What this calculator works out
Grass seed is sold by weight, but lawns are measured by area, so ordering the right amount comes down to converting one into the other at a sensible rate. This tool takes a lawn's shape and size, a job type that sets a sensible default sowing rate, and turns that into a seed weight in kilograms and pounds, then rounds it up into a bag count using a 1.4kg reference pack size, alongside an optional cost estimate.
It sits alongside the turf calculator and the topsoil calculator as one of the site's lawn-building tools. Seed and turf answer the same underlying question, how to cover a given area of ground in grass, but they're bought in completely different units and on completely different timescales, which is why each gets its own calculator rather than one shared page.
How the maths works
Area comes first, from whichever shape matches the lawn. A rectangle is length times width; a circle uses its diameter, halved to a radius and squared, times pi; and if you've already worked out the area some other way, you can type the total straight in and skip the shape step.
Seed weight is then area multiplied by the sowing rate, the amount of seed spread over each square metre. Working the default example through: a 10m by 5m lawn is 50 square metres; at a new-lawn rate of 35 grams per square metre, that's 1,750 grams, or 1.75kg. Divided by a 1.4kg reference bag and rounded up, since no supplier sells part of a bag, that's 2 bags.
The bag count exists to give a practical order figure rather than a raw decimal; the exact kilograms and pounds are shown alongside it, so a buyer working from a bulk 10kg or 20kg sack rather than small bags can still use the figure directly.
Choosing a sowing rate: two UK sources
The sowing rate is the one figure worth getting right, since it scales the whole order, and it varies more than most people expect between products and between a new lawn and an overseed. Two named UK suppliers were checked directly for this calculator. Johnsons Lawn Seed's own FAQ page states a new lawn rate of 35g per square metre and an overseeding rate of 25g per square metre. Rolawn's Medallion Lawn Seed is sold on the stated coverage of "28m² new lawn, 43m² overseeding" per 1kg pack, which works out to about 36g/m² for a new lawn and 23g/m² for overseeding, in close agreement with Johnsons' figures. This calculator defaults to the round Johnsons numbers, 35g/m² new and 25g/m² overseed, and both are fully editable if your own seed packet quotes something different.
Individual products vary either side of these figures depending on the seed mix, so the rate printed on your own packet always takes priority over a generic default; this calculator's rate field exists precisely so that figure can be typed straight in.
New lawn or overseeding, and why the rate roughly halves
"Overseed at half the new-lawn rate" is a common rule of thumb, and it's a reasonable starting point, but the two named UK suppliers above show the real figure sits a little higher than a strict half: Johnsons' 25g/m² overseed rate is about 71% of its 35g/m² new-lawn rate, and Rolawn's is about 65%. The reason isn't arbitrary. A new lawn is sown onto bare, cultivated soil with no competition and no existing grass to fall back on, so it needs a fuller rate to guarantee even establishment across every part of the area. Overseeding spreads new seed into an existing, already-rooted lawn, topping up thin patches and adding density rather than starting from nothing, so a lower rate still gets enough seed-to-soil contact without wastefully burying seed into ground that's already covered.
This calculator's job type selector sets its default rate from that logic, a fuller rate for bare soil and a lower one for overseeding, while leaving the figure fully editable for a specific product's own instructions.
US seeding rates by species
American grass seed guidance is usually given in pounds per 1,000 square feet rather than grams per square metre, and unlike the UK market, the rate varies significantly by species rather than sitting close to one figure. Scotts and Pennington's own product guidance, checked for this calculator, gives tall fescue at 8 to 10 lb/1,000ft² for a new lawn and roughly 4 to 6 lb/1,000ft² for overseeding; perennial ryegrass at around 5 to 9 lb/1,000ft² new and 3 to 5 lb/1,000ft² overseed; and Kentucky bluegrass, which has a much smaller, lighter seed, at just 2 to 3 lb/1,000ft² new and 1 to 2 lb/1,000ft² overseed. Switching the sowing rate field to lb/1,000ft² converts the same number both ways, so a US reader can type their species' published rate straight in and get a result in kilograms, pounds and bags exactly as a UK reader would.
Interestingly, converting the UK figures into the same units lines up closely with tall fescue: 35g/m² works out to roughly 7.2 lb/1,000ft², sitting inside the fescue range and a good way above the finer-seeded bluegrass rate, which is a useful sanity check either way.
When to sow, and what to expect from germination
Spring and autumn are the two windows most UK guidance points to, including Rolawn's own advice on when to sow. Autumn, roughly September to October, is generally rated the stronger of the two: the soil is still warm from summer, rain is more reliable, and grass gets to build roots over a mild autumn before it has to cope with a hard summer drought the following year. Spring, roughly April to May, is the second-best option and often the more practical one if autumn was missed, though a new spring lawn needs closer attention to watering through its first summer.
Germination itself needs soil temperatures consistently above about 8 to 10°C, and typically takes 7 to 21 days depending on the grass species, the weather and how consistently the ground is kept moist; fast-establishing types like perennial ryegrass are often visible within 7 to 10 days, while finer fescues and bluegrasses can take longer. Keeping the seedbed damp, not waterlogged, for that whole window matters more than almost anything else for a good result, since seed that dries out partway through germinating usually fails rather than just slowing down.
Worked example
Take a UK back garden lawn, 8m by 6m, so 48 square metres, being sown from bare soil at the default 35g/m² rate. That's 48 × 35 = 1,680g, or 1.68kg of seed, which rounds up to 2 bags of 1.4kg. At an example price of around £14 per kg for a mid-range hard-wearing mix, 1.68kg comes to roughly £23.50, or buying by the bag at around £18 each, 2 bags is £36; buying loose by weight is usually the cheaper route once the amount needed doesn't line up neatly with a bag size, which this calculator's two separate cost fields make easy to compare.
As a US aside, the same lawn measured in feet is about 26ft by 20ft, 520 sq ft. Sown with tall fescue at the Scotts new-lawn rate of 8 lb/1,000ft², that's 520 ÷ 1,000 × 8 = 4.16 lb of seed, which converts to 1.89kg, close to the UK example above since 8 lb/1,000ft² and 35g/m² sit in a similar band once converted, even though the two markets quote the rate in entirely different units.
Questions people ask
How much grass seed do I need?
Work out the lawn's area, multiply by the sowing rate for a new lawn or an overseed, and that gives the seed weight needed. This calculator does that step and rounds the result up into 1.4kg reference bags, alongside the exact kilograms and pounds.
Should I sow grass seed or lay turf?
Seed is considerably cheaper for the same area and offers more choice of grass mix, but takes several weeks to become a usable lawn and needs consistent watering while it germinates and establishes, staying vulnerable to weeds and heavy foot traffic in the meantime. Turf gives an established, walkable lawn within a few weeks and costs more per square metre; the turf calculator works out the same shape and size in UK rolls or US pallets if that's the better fit for the job.
When is the best time to sow grass seed?
Autumn, roughly September to October, and spring, roughly April to May, are the two windows most UK guidance recommends, with autumn generally rated the stronger of the two because the soil holds summer warmth and moisture without the risk of a hard summer drought straight after sowing.
Why does overseeding use roughly half the new-lawn rate?
A new lawn is sown onto bare soil with nothing established, so it needs a fuller rate to guarantee even coverage across the whole area. Overseeding tops up a lawn that's already rooted, so a lower rate still achieves good seed-to-soil contact. "Half rate" is a useful rule of thumb, though named UK suppliers' own figures run a little higher, around 65 to 71% of the new-lawn rate rather than an exact half.
Will birds eat grass seed before it germinates?
Yes, freshly sown grass seed sitting on top of the soil is an easy target for birds, and losses can be significant on an exposed lawn. Raking seed lightly into the top few millimetres of soil rather than leaving it fully exposed, and covering a small or high-risk area with garden fleece or netting until germination is visible, both cut losses substantially; keeping the seedbed consistently damp also helps it germinate faster and spend less time exposed.
How long does grass seed take to germinate?
Typically 7 to 21 days, depending on the grass species, soil temperature and how consistently the ground is kept moist. Soil needs to be consistently above about 8 to 10°C for germination to start; perennial ryegrass is usually one of the fastest species to show, often within 7 to 10 days, while finer fescues and bluegrasses can take longer.
This calculator uses sowing rates checked against two named UK suppliers, Johnsons Lawn Seed's published FAQ and Rolawn's Medallion Lawn Seed product page, plus published US per-species guidance from Scotts and Pennington for the lb/1,000 sq ft comparison. Individual seed products vary either side of these figures depending on the grass mix, so always check your own packet's stated rate and treat the figures here as a solid ordering estimate rather than a substitute for it.