Getting the seeding rate right is one of the highest-leverage decisions in arable farming. Drill too few seeds and you leave yield on the table through a thin canopy that cannot intercept enough light or compensate for plant losses. Drill too many and you waste money on seed, create an over-dense canopy prone to lodging and disease, and in crops like oilseed rape you can push the plant into excessive branching that delays harvest. This calculator puts the correct agronomic formula in your hands so you can dial in the optimal rate for your specific seed lot, seedbed conditions and target population.
How it works
The core agronomic formula for seeding rate is:
Seeding rate (kg/m²) = (Target plants per m² × TSW) / (Effective establishment × 1 000 000)
where TSW is the thousand seed weight in grams (the weight of exactly 1,000 seeds of your specific lot), and effective establishment is the fraction of seeds drilled that actually emerge as established plants. Effective establishment is itself the product of three independent factors:
Effective establishment = Field establishment (fraction) × Germination (fraction) × Purity (fraction)
Each of these is expressed as a decimal (so 90% becomes 0.90). Field establishment captures everything from seedbed tilth, sowing depth and soil moisture to slug and pest pressure after drilling. Germination and purity come directly from the seed test certificate on the bag and account for the fact that not every seed in the bag is capable of germination and not every particle in the bag is actually crop seed.
Multiplying all three together gives the true fraction of seed weight that converts into emerged plants. Divide the target population by that fraction to get the seeds you need to place per square metre, then multiply by the weight per seed (TSW in kg) to arrive at kg/m². Scale up by field area for total seed requirement.
The calculator also works in reverse: if you know how much seed you have and want to know what population that delivers (or what TSW or establishment the formula implies), switch the Solve-for dropdown to back-calculate the unknown.
Worked example — winter wheat
A farm has a 14.5-hectare field to drill with winter wheat. The seed bag shows:
- TSW: 48 g
- Germination: 96%
- Purity: 99%
The agronomist targets 300 plants/m² on a good seedbed with an estimated field establishment of 88%.
Effective establishment = 0.88 × 0.96 × 0.99 = 0.836
Seeds needed per m² = 300 / 0.836 = 359 seeds/m²
Seeding rate = (300 × 48) / (0.836 × 1 000) = 14,400 / 836 = 17.22 kg/ha
Wait — let us double-check by converting: 17.22 kg/ha × 0.836/48 × 1,000 = 300 plants/m².
Total seed for 14.5 ha = 17.22 × 14.5 = 249.7 kg ≈ 250 kg
At 25 kg per bag that is 10 bags. Enter these figures in the calculator and you will see exactly the same answer plus a breakdown of seeds per linear metre for your drill setting.
| Crop | Target plants/m² | TSW (g) | Typical rate (kg/ha) |
|---|---|---|---|
| Winter wheat | 300 | 45 | ~165 |
| Winter barley | 280 | 42 | ~150 |
| OSR (winter) | 60 | 4 | ~3.5 |
| Maize (grain) | 80 | 270 | ~23 |
| Field beans | 35 | 500 | ~225 |
Rates shown assume 95% germination, 98% purity and typical field establishment. Your seed bag TSW may differ — always use the actual figure from the label.
Formula note
The formula is universally accepted in European agronomy (NIAB, AHDB, DSV) and North American extension services (Purdue, NDSU). Some references express TSW in kg/1,000 seeds rather than g/1,000 seeds; in that case the divisor is 1 (not 1,000). Others substitute seeds/kg directly: seeds/kg = 1,000,000 / TSW(g), giving seeding rate (seeds/m²) × (1/seeds per kg). All three forms are mathematically identical — this tool uses the g/1,000 seeds convention because that is what appears on seed test certificates.