Buying seed by the bag and sowing it by feel works until it doesn’t. Sow too thin and the weeds win. Sow too thick and you pay for seed that crowds itself out.
The seed rate is the weight of seed you put down per hectare, and it’s easy to work out once you know four numbers. This seed rate calculator does the sums for you.
How to use the Seed Rate Calculator#
- Set your target population. Enter how many plants per square metre you want standing in the field.
- Enter thousand seed weight. This is on the seed label or lab certificate. It’s the weight of 1,000 seeds in grams.
- Enter germination and purity. Both are percentages from the same label.
- Estimate field establishment. Decide what share of viable seed will really become a plant in your conditions. 80 percent is a sensible starting guess.
- Enter the field area. The calculator gives the rate per hectare, the rate per acre and the total seed for your field.
How to calculate seed rate#
The method has two steps. First you work out how many seeds to sow, then you turn that count into a weight.
Seed rate (kg/ha) = seeds per m² × thousand seed weight ÷ 100 ÷ purity
Take the default numbers. You want 60 plants per square metre. Germination is 90 percent, establishment is 80 percent, purity is 98 percent and the thousand seed weight is 40 g.
- Seeds to sow: 60 ÷ (0.90 × 0.80) = 83.3 per m²
- Weight of those seeds: 83.3 × 40 g ÷ 1,000 = 3.33 g per m², which is 33.3 kg/ha
- Correct for purity: 33.3 ÷ 0.98 = 34.0 kg/ha
In acres that’s about 13.8 kg per acre. A 5 hectare field needs 170 kg.
Why the same bag gives different results#
Two bags of the same crop can weigh very differently for the same number of seeds. A large-seeded variety might have a thousand seed weight of 50 g while a small-seeded one sits at 35 g. If you sow both at the same kilograms per hectare, the small-seeded one ends up with far more plants. So a rule like “100 kg per hectare” can only ever be a rough guide.
Always use the thousand seed weight from the label on the seed you actually bought, not a figure from a textbook or a neighbour.
Germination, purity and establishment#
These three get mixed up, so here’s what each one means.
- Germination is the share of seeds that sprout under ideal lab conditions.
- Purity is the share of the bag that’s the crop seed itself, not weed seed, broken seed or dirt.
- Establishment is the share of viable seed that becomes a living plant in your actual field. This is the one nobody prints on a label, because it depends on your soil, your moisture, your sowing depth and your luck with the weather.
Multiply germination by purity and you get the pure live seed percentage. In the example that’s 0.90 × 0.98 = 88.2 percent. Some seed sellers quote this number directly.
What happens when germination drops#
Old or badly stored seed germinates worse, and the seed rate has to climb to compensate. Holding everything else at the default values:
| Germination | Seeds per m² | Seed rate |
|---|---|---|
| 95% | 78.9 | 32.2 kg/ha |
| 90% | 83.3 | 34.0 kg/ha |
| 80% | 93.8 | 38.3 kg/ha |
| 70% | 107.1 | 43.7 kg/ha |
Going from 90 to 70 percent germination costs you almost 10 extra kilograms per hectare. Over a large field that adds up quickly, and it’s a good reason to test old seed before sowing it.
When to sow more, and when not to#
The calculator gives you a baseline. Real fields are messier. Many growers raise the target population for late sowing, a rough seedbed or dry conditions, and lower it when the soil is fertile and moisture is reliable. How much to adjust is a local question, so check with an agronomist or your extension office.
One thing is worth saying plainly: a higher seed rate isn’t automatically better. Overcrowded plants compete for light and water, can lodge, and often yield no more than a properly spaced crop. You pay for the extra seed either way.
Frequently asked questions#
Work out the rate per hectare first, then multiply by 0.4047. A rate of 34 kg/ha is about 13.8 kg per acre.
It’s the weight in grams of 1,000 seeds. It varies by crop, variety and even by seed lot, so read it from your own seed label or certificate.
No. Past a certain point extra plants compete with each other and the yield stops rising, while your seed bill keeps growing.
Germination is measured in a lab under ideal conditions. Establishment is how many of those viable seeds become real plants in your field.
Yes, as long as you know the target plants per square metre and the thousand seed weight. It works for cereals, pulses, vegetables and grasses.