Grass Seed Calculator — Seed Weight & Bags by Area and Species

Real-world use cases

• Reseed a worn-out 250 m² back lawn in autumn

  Your back lawn is patchy after a dry summer and you want it thick again by spring. Add the lawn as a 25 m × 10 m rectangle, pick a cool-season mix, and choose Overseed because the turf is thin, not bare. The tool applies the half-rate (~15 g/m² instead of ~30), giving roughly 3.75 kg before the spare margin — so a single 5 kg bag covers it with a little to spare. Buying the "new lawn" amount here would have been two bags and double the cost.

• Cost out a brand-new front lawn from bare soil

  You have just had topsoil delivered and graded a 6 m × 8 m front lawn. Add it as one rectangle, keep the job on New lawn, and pick tall fescue. At 35 g/m² across 48 m² you need ~1.7 kg, plus a 5% edge margin. A 2.5 kg bag covers it. Because the number is exact you do not over-order, and the leftover seed (which loses germination in a year) is minimal.

• Convert a US seeding-rate guide to metric for a UK garden

  The lawn-care blog you are following quotes "7 lb per 1000 sq ft" but your seed bag and tape measure are metric. Switch the unit toggle to US, type the area in ft², and read the seed weight — or flip to metric and the tool restates the rate as ~34 g/m² so you can sanity-check it against the European bag instructions. No manual 4.88 conversion factor to remember.

• Split a mixed yard into measured patches

  Your garden is a 12 m × 9 m main lawn, a thin 4 m × 3 m strip down the side, and a circular 3 m bed you want to grass over. Add all three patches, set the side strip and bed however they need, and the tool sums the area to ~127 m². One seeding rate across all of it gives a single seed weight and bag count instead of three separate back-of-envelope sums.

• Decide whether a bigger bag is cheaper per use

  You need 6.2 kg of ryegrass. A 5 kg bag means two bags (10 kg) with 3.8 kg left to spoil; a 10 kg bag is one bag with the same leftover but usually a lower price per kg. Switch the bag size in the tool and watch the "you buy" figure — it shows the real weight you walk out with for each option, so you choose on price-per-used-kg, not on the sticker.

Common pitfalls

• Measuring the radius of a circular bed instead of the full diameter. The tool asks for the diameter across; entering the radius gives a quarter of the real area and you buy far too little seed. If you measured from the center to the edge, double it before typing.

• Using the new-lawn rate to overseed a thin lawn. Double the seed does not give double the lawn — overseeded ground only has so much room for new seedlings, so the surplus rots, cakes, and wastes money. Pick Overseed and let the tool halve the rate.

• Picking the wrong species and assuming all grass seeds the same. Kentucky bluegrass at ~15 g/m² and tall fescue at ~35 g/m² differ more than 2x because the seed size differs. Seeding bluegrass at the fescue rate buries the crowns and thins the stand.

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