The seed quantity calculator above works out how much seed a planting actually needs, which is always more than the number of plants you want. Between the seed you sow and the plant you harvest sit two losses: the germination rate printed on the packet, and everything that happens in a real seedbed that a laboratory germination test does not simulate. This tool applies both, then converts the answer into whole packets and an approximate weight so you can order.
Arb Digital publishes it as a planning tool. It handles the arithmetic of counts, rates and packets. It gives no planting advice: the spacings, the sowing depth, the timing and the crop choice are all inputs you bring from a published planting chart for your own climate and soil.
What This Seed Quantity Calculator Does
Working from a bed size and a spacing, it first computes how many plant positions that bed contains. Working from a plant count you already have, it skips that step. Either way it then divides by the effective establishment rate to get the number of seeds that need to be sown, multiplies by any thinning allowance, and converts the result into packets and grams.
The University of Nebraska–Lincoln Extension publication Understanding the Seed Packet makes the underlying point plainly: knowing how many seeds are in a package helps greatly when ordering seed and planning a garden, and the germination rate for each cultivar should be considered when working out how many seeds to plant for the final stand you want. Those two sentences are the whole logic of this page.
Boundary in one sentence: this page counts seeds and packets for a plant population you specify. Our plant population calculator gives the final plant density itself, and our grass seed calculator works in pounds per thousand square feet for broadcast turf rather than in counted seeds for discrete plants.
How to Use It
- Choose your starting point. A bed size plus a spacing, or a plant count you have already worked out.
- Enter the spacings from a published planting chart for your crop. The University of Maine Cooperative Extension planting chart for the home vegetable garden, for example, lists snap beans at two to four inches apart in rows twenty-four inches apart, and carrots at two to three inches apart in rows twelve to eighteen inches apart.
- Enter the packet germination percentage, which seed suppliers print on the packet or in the catalogue.
- Set your own field emergence allowance, which is the extra loss you expect between a laboratory germination test and your actual seedbed.
- Set the thinning allowance if you sow more than one seed per station, and read the packet count and weight.
The Formula and How It Is Calculated
Plant positions in a bed come from the area divided by the ground each plant occupies:
plants = bed area ÷ (row spacing × in-row spacing), with all three converted to the same unit.
The effective establishment rate multiplies the two loss factors:
establishment rate = packet germination × field emergence
And the seed requirement divides the plants by that rate, then applies the thinning allowance:
seeds = (plants ÷ establishment rate) × seeds sown per station
Worked example, which is the default loaded above. A bed 20 feet by 4 feet is 80 square feet. With rows 18 inches apart and plants 3 inches apart in the row, each plant occupies 1.5 × 0.25 = 0.375 square feet, so the bed holds 213 plants. Germination of 85 per cent multiplied by field emergence of 90 per cent gives an effective rate of 76.5 per cent, so 213 ÷ 0.765 = 279 seeds. At 200 seeds a packet that is two packets, and at 600 seeds per gram it is about 0.47 grams of seed.
Note the rounding rule: packets always round up, because you cannot buy four fifths of a packet, and the seed count rounds up too because a fractional seed does not exist.
Why Packet Germination Is Not Field Emergence
The germination percentage on a packet comes from a standardised laboratory test: seeds on moist paper at a controlled temperature, counted after a set number of days. It measures whether the seed is alive and capable of germinating. It does not measure whether it will do so in your garden.
A real seedbed adds losses the test never sees. Seed sown too deep may germinate and then exhaust itself before reaching the surface. Soil that crusts after rain can physically block emergence. Cold soil slows germination enough for seed to rot; hot soil can induce dormancy in some crops. Birds, slugs and soil insects take a share. Uneven moisture leaves patches that never come up at all.
That is why this tool asks for two figures rather than one. The packet number is a fact about the seed; the field emergence figure is your own judgement about your conditions, and it should be lower for a rough outdoor seedbed than for modules under cover. Keeping the two separate makes it obvious which part of the calculation is data and which part is assumption.
Seed Age, Storage and the Number That Quietly Drifts
Germination percentage falls with age, and the rate of decline varies enormously between species. Some seed remains highly viable for years in cool, dry storage; other species drop off sharply after a single season. The percentage printed on a packet applies to the season that packet was tested for, not to the packet that has been in a shed since.
If you are sowing older seed, the honest approach is a home germination test: count out ten or twenty seeds, roll them in damp kitchen paper, keep them warm, and count how many sprout after the expected number of days. Enter that figure instead of the printed one. It takes a week and it turns a guess into a measurement.
Storage conditions matter more than most people assume. Warmth and humidity are the two enemies, and the two combine, which is why a paper packet in a cool dry cupboard outlasts the same packet in a greenhouse by a wide margin.
Seeds by Count, Seeds by Weight, and Why Suppliers Use Both
Small-packet garden seed is usually sold by count, or by a weight so small that the count is what actually matters. Field and farm seed is sold by weight, because counting hundreds of thousands of seeds is impractical. The bridge between the two is the seeds-per-gram figure, and it varies across an enormous range: some brassica and lettuce seed runs into the high hundreds per gram, while beans and squash may be only a handful.
That is why this tool asks you to supply the figure rather than assuming one. If your supplier does not print it, weigh a counted sample — a hundred seeds on a kitchen scale accurate to a hundredth of a gram is enough to get within a few per cent, and the number is stable for that crop and variety from year to year.
One notational trap worth knowing: seed catalogues often abbreviate a thousand seeds as "1M", using the Roman numeral rather than the metric prefix. A packet listed as 5M contains five thousand seeds, not five million and not five milligrams.
Thinning, Overplanting and Where the Extra Seed Goes
Sowing more than one seed per station and thinning to the strongest is a long-established practice for crops where a gap in the row is worse than the work of thinning. The thinning allowance in this tool multiplies the seed requirement directly: sowing three seeds per station triples the seed you buy, which is worth knowing before you order.
There is a second, quieter form of overplanting: succession sowing, where you deliberately sow a fraction of the bed every fortnight so the harvest arrives in stages rather than all at once. That does not change the total seed for the season, but it does change how you buy it, since a large packet used over several sowings ages between them.
If you are planning the harvest end rather than the sowing end, our garden harvest yield calculator works from plant counts to expected produce, and the compost calculator and soil volume calculator cover what goes into the bed before the seed does.
Arb Digital publishes hundreds of free, no-signup tools covering gardening, biology, science, finance and everyday maths — no accounts, no stored data.
Browse All Free Tools Contact Arb DigitalCommon Mistakes to Avoid
- Buying seed equal to the number of plants you want. Germination and field losses mean you always need more, often twenty-five to fifty per cent more.
- Adding the two loss percentages instead of multiplying them. Eighty-five and ninety per cent give 76.5 per cent, not 75 and not 95.
- Using last year's packet germination figure for seed that has since spent a summer in a warm shed.
- Forgetting the thinning allowance, which multiplies the seed requirement by however many seeds go into each station.
- Mixing units — a bed measured in metres with spacings entered in inches. Set both fields consciously.
Related Free Tools From Arb Digital
For the plant density itself see the plant population calculator, and for broadcast turf seeding by weight the grass seed calculator. Discrete bulbs in a bed are handled by the bulb planting calculator, and yield estimation by the garden harvest yield calculator and the crop yield calculator. For soil and feeding volumes see the soil volume calculator and the fertilizer calculator. Browse the full free online tools hub for more.
Frequently Asked Questions
Work out the plant positions the bed holds at your chosen spacing, then divide by the combined germination and field emergence rate. A bed holding 213 plants at a 76.5 per cent establishment rate needs about 279 seeds.
Because not every seed becomes a plant. The packet germination percentage accounts for seed viability, and a further allowance covers what a real seedbed loses to depth, crusting, weather, pests and uneven moisture.
It is the result of a standardised laboratory test: seeds germinated on moist paper at a controlled temperature and counted after a set period. It shows the seed is viable; it does not predict emergence in a garden seedbed.
They multiply. Eighty-five per cent germination combined with ninety per cent field emergence gives an effective establishment rate of 76.5 per cent, not seventy-five.
Count out ten or twenty seeds, roll them in damp kitchen paper, keep them warm, and count how many sprout after the days the packet specifies. Use that percentage instead of the printed one.
Divide the number of seeds by the seeds per gram figure for that crop. Seed size varies hugely between species, so the figure must come from the supplier or from weighing a counted sample yourself.
No. Turf seed is broadcast and sold by weight per unit area, so a grass seed calculator works in pounds per thousand square feet. This page counts individual seeds for discretely spaced plants and converts to packets.
This calculator performs general planning arithmetic for educational use. It does not recommend spacings, sowing depths, timings or crop choices, all of which depend on your climate, soil and season. Take those figures from your local extension service or from the seed supplier's own guidance.
