Seed Rate Calculator
Work out how much seed a bed needs, in seeds, grams and packets, at your spacing.
Seed looks cheap until a rare variety runs out mid-row, and looks expensive when a whole packet goes into one short row. This calculator counts properly: plants that fit the area at the crop’s spacing, divided by your expected germination, plus a margin for the realities of soil, slugs and second thoughts, converted into grams using the crop’s seeds per gram, and into packets.
For a germination figure worth using, test your stored seed first with the Seed Germination Calculator method: 20 seeds on damp paper tells you the real percentage for that packet.
How this is worked out
Plants wanted = area divided by the square of the crop’s spacing. Seeds to sow = plants / germination rate x (1 + margin). Grams = seeds / seeds per gram for the crop, which is where the surprises live: a gram of celery holds about 2,000 seeds and a gram of beans about 3, so equal weights are meaningless and equal money is worse. Packet counts round up, because half a packet is a shopping trip.
Assumptions:
- Spacing is the square grid standard. Row sowing with wider paths uses less seed; broadcast beds of roots and salads use similar totals spread differently.
- Seeds per gram are mid-range published figures per crop; varieties vary by a quarter either way, which the margin absorbs.
- Germination is the tested figure for your seed lot, not the packet’s print from the year it was packed.
Worked examples
1. Carrots in a 4 x 8 ft bed. At 3 in spacing, about 512 plants wanted; at 85% germination plus 25% margin, roughly 750 seeds, under a gram at 800 seeds per gram. One small packet covers the bed twice over, which surprises everyone the first time.
2. Sweet corn in 200 sq ft. At 12 in spacing, 200 plants; at 85% plus margin, about 295 seeds at 4 per gram is roughly 74 grams: a real purchase, not a packet, which is why corn seed is sold by weight.
3. Tomatoes for 10 plants. The same maths gives about 15 seeds and a twentieth of a gram, which is why tomato packets hold 20 to 30 seeds and are priced like jewellery.
Seeds per gram: why packets differ
| Crop | Seeds per gram (approx) | What it means |
|---|---|---|
| Celery | about 2,000 | A pinch is a crop |
| Lettuce, basil | 700 to 800 | One packet, many seasons |
| Carrot, onion | 300 to 800 | Packets feel light because they are mostly future plants |
| Tomato, cabbage | about 300 | Count seeds, not grams, when sharing |
| Cucumber, squash | 8 to 35 | Packets hold a row or two |
| Peas, beans, corn | 3 to 5 | Buy by weight for any real area |
Seed swaps and saved seed make sense in seeds, not grams: ten tomato seeds is a fair trade unit, ten bean seeds is a mouthful.
Common mistakes
- Buying by packet count, not by content. Two packets of the same crop can hold 15 seeds and 300. Read the gram weight and the seeds per gram before judging value.
- Sowing the whole packet ‘to be safe’. Overdense rows cost thinning labour and invite disease. The margin in the calculator is the safety; the rest of the packet is next year’s seed, kept cool and dry.
- Trusting old seed at face value. Parsnip seed is nearly dead in year two; onion and leek fade fast too. The germination input exists for tested reality, not optimism.
- Ignoring thinning in the margin. Crops sown to a final stand (beans, corn) need little margin; crops thinned hard (carrots, beets, where each ‘seed’ is a cluster) need more.
Putting the result to work
Order seed in winter with these figures per bed and the seed bill shrinks to what the garden can plant. Label packets with the year and the tested germination, and store them cool, dry and dark. For when those seeds should go in, the Seed Starting and Planting Date Calculator has the dates, and the Succession Planting Planner multiplies the quantity by the season’s repeats.
How much seed do I need for this bed?
- Pick the crop; its spacing and seeds per gram are built in.
- Enter the area to sow.
- Set germination from a test of your actual seed, and a margin for losses.
- Read seeds, grams and packets to buy.
- Keep leftover seed cool and dry, labelled with the year and germination.
How many seeds are in a gram?
It spans three orders of magnitude: about 2,000 for celery, 700 to 800 for lettuce and basil, about 300 for tomato and cabbage, 8 to 35 for cucumbers and squash, and only 3 to 5 for peas, beans and corn. This is why seed is priced and traded by count for large seed and by weight for small.
How much extra seed should I allow?
A 25 percent margin over the germination-adjusted count covers thinning, patchy spots and a small resow for most crops in most beds. Crops thinned heavily (carrots, beets) justify 40 to 50 percent; precision sown crops in modules need barely 10.
Does old seed really germinate less?
Yes, at very different rates by crop. Parsnip is unreliable after one year; onions and leeks fade in two; beans and tomatoes hold four or more in good storage. A 20 seed damp paper test converts guesswork into the percentage this calculator wants.
Why does the calculator refuse potatoes and sweet potatoes?
Because they are not grown from botanical seed in gardens: potatoes are planted as tubers, sweet potatoes as slips, strawberries often as crowns. Their ‘seed rate’ is planting stock by count or weight, which follows spacing alone.
Is it cheaper to buy seed by weight?
For large seeded crops (peas, beans, corn) and for big areas, yes, substantially: packet pricing per seed can be ten times the bulk rate. For small seeded crops a single packet often holds several seasons of seed, and freshness beats bulk.
Seeds per gram are mid-range published figures per crop; spacing is the standard square grid. Germination adjustment and margins follow normal seed trade practice.
