Seed Rate Calculator
Work out seed rate in kg/ha from thousand grain weight (TGW), label germination, target plants/m², and optional field establishment. Add field area and bag size for total seed and bag estimates.
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Seed rate kg/ha = (seeds/m² × TGW) ÷ 100
Germination and establishment are applied first to work out adjusted seeds/m², then the seed rate in kg/ha follows from TGW.
Use the figure printed on your seed bag label where available.
Use the tested germination percentage from the seed label.
Typical guidance varies by crop, season, and locality. Use local agronomy advice rather than fixed recommendations.
Use 100% for a simple calculation, or reduce this where seedbed conditions are poor.
Enter a valid TGW, germination, target plants/m², and optional establishment to see adjusted seeds/m² and seed rate. Optional area and bag size add totals; invalid optional values are omitted from those lines.
Seed rate quick reference chart
Values are in kg/ha and do not include germination or field establishment adjustment.
| Seeds/m² | 35g TGW | 40g TGW | 45g TGW | 50g TGW | 55g TGW | 60g TGW |
|---|---|---|---|---|---|---|
| 150 | 52.5 | 60.0 | 67.5 | 75.0 | 82.5 | 90.0 |
| 175 | 61.3 | 70.0 | 78.8 | 87.5 | 96.3 | 105.0 |
| 200 | 70.0 | 80.0 | 90.0 | 100.0 | 110.0 | 120.0 |
| 225 | 78.8 | 90.0 | 101.3 | 112.5 | 123.8 | 135.0 |
| 250 | 87.5 | 100.0 | 112.5 | 125.0 | 137.5 | 150.0 |
| 275 | 96.3 | 110.0 | 123.8 | 137.5 | 151.3 | 165.0 |
| 300 | 105.0 | 120.0 | 135.0 | 150.0 | 165.0 | 180.0 |
| 325 | 113.8 | 130.0 | 146.3 | 162.5 | 178.8 | 195.0 |
| 350 | 122.5 | 140.0 | 157.5 | 175.0 | 192.5 | 210.0 |
| 375 | 131.3 | 150.0 | 168.8 | 187.5 | 206.3 | 225.0 |
| 400 | 140.0 | 160.0 | 180.0 | 200.0 | 220.0 | 240.0 |
| 425 | 148.8 | 170.0 | 191.3 | 212.5 | 233.8 | 255.0 |
| 450 | 157.5 | 180.0 | 202.5 | 225.0 | 247.5 | 270.0 |
How to calculate seed rate
Seed rate in kg/ha links how many plants you want to establish per square metre with the size of each seed. Two adjustments matter in practice: laboratory germination on the label (how many seeds are viable) and expected field establishment (how many of those viable seeds actually produce a plant in your seedbed). Together they decide how many seeds you must sow per m² before you convert to kg/ha using thousand grain weight (TGW).
Seed rate formula
Working in two steps keeps the biology separate from the mass unit:
- Seeds/m² (adjusted) = target plants/m² ÷ (germination × establishment), with germination and establishment expressed as decimals (for example 92% → 0.92).
- Seed rate kg/ha = (seeds/m² × TGW) ÷ 100
The calculator above follows the same order: adjust for germination and establishment to get seeds/m², then multiply by TGW and divide by 100 for kg/ha.
What is thousand grain weight?
Thousand grain weight (TGW) is the mass in grams of one thousand seeds from a seed lot. It varies with crop species, variety, growing season, and cleaning/grading, so two bags labelled for the same variety can differ. A higher TGW means heavier seeds, so you need more kilograms per hectare to achieve the same number of seeds per square metre.
Germination and establishment adjustment
Germination on the seed certificate describes how seeds perform under laboratory conditions. Establishment in the field is usually lower and depends on drilling date, seedbed quality, pests, and weather. Reducing the establishment percentage in the calculator increases the number of seeds you sow to reach the same target plant population.
Typical TGW ranges
Ranges below are indicative only. Always use the TGW printed for your seed lot.
Winter wheat
Typical TGW: 35–55 g
Winter barley
Typical TGW: 38–52 g
Spring barley
Typical TGW: 38–52 g
Oats
Typical TGW: 28–42 g
Oilseed rape
Typical TGW: 3–6 g
Your seed label
Bag labels override any typical range.
Seed rate guidance by crop
The notes below are practical context only, not variety-specific targets.
Winter wheat seed rate
Target ears/m² and drilling date usually drive plant counts more than a single magic number. Early drilling in good conditions often sits at the lower end of typical guidance; later slots or difficult seedbeds may need higher plant numbers or a lower establishment assumption. Variety, rotation, and local advice should guide the target.
Barley seed rate
Winter and spring barley both need enough plants to till and achieve ear numbers without overcrowding weak plants. TGW moves seed rate in kg/ha sharply, so re-run the calculator whenever you change seed lot.
Oats seed rate
Oats vary in grain size and panicle structure; TGW from the bag is especially important. Check lodging risk and seed spacing with your agronomist where pushing populations.
Oilseed rape seed rate
OSR TGW is much smaller than cereals; a few grams shift in TGW changes plants/m² quickly. Establishment assumptions often dominate in marginal autumns. Adjust establishment rather than chasing extreme targets without local reasoning.
Frequently asked questions
What is the formula for seed rate in kg/ha?
Adjusted seeds/m² equals target plants/m² divided by (germination × establishment) as decimals. Seed rate kg/ha is (adjusted seeds/m² × TGW in grams) ÷ 100.
How do I calculate seed rate from TGW?
First work out how many seeds you need per square metre after germination and establishment, then multiply that by TGW in grams and divide by 100 to convert to kilograms per hectare.
What does TGW mean?
TGW is thousand grain weight: the weight in grams of one thousand seeds. It varies by crop, variety, and seed lot, which is why labels should be read each season.
Is seed rate the same as plant population?
Not exactly. Seed rate is kg/ha of product sown. Plant population is established plants/m² or ears/m² after germination and field losses. The calculator links a target plant number to seed rate using those adjustments.
How does germination percentage change seed rate?
Lower germination means fewer viable seeds per kilogram, so you sow more seeds per square metre and the seed rate in kg/ha rises if everything else stays the same.
Should I increase seed rate for late drilling?
Often yes for cereals in terms of plant targets or establishment assumptions, but it is not a universal rule: variety, soil, and regional practice matter. Use local agronomy advice alongside the calculator.
Can I use this calculator for wheat and barley?
Yes. Select winter or spring wheat or barley in the crop list, enter label TGW and germination, then set your target plants/m² and establishment to match your situation.
Why is my seed bag TGW different from typical values?
Each seed lot is tested and graded separately. Growing conditions, cleaning, and variety choice all shift TGW. The bag label overrides any general range on this page.
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