Vegetable Seed Calculator

Seed packets are sold by weight and hope; gardens are planted by rows and spacing. This calculator converts between the two dialects: enter your row length, the in-row spacing for the crop, the number of rows, and the packet’s germination percentage, and it returns the seeds to sow (germination already allowed for), the plants that should result, and the weight of seed to buy if you tell it how many seeds run to a gram. It is the arithmetic that turns “one packet” into a decision instead of a guess.

The Two Divisions That Size a Seed Order

Plants from geometry: a row’s plant count is its length divided by the in-row spacing — 25 ft is 300 inches, and at 12-inch spacing that is 25 stations per row. Seeds from honesty: not every seed germinates, so the sowing count is plants ÷ germination rate — at the packet’s 85%, 25 ÷ 0.85 = 29.4, rounded up to 30 seeds, the calculator’s default answer. The two divisions generalise across the whole seed catalogue: multiply by rows for beds, swap in each crop’s spacing (2 inches for carrots, 18 for courgettes), and read the germination from the packet’s tested figure (a legal datum on commercial seed, not a hope). The weight estimate then converts count to commerce through seeds-per-gram, a number that spans orders of magnitude across crops — roughly 300 seeds per gram for tomato and 150 for peppers at the small end, against 2–4 per gram for beans and peas at the large end — so small-seeded crops are bought by the thousand and large-seeded ones by the handful, and the calculator’s gram field is where that difference stops being abstract.

Worked Examples: A Bed of Lettuce, A Row of Beans

Lettuce, intensively: four rows of 10 ft, 8-inch spacing, germination 90%. Plants per row = 120/8 = 15; bed total 60; sow = 60/0.90 = 67 seeds; at lettuce’s roughly 800 seeds per gram, the seed weighs under a tenth of a gram — which is why lettuce seed is pelleted for machinery and why a single packet is a multi-year supply for a home bed, with seed life, not seed count, the binding constraint. Runner beans: one 25 ft row at 12-inch spacing, germination 85%: 25 plants wanted, 30 seeds sown — the defaults — and at about 1 seed per gram for large runner beans, 30 g of seed: a meaningful fraction of a 100 g packet, and a very different purchasing problem from the lettuce. The contrast is the calculator’s sermon: spacing and germination set the count, but crop seed size sets the cost, and planning both on one form is how a seed order survives contact with a real catalogue.

Germination: The Percentage With a Date On It

The germination figure is a measurement with a shelf life. Commercial packets carry a tested percentage for the packing season; the same seed stored warm and damp declines measurably year on year, while seed kept cool, dry and dark (the refrigerator’s crisper in a sealed jar is the household optimum) holds most viability for years beyond the printed date — crop-dependent: onion and parsnip seed are famously short-lived (a year or two), cucurbits and brassicas patient (four to five), tomato in between. The gardener’s audit is the damp-paper-towel test: ten seeds, a week of warmth, count the sprouts, and enter that percentage in this calculator for old seed — a ten-seed experiment that converts the packet’s history into a current number. Sowing rate follows immediately: tired seed is not discarded, it is sown thicker, by exactly the division this page performs, until the test says viability has fallen past economical rescue (a working floor of about 50% for most crops).

Succession, Thinning and the Seeds You Sow to Remove

Two practices deliberately overshoot the calculator’s plant count, and it is worth knowing they are features. Succession sowing repeats a small sowing every two to three weeks for crops eaten young (lettuce, radish, spinach, beans); the calculator prices one sowing, and the season’s order is that figure times the number of successions intended — the planning act is scheduling the repeats, not enlarging one heroic sowing that matures all at once and bolts in unison. Station sowing with thinning — beetroot, carrots, salads sown in place — puts two to three seeds per station against germination and slug risk and removes the extras by hand; the calculator’s germination division approximates the seed need, but the honest multiplier for difficult, small-seeded, direct-sown crops in open ground is experience’s: sow the calculator’s number, then add a quarter for the field’s opinion. Indoor-raised transplants invert the waste: one seed per cell at known germination, the calculator at its most precise — the tray is the one place in gardening where arithmetic rules unchallenged.

Sweet Corn: The Block Planting Exception

One crop rewrites the row logic entirely. Sweet corn is wind-pollinated and fails in single rows — pollen falls where the wind puts it, and a lone row sheds most of its fertility on the path. The remedy is the block: plant at least four short rows together. Price a modest 10 × 10 ft block at 12-inch spacing both ways: 10 rows of 10 plants = 100 plants wanted; at 90% germination, sow = 100/0.90 = 112 seeds; at corn’s roughly 5 seeds per gram, about 22 g of seed. The calculator reaches the block by treating each row in turn (rows = 10, length = 10) — a reminder that its three fields describe any layout the gardener can decompose into rows, including the one layout where rows are a pollination strategy rather than a convenience.

Pelleted and Taped Seed: Precision You Buy by the Seed

Two products exist to defeat small seed’s uncountability. Pelleted seed coats each lettuce or petunia seed in clay until it is handleable and singable by machine or by hand — sold by exact count, priced per thousand, and entered in this calculator with germination near the packet’s laboratory figure because pellets are sold on viability. Seed tape pre-spaces seed in a biodegradable strip — the calculator’s geometry performed at the factory, priced by the row-foot. Both convert the weight dialect back into the count dialect this page speaks; both are worth their premium exactly where the calculator’s numbers are smallest (a tenth of a gram of lettuce seed is unweighable on a kitchen scale) and pointless where seed is large, cheap and easily fingerspaced.

Related Tools

More from the Biology Calculators hub: the Bulb Spacing Calculator for the same density logic in autumn, the Water Soluble Fertilizer Calculator for feeding the rows these seeds become, and the live Acres Per Hour Calculator if the rows ever outgrow the garden.

Frequently Asked Questions

How do I find the right in-row spacing?

From the packet or a spacing chart for the crop — but read it as the final spacing the mature plant needs (its canopy and root run), not the sowing spacing. Typical figures: carrots 2 in, lettuce 8–12, beans 6–12 by type, cabbage 18, courgette 24–36. The calculator converts the figure you choose into counts; crowding below the recommendation trades plant size for plant number at an exchange rate experienced growers respect.

My packet says “sow 2–3 seeds per station”. How does that fit?

Station sowing is the germination division done by folklore: instead of computing plants ÷ rate, you sow a small cluster per final position and thin to the strongest. The seed cost is the cluster size times stations (for 25 stations at 3 per station, 75 seeds), which the calculator reaches by entering germination as roughly 33–50% — the trick is acknowledged as an approximation, and for cheap seed in risky ground it is often the rational one.

How many seeds are in a packet?

By count for large seed (50–200 beans or peas is common), by weight for small (“0.5 g” of lettuce is ~400 seeds; tomato packets often state ~30–300 by count). The seeds-per-gram field converts: lettuce ≈ 800/g, tomato ≈ 300/g, pepper ≈ 150/g, cabbage ≈ 250–300/g, carrot ≈ 800/g, beans ≈ 2–4/g, sweet corn ≈ 4–6/g — orientation figures; your packet’s own count, divided by its weight, is the exact one for your order.

Does old seed need different spacing?

No — spacing is the plant’s requirement and does not change; the sowing rate changes, through the germination field. Test the old lot (ten seeds on damp paper), enter the measured rate, and the calculator thickens the sowing by exactly the viability lost. Below about 50% germination most growers replace the lot, because patchy emergence costs more in bed-weeks than the seed saves.

Can I use this for lawns, cover crops or wildflower mixes?

Those are sold and sown by weight per area (grams per square metre), a different convention — the same family as this page’s weight field, but entered directly by rate rather than by station geometry. If you know a mix’s recommended rate, multiply by the area; station arithmetic like this calculator’s serves row and bed crops, where individual plants have addresses.

What about transplant trays?

Enter the tray as the “row”: a 40-cell tray is 40 plants wanted; rows = number of trays; germination at tray conditions is usually at the top of the packet’s range (warmth and moisture are controlled), and one seed per cell makes the count exact. Add a small percentage for a spare tray against the slugs’ review of your hardening-off arrangements.