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How the Seed Amount Is Calculated

The underlying method is: required seed (kg/decare) = target plants per m² × thousand-seed weight (grams) ÷ 1000 ÷ germination rate (%) ÷ purity rate (%). Thousand-seed weight (TSW) is the mass of 1,000 seeds in grams, and it varies enormously — not just between species but between varieties of the same crop. A wheat variety might have a TSW of 35 g while another runs 45 g; sowing both at the same kg/decare rate would put very different numbers of actual seeds in the ground.

Worked example: targeting 450 plants/m² of a wheat variety with a 40 g TSW, 92% germination and 98% purity: 450 × 40 ÷ 1000 ÷ 0.92 ÷ 0.98 ≈ 19.9 kg/decare — right in the middle of the typical 18-22 kg/da range. If the same target is sown with a lower-germination lot (80%) instead, the rate rises to 450 × 40 ÷ 1000 ÷ 0.80 ÷ 0.98 ≈ 22.9 kg/decare — nearly 3 kg/decare more just to compensate for the weaker lot, without changing the target plant density at all.

What to Know

The single biggest source of error is using an average or guessed thousand-seed weight instead of the value printed on the seed bag's label. Because TSW enters the formula directly and swings widely across varieties, a 20% guess-vs-actual TSW error produces roughly a 20% seed-quantity error — enough to under- or over-sow a field noticeably. Always read the certified seed tag for TSW, germination % and purity % rather than relying on textbook averages, especially for legumes and oilseeds where variety-to-variety TSW differences can be large.

• Broadcast (not drilled) sowing needs a higher rate than drilled sowing for the same target stand — seed-soil contact is worse and losses to birds/wind are higher.
• Late sowing, coarse or dry seedbeds, and marginal soil conditions all push the rate toward the upper end of the typical range.
• Row crops sown by seed count (maize, sunflower) work differently from broadcast small grains — the calculator handles both approaches.