Seed Rate Calculator
Work out sowing rate from target plant population, seed weight and germination.
Please note: Planning figures based on published averages. Soil type, climate, variety, season and local practice all shift these results substantially. Base real decisions on a soil test, local extension guidance and the product label — particularly for fertiliser, lime and any crop protection product, where the label is a legal document.
What the Seed Rate Calculator does
Sowing rate works backwards from the plant population you want in the field. Not every seed germinates and not every seedling establishes, so the seeds sown must exceed the target by the product of those two losses — then thousand seed weight converts a seed count into kilograms.
Formula
Seeds/m² = Target plants ÷ (Germination × Establishment)Rate (kg/ha) = Seeds/m² × TSW ÷ 1,000 × 10Total seed = Rate × Area
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Target plant population | plants/m² | Yes | Accepts more than 0. |
| Crop | one of 9 options | Yes | — |
| Thousand seed weight | g | In some modes | Accepts more than 0. Shown Crop is Custom seed weight. |
| Germination | % | Yes | — |
| Field establishment | % | Yes | The share of viable seed that becomes a plant in the field. |
| Area | number | Yes | Accepts more than 0. |
| Area unit | one of 4 options | Yes | — |
| Seed price per tonne | selected currency | Optional | Accepts 0 or more. |
| Currency | one of 10 options | Optional | — |
How to use it
- Choose Crop and Area unit.
- Enter Target plant population, Germination, Field establishment and Area.
- Fill in the remaining inputs the form shows for your choice.
- Optionally add Seed price per tonne.
- Select Calculate.
Worked example
Targeting 250 wheat plants per square metre with 45 g TSW, 95% germination and 85% establishment.
- Target
- 250 plants/m²
- TSW
- 45 g
- Germination
- 95%
- Establishment
- 85%
Survival is 80.75%, so 309.6 seeds/m² must be sown. At 45 g per thousand that is 139.3 kg/ha.
Reading the result
- "Extra seed for losses" is the flip side of survival rate — because the relationship is a ratio, a small drop in establishment requires a disproportionately larger seed-rate increase than intuition suggests.
- The "rate at different TSW" table shows seed weight moves the seed rate roughly proportionally even though the target plant population has not changed — worth rechecking TSW every season rather than reusing an old rate.
Assumptions and limitations
- Germination and establishment are both single flat percentages applied uniformly across the whole field, though real establishment varies by zone with drainage, compaction and seed depth.
- The custom TSW field only takes effect when "custom" is selected as the crop — choosing a named crop always uses that crop's built-in TSW preset instead, so switching crop selection silently changes which weight figure drives the calculation.
Common mistakes
- Assuming a higher target plant population always means proportionally higher yield — sowing more thickly raises the seed rate calculated here, but does not translate linearly to yield once crowding brings its own competition effects this calculator does not model.
Frequently asked questions
What is thousand seed weight?
The mass in grams of 1,000 seeds. It varies by variety, season and grain fill, and it is what converts a target seed count into a weight you can drill.
Why is establishment lower than germination?
Germination is measured in a laboratory. In the field, seedbed quality, moisture, depth, pests and weather all take a further toll — typically 10–20%.