Article — Vegetable Seed Calculator
Vegetable seed calculator: how many seeds to plant per row
A vegetable seed calculator turns row length or bed area into a seed count by working through three numbers: target plant population, seed germination rate, and a safety margin for losses. The base formula is seeds = (target plants ÷ germination rate) × (1 + margin). For 10 m of carrots at 7 cm spacing, that means 143 target plants. At a typical 70 percent carrot germination and 30 percent margin, sow about 266 seeds. The calculator above handles the math and includes spacing presets for 12 common vegetables — carrot, beet, cabbage, tomato, zucchini, lettuce, peas, beans, radish, onion, spinach, and pepper.
Seed math matters because seed is cheap relative to the cost of replanting a bed that fails to germinate. Over-seeding slightly is always better than under-seeding. Thinning takes minutes; replanting wastes a whole growing season.
How to calculate vegetable seeds
The math has three steps. First, calculate the target plant count from row length or bed area divided by spacing. Second, divide the target by the germination rate (as a decimal) to account for seeds that fail to sprout. Third, multiply by (1 + safety margin) to cover losses from weather, pests, and over-thinning.
Target plants (row) row length / spacingTarget plants (bed) area / (spacing × row spacing)Seeds to sow target / germ × (1 + margin)Direct-seed margin 30%Transplant tray margin 15%Sowing depth 2 to 3 × seed diameterCarrot per meter ~26 seeds (7 cm spacing)Lettuce per meter ~6 seeds (25 cm spacing)The same formula scales to area-based planting. For a 1 m² bed of carrots at 7 cm in-row and 30 cm between rows: 10,000 ÷ (7 × 30) = 48 plants/m². Multiply by your bed area and run the same germination + margin math.
Germination rate explained
Germination rate is the percent of seeds in a lot that sprout under standard test conditions (moist paper, 20 to 25°C, light per crop preference, 7 to 14 days). Seed companies test every lot and print the result on the packet. Most countries require seed labeling to show germination rate and the test date.
First-year fresh seed runs 70 to 95 percent. Beans and lettuce germinate near the top; carrots, onions, and parsnips are notoriously poor (60 to 80 percent). Germination drops with seed age: subtract about 10 percentage points per year for most vegetables. Bean and pea seed holds up well (3 to 4 years viable). Onion, parsnip, and corn seed loses viability after 1 to 2 years.
The longest-recorded viable vegetable seed is a 2,000-year-old date palm seed recovered from the Masada fortress in Israel. It germinated successfully in 2005, producing a tree now nicknamed "Methuseleh." For practical garden purposes, seed storage longevity ranges from one year (onion, parsnip) to ten or more years (tomato, melon) when kept cool and dry. The University of California seed-saving guidelines and the Svalbard Global Seed Vault both target storage conditions of −18°C and 15 percent humidity for maximum longevity.
Vegetable seed spacing by crop
Spacing varies hugely by crop. Small leafy greens and root crops (carrot, radish, lettuce thinnings) need only 5 to 25 cm between plants. Brassicas (cabbage, broccoli, cauliflower) need 30 to 50 cm because they grow large heads. Solanaceae (tomato, pepper, eggplant) need 45 to 90 cm because they are large branching plants. Cucurbits (squash, zucchini, melon) need 75 to 120 cm because they vine widely.
Row spacing follows similar logic but also depends on equipment. Hand-cultivated beds run rows 25 to 45 cm apart. Tractor-cultivated rows run 60 to 100 cm to accommodate cultivator widths. Drip-irrigated raised beds often crowd rows to 30 cm because no equipment access is needed between rows once the bed is planted.
Planting depth for vegetable seeds
The classic rule is plant seeds at 2 to 3 times their diameter. Small seeds (lettuce, carrot, celery) at 0.5 to 1.5 cm. Medium seeds (beet, radish, spinach, basil) at 1.5 to 2.5 cm. Large seeds (peas, beans, corn, squash, melon) at 2.5 to 4 cm. Very deep planting wastes the seed's stored energy reaching the surface; very shallow planting risks drying before germination.
Some seeds need light to germinate and must be barely covered or surface-sown. Lettuce, celery, dill, and many herbs fall in this group. Press the seed lightly into the soil surface and cover with a thin layer of vermiculite or fine compost. The opposite is true for beans and cucurbits, which germinate in darkness and benefit from full burial at the recommended depth.
Direct seed vs transplant
Direct seeding sows seed directly into the final bed. Best for root crops (carrots, beets, parsnips, radishes) and large-seeded crops (beans, peas, corn, squash) that resent transplanting. Direct seeding is simple but uses more seed (30 percent margin) and exposes seedlings to weather.
Transplanting raises seedlings indoors in cells, then moves the strongest into the bed. Best for brassicas, nightshades (tomato, pepper, eggplant), and lettuce. Transplanting uses far less seed because cell-tray conditions are nearly ideal and you select the best seedlings. The trade-off is more labor and more equipment (cell trays, seed-starting mix, indoor lighting or a greenhouse).
Soil that is too cold halts germination and exposes seeds to fungal pathogens (Pythium, Rhizoctonia, damping-off). Cool-season crops (peas, spinach, lettuce, carrots) germinate at 5 to 15°C. Warm-season crops (beans, corn, melons, squash) need 18 to 25°C minimum. Planting beans into 12°C soil routinely produces 30 to 50 percent stand loss, not because the seed is bad but because slow germination invites soil pathogens. Use a soil thermometer at 5 cm depth and wait for sustained warmth before sowing warm-season crops.
Testing old vegetable seed
Test germination of any vegetable seed older than two years before sowing a large bed. The method: count out 10 seeds, place them on a damp paper towel, fold the towel and seal in a plastic bag, leave at room temperature for the germination days listed on the original packet (usually 7 to 14). Count how many sprouted. That number times 10 is your germination percent.
Above 80 percent: use as fresh seed. 60 to 80 percent: usable, increase safety margin to 40 percent. 30 to 60 percent: usable only for thick direct sowing with heavy thinning, or as a last resort. Below 30 percent: discard. The cost of seed is trivial compared to the cost of a failed planting that loses a season.
Vegetable seed storage
Vegetable seed stores best cold and dry. Standard recommendation: temperature below 10°C, relative humidity below 50 percent, away from light. Most home gardeners use a sealed jar in a refrigerator with a silica gel desiccant packet. Properly stored, lettuce and beet seed remain viable 4 to 5 years, tomato and pepper seed 5 to 10 years, beans and peas 3 to 4 years.
The Harrington rule says seed longevity halves for every 1 percent increase in seed moisture content or every 5°C increase in storage temperature. So 5°C cooler doubles shelf life; another 5°C doubles it again. Seed banks like Svalbard run at −18°C for this reason — at that temperature most vegetable seed remains viable for decades.
Thinning after emergence
Direct-seeded crops nearly always need thinning. Sow at the seeds-to-plant ratio the calculator returns, then thin to final spacing after the 2-leaf stage. Thin by snipping unwanted seedlings at soil level with scissors — pulling damages the roots of seedlings you want to keep. Carrots especially benefit from two thinnings: first to 2 to 3 cm spacing, then to final 5 to 8 cm after another 2 to 3 weeks.
Some crops self-thin in practice. Lettuce planted at full density can be harvested progressively, taking outer leaves or whole heads as they reach edible size. Radishes and salad greens often double as their own thinning crop.
- Direct-seed margin = 30% extra over target plants
- Transplant tray margin = 15% (controlled conditions)
- Sowing depth = 2 to 3 × seed diameter
- Fresh seed germination = 70 to 95% commercial
- Age penalty = ~10 percentage points per year
- Light-required seeds = lettuce, celery, dill
- Soil temp (cool-season) = 5 to 15°C
- Soil temp (warm-season) = 18 to 25°C minimum