Two questions, two modes
Not every seed makes a plant, so you always sow more seeds than the plants you want. Garden mode counts seeds for plants, cells, pots and holes. Field or food plot mode works in pounds or kilograms per acre or hectare.
Garden mode: seeds to sow for the plants you want
Sowing one seed per cell, you need plants ÷ germination seeds. Sowing two or three per cell and thinning to the strongest fills more cells, because a cell only stays empty when every seed in it fails.
seeds = cells × seeds per cell, where cells = plants × (1 + extra) ÷ (1 − (1 − germination)seeds per cell)
Worked example. You want 12 tomato plants, sow 2 seeds per cell and add 10% spares. At 75% germination a cell with two seeds comes up 94% of the time (1 − 0.25 × 0.25). 12 × 1.1 ÷ 0.9375 = 14.1, so you sow 15 cells and 30 seeds. The table next to the result shows the trade-off for the same 12 plants: 18 seeds sown singly, 30 at two per cell, 42 at three and 56 at four.
Where the defaults come from:
- Germination. The starting value is the federal minimum for that crop under the Federal Seed Act (7 CFR 201.31): 75% for tomato, squash, broccoli and sweet corn, 80% for lettuce, cucumber and peas, 55% for carrot and pepper, 50% for okra and chives. Packets below the minimum must say "Below Standard". If your packet states a tested germination, use that instead.
- Seeds per cell. University of Maryland Extension advises two or three seeds per container or cell, then removing the weaker seedlings, and two to four seeds per hill for cucumber, squash and melons.
- Leftover seed. Johnny's Selected Seeds lists how long stored seed stays usable: onion 1 to 2 years, tomato 3 to 7. Keep °F plus % relative humidity under 100.
Fill a bed or Fill a row counts plants on a square grid at the crop's spacing first. The plant spacing calculator draws that bed, and the seed starting calculator tells you when to sow.
Field or food plot: pounds per acre from a target stand
Row crops are planned by the plants standing at harvest. Penn State Extension gives germinable seeds per acre = seeds per pound × pounds per acre × germination; solved for pounds, with field emergence added:
lb/acre = target plants per acre ÷ (seeds per lb × germination × emergence)
Soybeans. Iowa State and the Illinois Agronomy Handbook both put full yield at about 100,000 plants per acre, and Illinois gives 2,800 seeds per pound as typical. With 90% germination and 85% emergence that is 130,719 seeds, or 46.7 lb per acre, which sits inside Iowa State's advice to sow 125,000 to 140,000 seeds. For 40 acres you need 1,867 lb, about 38 units of 140,000 seeds. K-State adds 15% when a grain drill replaces a row planter: 150,327 seeds and 53.7 lb per acre.
Corn. Purdue's R.L. Nielsen recommends final stands of at least 30,000 plants per acre and assumes 95% emergence. Seed corn is sold by count, in units of 80,000 kernels. With 90% tag germination on top, the calculator asks for 35,088 seeds per acre; 20 acres take 8.77 units, so you buy 9. If your tag shows 95% or more, enter it and the rate moves toward Nielsen's 33,000.
Wheat and canola. K-State uses 15,000 wheat seeds per pound and final stands of 600,000 to 720,000 plants per acre; at 15 plants per square foot that is 53.8 lb per acre. Illinois aims higher, at 1.5 million seeds per acre. The Canola Council of Canada works from thousand seed weight: 5 plants per square foot at 5 g and 50% survival is 4.8 lb per acre.
Published rates for cover crops, forage and wildflowers
Cover crops and clover are sown at a published rate, higher when broadcast. From SARE's Managing Cover Crops Profitably:
| Crop | Drilled, lb/acre | Broadcast, lb/acre |
|---|---|---|
| Cereal rye | 60 to 120 | 90 to 160 |
| Oats | 80 to 110 | 110 to 140 |
| Crimson clover | 15 to 20 | 22 to 30 |
| Red clover | 8 to 10 | 10 to 12 |
| White clover | 3 to 9 | 5 to 14 |
For grain, the Illinois Agronomy Handbook drills oats at 64 to 96 lb (half to one bushel more broadcast) and spring barley at 96 lb.
Alfalfa is 12 to 15 lb of pure live seed per acre in the same handbook. Pure live seed is purity × germination: at 90% and 80% that is 72%, so 13.5 lb means 18.8 lb from the bag.
We found no extension rate for wildflower mixes. American Meadows says 1 lb covers 1,000 to 2,000 square feet, so the calculator starts at 0.75 lb per 1,000 square feet (32.7 lb per acre); Prairie Moon Nursery designs native mixes for at least 100 seeds per square foot. For microgreens, Johnny's trial used about 10 g of kale seed per 10 by 20 inch tray.
Checking the drill
Penn State Extension's method: turn the drive wheel for a measured distance, catch and weigh the seed from all openers, and divide by the area. Working width is opener spacing times the number of openers.
lb/acre = seed caught (lb) × 43,560 ÷ (distance ft × width ft)
In their example, 2.3 lb over 200 feet with a 10 foot drill (0.046 acre) is about 50 lb per acre; the calculator shows 50.1. The dot picture shows how many seeds should land per square foot, which you can count behind the drill.