That is why the amount you spread depends on the product: to deliver the same nitrogen you need twice as much 10-10-10 as 20-20-20.

The sum is the nitrogen you want divided by the first number as a decimal. The University of Maryland Extension gives 2 lb of nitrogen per 1,000 square feet as the general figure for vegetables, and 3 lb for heavy feeders such as tomatoes, broccoli and beets. With 10-10-10 that is 2 ÷ 0.10 = 20 lb per 1,000 square feet. The calculator scales it to your bed, shows the phosphate and potash that come along with it, and can split the total into several smaller feedings.

Two cautions. A soil test beats any general figure: if it shows plenty of phosphorus and potassium, a nitrogen-only product is the better choice. And the product label always wins. If it says to use less, use less — more fertilizer is not more food, and extra nitrogen tends to give leafy plants and little fruit.

If you think in ratios rather than percentages, the ratio calculator will simplify an N-P-K like 16-16-8 to 2:2:1.

How much seed to buy

Not every seed comes up. Federal seed law sets a minimum germination rate for each vegetable sold across state lines — 80 percent for lettuce, 55 percent for carrots and peppers — and packets usually print their own tested figure, which is often higher. The calculator starts from the federal minimum and you can type the packet's number over it.

Seeds needed are the plants you want, times the seeds you drop in each spot, divided by the germination rate. Sixty carrots at 55 percent need about 110 seeds. If you like to sow two or three seeds a spot and thin to the strongest, raise the "seeds per spot" box and the seed count follows. For crops you set out as transplants — tomatoes, peppers, cabbage — the plant count is simply the number of starts to buy or grow.

Why the harvest is a range

Yield depends on the variety, the weather, the soil, pests, and how often you pick. Extension services publish averages anyway, usually per 10 or 100 feet of row, and they are useful for planning as long as nobody treats them as a promise. They also disagree. For tomatoes, K-State lists 10 lb per 10 feet of row and LSU AgCenter 250 lb per 100 feet, which is 25 lb per 10 feet.

So the calculator shows each published figure for your crop, scaled to your row length, and the range between them. Twenty feet of tomato row comes out at 20 to 50 lb. Where one source counts heads, ears or fruits and another weighs pounds, both are shown as they were published — the calculator never guesses how much a head of cabbage weighs to force them into one number.

If you plant in a bed rather than in rows, the calculator uses the plants times the in-row spacing as a row-equivalent length. It is a fair approximation, not a measurement, and the result says so.

A worked example: one 4 x 8 raised bed

Load the example and you get a 4 x 8 ft bed, 32 square feet, with 24 feet of edge for boards or border. At 12 inches square spacing it holds 32 plants. Filled 12 inches deep it needs 32 cubic feet of mix, 1.19 cubic yards, or sixteen 2 cubic foot bags. Two inches of mulch on top is another 5.33 cubic feet.

At 1 inch of water a week on drip, it needs about 22 gallons; with a sprinkler on a hot day, about 40. At the general 2 lb of nitrogen per 1,000 square feet, 10-10-10 comes to 0.64 lb for the season — about 1.3 cups at half a pound a cup.

None of those figures needed a guess. Change the depth, the spacing or the product and each one moves, with the working shown beneath it.

Common planning mistakes

Measuring the outside of the frame. Measure inside the boards. The difference is small on one side and adds up across a whole bed of soil.

Mixing units. Depth in inches times length in feet gives a nonsense volume. The calculator converts everything first, which is the main reason to use one.

Dividing area by spacing squared. It ignores the edges and overcounts, sometimes by a lot in a small bed.

Spreading fertilizer by the bag, not by the nitrogen. Ten pounds of 20-20-20 and ten pounds of 5-10-5 are very different doses.

Forgetting the rain. A good storm can cover a whole week's water. Check a rain gauge before you water.

What the calculator cannot know

It cannot see your soil, your sun or your weather. Spacing, water and fertilizer figures are general starting points from extension publications, and the right number for your garden can differ with variety, soil type, climate, pests and care. Planting dates are left out on purpose, because they depend on your local frost dates and one national answer would be wrong for most of the country — your county extension office publishes them for your area.

Costs are only as good as the prices you type in. Water prices vary a lot between utilities, so the page asks for yours rather than assuming one. For percentage questions it does not cover, such as a sale price on soil, the percentage calculator does the same kind of sum. More everyday tools are in the home and garden calculators.

Frequently asked questions