Seed Rate Calculator

What Is Seed Rate and Why It Matters

Seed rate is the amount of seed — usually measured in kilograms per hectare or per acre — that a farmer sows to establish a healthy, well-spaced crop stand across a field. Sowing too little seed leaves gaps in the field that weeds quickly fill and reduces overall yield potential, while sowing too much wastes expensive seed, increases competition between plants for light, water and nutrients, and can raise the risk of lodging and disease in dense stands. Getting seed rate right is one of the simplest, highest-leverage decisions a farmer makes each season, and it depends on more than just the crop — it depends on the specific seed lot's size, quality, and how it will be sown.

This free seed rate calculator uses the standard agronomic formula to translate a target plant population — or your row and plant spacing — directly into a weight of seed, adjusted for the 1000-seed weight, germination percentage, and purity of your seed lot.

The Seed Rate Formula Explained

The core relationship connects four things: how many plants you want per hectare, how much each seed weighs, and how many of those seeds will actually become healthy plants.

Seed Rate (kg/ha) = [Plant Population/ha × 1000-Seed Weight (g)] ÷ [100 × Germination% × Purity%]

Working through this logic step by step: if you want, say, 2 million plants established per hectare, you first need to account for the fact that not every seed sown will germinate and grow — a seed lot with 85% germination means roughly 15 in every 100 seeds won't come up. Purity accounts for inert matter, weed seed, or other crop seed mixed into the lot, which further reduces the proportion of viable seed by weight. Dividing your target population by germination and purity gives the actual number of seeds you need to sow; multiplying that by the weight of an individual seed (derived from the 1000-seed weight) gives you the total weight of seed required.

Calculating Population from Row & Plant Spacing

For precision-planted row crops like maize, cotton, groundnut, and sunflower, it's often easier to think in terms of spacing rather than population directly. This calculator's second mode does that conversion for you:

Plant Population/ha = 10,000 ÷ (Row Spacing (m) × Plant Spacing (m))

One hectare equals 10,000 square metres, so dividing that by the area each individual plant occupies (row spacing multiplied by plant spacing) gives the total number of plants that will fit across the field at that spacing.

How to Use This Calculator

1. Choose a crop preset for typical reference values, or select "Custom" to enter your own figures.
2. Pick your calculation method: Plant Population (direct entry) for broadcast or drilled crops, or Row & Plant Spacing for precision-planted row crops.
3. Enter your seed lot's 1000-seed weight, germination percentage, and purity percentage — these are usually printed on the seed bag tag or seed test certificate.
4. Optionally enter your field area in hectares or acres to see the total quantity of seed you'll need to procure.
5. Tap "Calculate Seed Rate" to see your result in kg/ha, kg/acre, and total seed required.

Typical Seed Rate for Major Crops

Factors That Affect Seed Rate Beyond the Formula

• Sowing method: Broadcast sowing typically needs a higher seed rate than precision drilling or dibbling, since seed-to-soil contact is less consistent and losses to birds, insects, or uneven depth are higher.
• Seed size and vigour: Larger, more vigorous seeds generally establish more reliably and may allow a slightly lower rate than smaller or older seed lots of the same variety.
• Soil moisture and sowing time: Sowing into marginal soil moisture, or later than the optimal window, often reduces field emergence, which some farmers offset with a modestly higher seed rate.
• Seed treatment: Fungicide or insecticide seed treatment can improve seedling establishment and survival, sometimes allowing a lower effective seed rate.
• Cropping system: Intercropping or relay cropping arrangements typically use reduced populations of each component crop compared to a pure stand.
• Soil fertility and irrigation: Well-irrigated, fertile fields can often support and benefit from a slightly higher plant population than rainfed or lower-fertility fields.

What Happens With the Wrong Seed Rate?

Overseeding

Sowing more seed than needed wastes money on seed that's simply not required, and it can also crowd the crop stand — increasing competition for light, water and nutrients between plants, raising humidity within the canopy that favours fungal disease, and increasing the risk of lodging in cereals as plants grow tall and thin while competing for light.

Underseeding

Too little seed leaves gaps in the field that weeds are quick to occupy, reduces the crop's overall ability to capture sunlight and outcompete weeds, and typically results directly in lower yield potential for the season, since final yield is closely tied to achieving the target plant population.

Worked Example

Consider a farmer growing maize with a target population of 83,000 plants per hectare (achieved with 60cm row spacing and 20cm plant spacing), a seed lot with a 1000-seed weight of 280g, 90% germination, and 98% purity:

Seed Rate = (83,000 × 280) ÷ (100 × 90 × 98) ≈ 26.3 kg/ha

For a 2-hectare field, that works out to roughly 52.6 kg of seed to procure and sow — a number this calculator produces instantly once you enter the same values.

Tips to Optimise Your Seed Rate

• Always check the seed tag. Germination and purity percentages are usually printed on certified seed bags — use the actual figures rather than assumed defaults whenever possible.
• Conduct a simple germination test. For farm-saved seed without a certificate, soak and count germination from a sample of 100 seeds to get a realistic percentage.
• Match seed rate to sowing method. If switching from broadcasting to precision drilling, you can often safely reduce your seed rate since establishment losses drop.
• Recalculate each season. Seed lot characteristics like TSW and germination can change year to year — don't assume last season's seed rate still applies to new seed stock.
• Buy a small buffer, not a large one. A 5–10% buffer over the calculated amount covers minor losses without significant waste.

Final Thoughts

Use this calculator as your starting baseline each season, adjust it against your local extension recommendations and your own field experience, and recheck it whenever you switch seed lots, sowing methods, or crop varieties. Getting the seed rate right is one of the most cost-effective decisions in the whole growing season — bookmark this page and return to it before every sowing.