Grass seed needed is your lawn area times the seeding rate. A new lawn of 50 square metres at 35 grams per square metre needs 50 times 35, which is 1750 grams, or 1.75 kilograms. Enter your lawn size and purpose to get the seed amount and coverage.
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How to Use the Grass Seed Calculator
- Enter the length and width of the lawn area in metres.
- Choose your purpose: a new lawn, overseeding or patch repair.
- The right seeding rate is applied for that purpose.
- Read the seed needed in kilograms, plus the area and grams.
| Result | What it means |
|---|---|
| Grass seed needed | The total seed to buy, in kilograms. |
| Lawn area | The area you are seeding, in square metres. |
| Seed in grams | The same amount in grams, handy for small patches. |
| Rate used | The grams of seed per square metre for your purpose. |
What Is a Grass Seed Calculator?
A grass seed calculator works out how much seed to buy for a lawn, based on its area and the job you are doing. Sowing too little leaves thin, patchy grass that weeds invade; sowing too much wastes seed and makes seedlings compete and thin themselves out.
Seed is sold by weight, and coverage is given as a rate per square metre, so the calculator turns your lawn size into the exact weight of seed you need. Getting this right the first time means an even, hard-wearing lawn that establishes quickly, rather than one you have to patch and reseed a few weeks later.
How Does the Grass Seed Calculation Work?
seed = lawn area x seeding rate- Multiply length by width for the lawn area in square metres.
- Choose a seeding rate in grams per square metre for your purpose.
- Multiply area by rate for the seed in grams.
- Divide by 1000 to get kilograms.
New lawns need the highest rate because there is no existing grass, while overseeding an established lawn needs less, and small repairs use a heavier rate to fill fast.
Grass Seed Example
You are sowing a brand new lawn 10 metres by 5 metres, an area of 50 square metres. At a new-lawn rate of 35 grams per square metre, you need 50 times 35, which is 1750 grams, or 1.75 kilograms of seed. If you were overseeding an existing lawn of the same size, the lower 25 gram rate would need just 1.25 kilograms instead. Buying seed in that 1.75 to 2 kilogram range, rather than a random bag, means you finish the job without a second trip and without a large surplus going stale in the shed.
What Affects How Much Seed You Need
Purpose
New lawns, overseeding and patch repair all use different rates, which is the biggest driver of how much seed you buy.
Grass Type
Fine fescues are sown more thickly than ryegrass, so check the rate on your chosen seed mix and adjust if needed.
Lawn Area
Seed scales directly with area, so measure carefully and split odd shapes into rectangles.
Sowing Conditions
Poor soil, birds and heavy rain can cost you seed, so a slightly higher rate improves establishment on tough sites.
Seeding Rates Compared
Rates vary by job. These are common starting points for cool-season lawns.
| Purpose | Rate | Seed for 100 m2 |
|---|---|---|
| New lawn from bare soil | 35 g/m2 | 3.5 kg |
| Overseeding a thin lawn | 25 g/m2 | 2.5 kg |
| Patch repair | 50 g/m2 | 5.0 kg |
Always check the rate on your seed bag, as different grass mixes and brands recommend their own coverage.
When to Use a Grass Seed Calculator
Starting a New Lawn
Buy the right amount of seed for bare ground in one trip.
Overseeding in Autumn
Top up a tired lawn at the lighter rate to thicken it before winter.
Fixing Bare Patches
Work out the small amount of seed a repair really needs.
Common Grass Seeding Mistakes
1. Guessing the Area
Under-measuring leaves you short; measure length and width and split complex shapes.
2. Sowing Too Thickly
Overdense seed makes seedlings compete, giving weaker grass, not a lusher lawn.
3. Using the Wrong Rate
Applying a new-lawn rate when overseeding wastes seed and money.
4. Ignoring the Bag Rate
Different mixes vary, so always cross-check against the coverage printed on your seed.
Accuracy and Limitations
The area and seed maths are exact, and the rates are typical for common lawn seed. Your bag may recommend a slightly different rate for its mix.
What it calculates accurately
- Lawn area from length and width
- Seed weight for the chosen rate
- Coverage for new, overseed and repair jobs
What it does not do
- Set the exact rate for every seed mix
- Handle round or irregular lawns directly
- Allow for seed lost to birds or weather
How We Calculate the Result
Frequently Asked Questions
How much grass seed do I need?
Multiply your lawn area by the seeding rate. A 50 square metre new lawn at 35 grams per square metre needs 1750 grams, or 1.75 kilograms.
What is the seeding rate for a new lawn?
A common rate for a new lawn from bare soil is about 35 grams per square metre, though fine grass mixes can be sown a little more thickly.
How much seed for overseeding?
Overseeding an existing lawn uses a lighter rate, around 25 grams per square metre, because the established grass fills much of the space already.
How much seed to repair a patch?
Patch repair uses a heavier rate, about 50 grams per square metre, so the bare spot fills in quickly and resists weeds.
Can I sow too much grass seed?
Yes. Over-sowing makes seedlings compete for light, water and nutrients, giving weaker, thinner grass rather than a denser lawn.
Does grass type change the rate?
Yes. Fine fescues are sown more thickly than ryegrass, so always check the recommended rate on your seed bag.
When is the best time to sow grass seed?
Early autumn and mid to late spring are ideal, when soil is warm and moist and there is less competition from weeds.
How do I measure an odd-shaped lawn?
Split it into rectangles, work out each area, and add them together, or measure the average length and width.
Is my data saved?
No. The calculator runs entirely in your browser and nothing you enter is stored or sent anywhere.
Sources
- Lawn care and seeding (Wikipedia).
- Sowing and germination (Wikipedia).
- Common lawn grasses (Wikipedia).
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Author
Shakeel Muzaffar is the Founder and Editor-in-Chief of MultiCalculators.com, bringing over 15 years of experience in digital publishing, product strategy, and online tool development. He leads the platform's editorial vision, ensuring every calculator meets strict standards for accuracy, usability, and real-world value. Shakeel personally oversees content quality, formula verification workflows, and the platform's commitment to publishing tools that are genuinely useful for students, professionals, and everyday users worldwide.
