How much grass seed do I need?

How to measure for grass seed

Measure the area you are going to sow. For a rectangular lawn, take the length (A) and the width (B). For a round patch, measure straight across through the middle; for an oval, the longest length and the widest width. An L-shaped lawn is the full rectangle minus the missing corner, and a wedge is a triangle. Add each piece with Add another area.

When you are patching or overseeding, measure only the part you will sow, not the whole garden: a thin strip along a path needs far less seed than the lawn it sits in. You don’t need to measure to the centimetre, but round up rather than down.

Feet and inches work too: switch the units at the top of the calculator. UK packets give sowing rates in grams per square metre, so the calculator converts your measurements before it works out the seed.

Grass seed sowing rates

Each mix has its own rate, and the packet you buy is the best guide. These are typical rates from UK seed brands, a specialist wildflower grower and the RHS. Where there is a range, the button applies the lower figure; raise it in step 3 if your packet asks for more.

The two lawn ranges come from different brands: Johnsons prints the lower rates on its packets, and The Grass People recommend the higher ones. Both are sound advice, so follow the packet you actually buy.

How much grass seed per square metre?

Grass seed per m² is simply the sowing rate on the packet, in grams. For a new lawn on bare soil, UK brands give 35–50 g/m²; for thickening an existing lawn, 25–35 g/m². Multiply the rate by the area and you have the grams to sow. The calculator does that sum, adds any extra you ask for, and then chooses the packs.

Sowing much more thickly than the packet says won’t give you a lawn any sooner. The seedlings crowd each other and the extra seed is wasted, so keep to the rate unless the packet gives a range. A little extra makes sense where birds take a share, which is what Extra seed in step 4 is for.

How much seed to overseed a lawn

To overseed is to sow new seed into an existing lawn to thicken it up. Because there is already grass there, the rate is lower, typically 25–35 g/m². Choose Overseeding in step 1 and measure only the thin or bare parts if the rest of the lawn is fine.

Seed has to reach the soil to grow, so rake out moss and dead grass first, sow, then rake lightly or top-dress so the seed is just covered. Keep it moist until it is up: seed that dries out in an established lawn struggles against the grass around it.

Wildflower seed

Wildflower seed goes on far more thinly than lawn seed. The RHS and specialist growers suggest 1.5–2 g/m² for flowers on their own, and 4–5 g/m² for a meadow mix with fine grasses. It is sold by weight from 100 g upwards, so the calculator rounds up to whole 100 g lots.

With so little seed per square metre it is hard to spread evenly. Mix it with a few handfuls of dry sand so you can see where it lands, and sow half walking one way and half walking across. Wildflower meadow and Wildflowers with grasses in step 1 both switch the calculator to wildflower seed.

Lawn seed calculator: packs and left-over seed

As a lawn seed calculator, the last job is choosing packs. Lawn seed comes in small boxes for patching and bigger bags for whole lawns, and the calculator picks the fewest packs that cover what you need, as long as less than one small box is left over. Where two choices need the same number of packs, it takes the one with less seed to spare. Sometimes one large bag beats several small boxes; sometimes a bag plus a box is the closest fit.

A little left over is no bad thing. Keep it cool, dry and sealed, and use it to patch worn spots or bare edges later.

Worked example: overseeding a lawn

Here is the full calculation for thickening up a tired back lawn at the Overseeding rate, with no extra seed. Every line comes from the same code that runs the calculator above, so the figures always match.

1. Area 12 m × 6 m = 72.00 m²
2. Seed 72.00 m² × 25 g/m² + 0% = 1.8 kg
3. Packs the fewest packs that cover 1.8 kg, with less than the smallest pack left over = 1 × 2 kg pack

On the list:

• 12 kg pack of grass seed1.8 kg needed

Notice that the calculator doesn’t just add up the smallest packs: one larger bag does the job of a stack of small boxes, with less than a small box of seed to spare. If the old grass is coming up first and you are really starting from bare soil, switch to New lawn and the seed goes up accordingly.

UK grass seed pack sizes

Lawn seed is sold by weight. Garden centres stock small boxes that cover a patch or a small lawn, and seed merchants sell larger bags for new lawns and big gardens. Wildflower seed comes by the gram from specialist growers, with a minimum order.

If a box says how many square metres it covers, check which rate that assumes: a box that covers a good area at the overseeding rate covers less on bare soil.

Common mistakes when buying grass seed

• Using the overseeding rate on bare soil. A new lawn needs the higher rate. Pick New lawn in step 1 if there is no grass there now.
• Sowing wildflowers at lawn rates. Wildflower seed goes on at a small fraction of the lawn rate, and too much lets the strongest plants smother the rest.
• Measuring the whole garden for a patch. When overseeding or repairing, measure only the area you will actually sow.
• Sowing onto moss or hard ground. Seed that sits on thatch or compacted soil dries out before it roots. Rake first so it reaches the soil.
• Mixing up the units. A lawn measured in feet but typed in as metres asks for about ten times too much seed. If the answer looks huge, check the units.

What else you'll need

• Topsoil for a new lawn on poor or uneven ground: the topsoil calculator works out the bags for the seedbed.
• Turf instead of seed if you want a usable lawn sooner: the turf calculator gives the rolls and pallets.
• A seed spreader, or a marked pot and a steady hand, to sow evenly in two directions.
• A rake and a board or light roller to cover the seed lightly and firm it in.
• Netting or fleece if birds are a problem while the seed comes up.

Related calculators

• turf calculatorRolls and pallets of turf for a new lawn.
• topsoil calculatorNew lawns, borders and veg beds, in bags or bulk.
• plant spacing calculatorHedge plants per metre, plants and bulbs per m².
• compost calculatorLitres and bags for mulching and improving soil.