Its time to overseed your lawn and you now need to work out how much grass seed you need to buy. A new lawn and an existing lawn being overseeded can need different quantities, even when you use the same seed mixture. Its important to get the rate right; not enough and the lawn will be thin; too much seed and they’ll compete for water and nutrients resulting in weak growth.
Read on to see how to go about working out how much you need. And if you haven’t already bought seed then make sure to take advantage of my discount code FORMBY10 to get yourself 10% off at Hurrells Seeds.
Calculate how much grass seed you need for your lawn
The first step is to measure the size of your lawn(s). For a rectangle, it’s simply a case of multiplying length by width. For a more irregular shape, try splitting it into smaller rectangles and triangles, add the areas together and subtract any areas of beds, paving and anywhere else you won’t be sowing. For a circular lawn, use 3.14 × radius × radius. Personally, for any shape lawn I just use the measure tool on Google Earth.
Once you have the lawn area, you need to find the packet’s new lawn or overseeding rate. Do not borrow a rate from a different mixture. Where a supplier gives pack coverage instead, divide the pack weight in grams by the stated square metres. A 5,000g pack covering 200m² works out at 25g/m².
To turn the area into a shopping quantity: lawn area in square metres × the packet rate in grams per square metre ÷ 1,000 = kilograms of seed. For example, 100m² at 25g/m² needs 2.5kg. Weigh that amount before you start; a 5kg bag is not an instruction to spread all 5kg.
Grass seed coverage: quantities by lawn size
The following figures are calculated from the current HM3 and HM4 pack coverages: 5kg for 140m² of new sowing, or 200m² of overseeding. They are approximate buying quantities, rounded to the nearest 0.01kg. The current packet instructions take priority.
| Area to sow | New lawn: approx. | Overseeding: approx. |
|---|---|---|
| 25m² | 0.89kg | 0.63kg |
| 50m² | 1.79kg | 1.25kg |
| 75m² | 2.68kg | 1.88kg |
| 100m² | 3.57kg | 2.50kg |
| 150m² | 5.36kg | 3.75kg |
| 200m² | 7.14kg | 5.00kg |
For example, a 5kg pack is enough for a 100m² new lawn using these coverages, but it is short of the calculated 5.36kg for 150m². Check the available HM4 pack sizes or HM3 pack sizes before ordering. Buying a larger bag does not mean you should spread the whole bag.
How far do 5kg, 10kg and 20kg packs go?
For 100m² of overseeding at that example rate, a 5kg bag leaves 2.5kg unused. That can be handy for later repairs, but it is not a reason to sow twice as much today. For 150m² of new sowing using the listed coverage, you need about 5.4kg, so one 5kg bag falls slightly short.
Compare pack combinations and delivered prices. The cheapest bag is not always the cheapest way to buy the measured quantity, and a large surplus saves money only if you have a use for it and somewhere suitable to store it. Keep a note of the mix and batch with any leftovers.
What if the lawn has bare patches as well?
Calculate bare soil and thin existing grass separately. In this example, a 100m² lawn contains 80m² of thin grass and 20m² of bare ground. Using the coverage-derived HM4 rates, the existing grass needs 2kg and the bare area about 0.71kg: roughly 2.71kg altogether.
Count each area once. Do not include the bare patches in the overseeding area and then add the new-lawn amount on top. Put the two weighed portions into separate containers, marked for thin grass and bare soil, so each quantity reaches the right place.
Choose a suitable mixture before choosing the bag size
Hurrells HM4 General Amenity has an equal split of dwarf perennial ryegrass and strong creeping red fescue. It is a straightforward shortlist option for general-purpose lawns, including new sowing and repairs.
Hurrells HM3 Hard Wearing Lawn has 30% dwarf perennial ryegrass alongside several fescues, meadow grass and browntop. It offers a different mixture balance; the right choice depends on the lawn’s conditions and intended use, rather than whichever bag covers the most ground. For busy family gardens, see our grass seed guide for dogs and children.
Start with our wider lawn seed buying guide if you are still choosing the mix. Deep shade, dry sandy soil and heavy wear need consideration before you order.
If you prefer to buy through Amazon, you can view the available options here: Hurrells Seeds Amazon Store.
Get the measured seed onto the lawn evenly
When I repaired a worn, high-traffic patch with Hurrells HM3, preparing the surface and keeping the new seed damp mattered as much as buying the right amount. I would measure the area first, use the current pack rate to buy enough seed, then put it down evenly rather than emptying the bag in the thinnest-looking spots. A suitable mix and steady aftercare are a better buy than surplus seed left in storage.
Divide the lawn into manageable sections and give each its share of the weighed seed. That lets you spot a section taking too much before the bag is empty. Where the packet allows crossing half-rate passes, use half the seed in one direction and half across it. Turning a dial to half its number does not prove it is applying half the rate; check the output with the actual mixture. My spreader calibration guide explains how.
Extra seed cannot compensate for poor soil contact or a seedbed that dries out. Follow our overseeding steps and our new-seed watering guide to give the quantity you buy a better chance of establishing.
Both HM3 and HM4 specify soil temperatures of 10°C or above. Check local conditions and when to sow grass seed in the UK rather than relying on the calendar alone. The RHS recommends spring or autumn for lawn repairs; follow the RHS lawn repair guidance alongside your seed instructions.
Quick answers before ordering
How much grass seed for 100m²?
Using the listed HM3 / HM4 coverage, allow about 3.57kg for a new lawn or 2.5kg for overseeding. Other mixes can differ.
Should I use the whole 5kg bag?
Only if your measured area and the specified rate call for it. Weigh the required amount; more seed is not a shortcut to a better lawn.
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Sources checked 18 September 2026: Hurrells HM3 listing, Hurrells HM4 listing and RHS lawn repairs. Coverage figures are supplier estimates; always check the current packet before sowing.
