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Seed Calculator
Use this free seed calculator to plan a vegetable garden before you buy seed. Pick a plant, enter your row length and number of rows, and it works out how many seeds you need, how many packets to buy, and how many plants you can expect after germination.
Tomato preset: 24 in between plants, 36 in between rows, 25 seeds per packet, 85% germination.
| Amount | Value |
|---|---|
| Seeds needed | 24 |
| Packets to buy | 1 |
| Expected plants after germination | 20 |
| Planting spots per row | 5 |
| Total planting spots | 20 |
| Number of rows | 4 |
| Total row length | 40 ft |
| Approx. bed width | 12 ft |
How it works
A seed calculator turns a garden plan into a shopping list. It starts with the planting spots: the row length in inches divided by the in-row spacing. A 10-foot row is 120 inches long, so at 24 inches between tomato plants it holds 5 spots. Multiply the spots per row by the number of rows, and you have the total number of plants you want in the ground. That is the target the rest of the math works back from.
Germination is the reason the seed count is higher than the plant count. Not every seed sprouts, so you sow extra seed to make up for the ones that fail. The germination rate is the share that usually sprouts, taken from the seed packet when it is printed there. Divide the total spots by the germination rate as a decimal to get the seeds needed. For 20 tomato spots at 85 percent germination, that is 20 ÷ 0.85 = 23.53, rounded up to 24 seeds. Sowing those 24 seeds should give about 20 plants, because 24 × 0.85 = 20.4.
Total spots = spots per row × number of rows
Seeds needed = total spots ÷ germination rate, rounded up
Packets to buy = seeds needed ÷ seeds per packet, rounded up
Expected plants = seeds needed × germination rate
Each plant in the menu comes with a preset for spacing, seeds per packet, and a typical germination rate, so you can get a sensible answer with one click. Tomatoes sit 24 inches apart in the row with 36 inches between rows, peppers 18 inches apart with 24 inches between rows, and zucchini needs 36 inches in the row and 48 inches between rows because the plants spread so wide. Small seeds are the opposite: carrots and radishes sit 2 inches apart, lettuce 8 inches, and spinach 3 inches. Packets vary just as much, from 25 seeds for tomatoes to 500 for lettuce and carrots, which is why the packet count can surprise you. Change any preset value if your packet says something different — the packet in your hand always beats a general preset.
The row length and the number of rows are the two numbers that set the size of the job. Total row length is simply row length times rows: four 10-foot rows give 40 feet of row to plant, weed, and water. The between-row spacing sets how wide the bed needs to be, roughly rows times the between-row spacing. Four rows at 36 inches need about 12 feet of width.
Use the result as a plan, not a promise. Germination changes with soil temperature, moisture, seed age, and how deep the seed is sown. Seed sown too deep or into cold, wet soil germinates worse than the packet rate, while fresh seed sown at the right depth in warm, damp soil can beat it. Keep a few spare seeds from the packet for filling gaps a week or two after sowing, and thin crowded seedlings to the spacing the calculator used, keeping the strongest plant in each spot.
Worked example
You are planting tomatoes in 4 rows, each 10 feet long, with 24 inches between plants in the row and 36 inches between rows. The packet holds 25 seeds and lists 85 percent germination.
Step 1: Spots per row — 10 feet is 120 inches, and 120 ÷ 24 = 5 spots per row.
Step 2: Total spots — 5 spots × 4 rows = 20 planting spots.
Step 3: Seeds needed — 20 ÷ 0.85 = 23.53, rounded up to 24 seeds.
Step 4: Packets to buy — 24 ÷ 25 = 0.96, rounded up to 1 packet.
Step 5: Expected plants — 24 × 0.85 = 20.4, or about 20 plants.
Step 6: Total row length — 10 feet × 4 rows = 40 feet of row, and the bed is about 4 × 36 inches ÷ 12 = 12 feet wide.
The result is 24 seeds, 1 packet, and about 20 expected plants from 4 rows of 10 feet.
FAQ
How many seeds do I need per row?
Divide the row length in inches by the in-row spacing, then allow for germination. A 10-foot row of tomatoes spaced 24 inches apart has 5 planting spots, and at 85 percent germination you need 6 seeds for that row, because 5 divided by 0.85 is 5.88, rounded up to 6.
What germination rate should I use?
Use the rate printed on your seed packet when you have it, because it is tested for that batch of seed. If the packet does not list one, start with 85 percent for most vegetables, 80 percent for carrots and onions, and 90 percent for lettuce, cucumbers, and beans. Older seed usually germinates worse, so lower the rate by 10 to 15 points for seed more than two years old.
How many packets of seeds should I buy?
Divide the seeds needed by the seeds per packet and round up to a whole packet, because packets are only sold whole. For example, 24 tomato seeds divided by 25 seeds per packet is 0.96, so you buy 1 packet. Buying one extra packet is sensible for a large planting, since a torn or damp packet can spoil the seed inside.
How far apart should I space my seeds?
Follow the in-row spacing for the plant, because crowded plants compete for light, water, and food and stay small. Tomatoes need about 24 inches between plants in a row, lettuce about 8 inches, and carrots about 2 inches. Leave the full between-row spacing too, so you can walk, water, and weed without stepping on the roots.
Can I use this seed calculator for starting seeds indoors?
Yes. Enter the number of cells or pots you plan to fill as your planting spots, or work out the spots from the tray size and spacing, and the calculator will give you the seeds and packets needed. Sow one or two seeds per cell, keep the soil damp and warm, and thin to the strongest seedling in each cell once the first true leaves appear.