Seed Rate Calculator
Enter the area and the seeding rate to get the seed to buy, buffer included.
What this tool does
Multiply the area by the seeding rate and add the germination buffer - 10% is a common allowance for seed that will not sprout. 50 m² at 4 g/m² is 200 g of seed, and 10% on top takes it to 220 g. The second mode works from plant spacing instead, turning the bed dimensions into a plant count and then into seeds.
Multiplies the area by the seeding rate and adds a germination buffer to give the seed to buy.
Works from plant spacing as well, turning bed length, width and spacing into a seed count.
Reads rates in grams per square metre or pounds per 1,000 square feet, and switches units for you.
Splits the total into seed and buffer, so the extra you pay for stays visible.
How to use it
- Choose metric (square metres and grams) or imperial (square feet and pounds).
- Choose seed per area, or work from plant spacing.
- Enter the area and the rate, or the bed size and the spacing between plants.
- Set the germination buffer - the share of seed you expect not to grow.
- Read the total seed and copy it for your order.
Why use this tool
Seed is sold by weight and planted by area, and almost nobody buys the right amount the first time. Too little and half a row is empty in May; too much and the surplus sits in a drawer until it is old. Converting the rate on the packet into a weight, and adding the allowance for seed that never comes up, turns a vague guess into an order you can place once.
The buffer is the part people leave out. Germination is a percentage, not a promise, and drilling, broadcasting and hand-sowing all waste different amounts before a single seed reaches soil.
Why we built it
We built this seed rate calculator because the two directions of the problem - given an area how much seed, and given a bed how many plants - are usually handled by two separate rules of thumb. Here the area mode multiplies through and shows the buffer as its own line, while the spacing mode counts stations first and then applies seeds per station. The Plant Spacing Calculator does the counting on its own, and the Fertilizer Calculator works the same area times rate logic for the bag rather than the seed.
For the bed itself, the Topsoil Calculator gives the volume to fill it, and the Percentage Calculator checks the buffer figure you have been quoted.
Who this tool is for
- Gardeners ordering grass seed or cover crop by the kilo
- Farmers checking a drill rate against a supplier's recommendation
- Market gardeners working a new bed out from a spacing chart
- Anyone comparing a metric packet with an imperial recommendation
How it works
The area mode is one multiplication and one addition: seed = area × rate, buffer = seed × buffer % ÷ 100, total = seed + buffer. The spacing mode counts planting stations with the same floor-plus-one rule the plant spacing calculator uses, multiplies by the seeds you sow per station, then applies the buffer and rounds up to a whole seed.
| Step | Calculation |
|---|---|
| Seed for the area | area × rate |
| Buffer | seed × buffer % ÷ 100 |
| Total seed | seed + buffer |
| Plants from spacing | (floor(length ÷ spacing) + 1) × (floor(width ÷ spacing) + 1) |
| Seed from spacing | plants × seeds per station × (1 + buffer %) |
| Rounding | weights stay as they are; seed counts are rounded up |
A buffer of 10% covers a normal germination shortfall. Use 15-20% for old seed, cold soil or broadcasting, and 0% only when you have a recent germination test.
A worked example
Press Load sample and the calculator opens on the simplest possible case: a 50 m² area sown at 4 g/m² with a 10% buffer. The inputs are exactly these:
| Input | Value |
|---|---|
| Units | Square metres and grams |
| Mode | Seed per area |
| Area | 50 m² |
| Seed rate | 4 g/m² |
| Germination buffer | 10% |
With those values the calculator returns:
- Area: 50 m²
- Rate: 4 g/m²
- Seed before buffer: 200 g
- Buffer (10%): 20 g
- Total seed: 220 g
- In kilograms: 0.22 kg
So one 250 g packet covers the job with a little to spare, and the 20 g of buffer is visible as its own line rather than hidden inside the total. Switching the units to imperial converts the same plot to square feet and the rate to pounds per 1,000 square feet without changing what is being asked.
What you enter, and what you get back
| Item | Details |
|---|---|
| Units | Square metres and grams, or square feet and pounds. |
| Mode | Seed per area, or seed from plant spacing. |
| Area and rate | The ground to cover and the rate printed on the packet. |
| Germination buffer | The percentage added for seed that does not grow. |
| Result | Seed before buffer, the buffer itself, the total weight and the count in seeds. |
| Copy | The whole calculation as plain text for your order. |
Issues you may run into
| What you see | Why it happens | What to do |
|---|---|---|
| The total is far too small | The rate was per 1,000 m² or per hectare rather than per m². | Check the denominator on the packet and enter the rate for the area you typed. |
| Buffer looks like a mistake | It is being read as part of the rate rather than an addition. | The buffer line is separate: seed before buffer plus buffer equals the total. |
| Seed count does not match the weight | Seeds per gram differ between crops. | Use the weight for ordering and the count only as a guide for spacing mode. |
| Nothing is calculated | The area or the rate is blank or zero. | Enter an area and a rate greater than zero; the buffer may stay at 0%. |
Assumptions and limitations
- The rate you enter is the rate it uses - it does not adjust for soil, season or seed age.
- It returns seed weight and seed count, not the number of plants that survive to harvest.
- Broadcast and drilled seed waste different amounts, so the buffer should suit your method.
- Unit conversions are rounded for display, so check the final figure against the pack size.
Frequently asked questions
How do I calculate a seed rate?
Multiply the area by the rate printed on the packet, then add the germination buffer. For 50 m² at 4 g/m² the seed is 200 g, and a 10% buffer takes the order to 220 g.
What germination buffer should I add?
10% suits most situations. Use 15-20% for old seed, cold or dry soil and broadcasting, and reduce it towards zero when you have a recent germination test for that batch.
How much seed do I need for 50 square metres?
It depends on the rate. At 4 g/m² the raw need is 200 g, and with a 10% buffer the calculator orders 220 g. Change the rate to your packet's figure and the total updates.
Can I work out the seed from plant spacing?
Yes. Switch to the spacing mode, enter the bed length, width and spacing and the seeds you sow per station, and the calculator counts the stations, adds the buffer and rounds up to whole seeds.
What is the difference between g/m² and kg/ha?
They describe the same thing at different scales: 1 g/m² is 10 kg/ha. The calculator keeps the rate in whichever unit you choose and converts the area to match it.
Is the seed rate calculator free?
Yes, completely free with no account and no upload. Everything is worked out in your browser, so your sowing plan never leaves the page.