The rate depends on the job, not the lawn
Grass seed rates are quoted in grams per square metre and they differ substantially between sowing a new lawn, overseeding an existing one, and patching a bare spot. Using one rate for all three wastes seed in one direction and produces a thin lawn in the other.
A new lawn from bare soil needs around 35 g/m². It is starting from nothing and needs enough seedlings to form a closed sward before weeds colonise the gaps. Overseeding an existing lawn needs less, around 25 g/m², because the existing grass is already holding the ground and the new seed is filling in thin areas.
Patch repair goes the other way, around 50 g/m². A bare patch is bare for a reason and usually gets walked on, so it needs to establish quickly and thickly enough to survive that. Sowing patches at the new-lawn rate is a common reason they never quite fill in.
| Job | Rate | For 100 m² | For 1,000 ft² |
|---|---|---|---|
| New lawn from bare soil | 35 g/m² | 3.5 kg | 7.2 lb |
| Overseeding | 25 g/m² | 2.5 kg | 5.2 lb |
| Patch repair | 50 g/m² | 5.0 kg | 10.3 lb |
Fertiliser is sold by analysis, not by nitrogen
This is where lawn feeding arithmetic goes wrong. Recommendations are given in grams of actual nitrogen per square metre, but fertiliser bags are sold by total weight with an analysis printed on them — 20-5-10 means 20% nitrogen, 5% phosphorus, 10% potassium.
So a lawn needing 3 g/m² of nitrogen does not need 3 g/m² of fertiliser. It needs 15 g/m² of a 20% product, because only a fifth of the bag is nitrogen. Applying the nitrogen figure as a bag weight under-feeds by a factor of five, which is the usual reason a fed lawn looks no different afterwards.
The calculator does this conversion and shows both figures. Check the analysis on your own bag — products vary from about 10% to about 30% nitrogen, and the bag weight changes proportionally.
Seed needs soil contact and daily water
Two things kill more newly sown grass than anything else, and neither is about the seed.
The first is soil contact. Seed lying on top of thatch, or on loose fluffy soil, does not germinate reliably — it needs to be in contact with firm moist soil. For a new lawn that means raking to a fine tilth and firming before sowing, then raking lightly to cover. For overseeding it means scarifying first to open the thatch, otherwise the seed never reaches soil at all.
The second is water. Grass seed must not dry out between germinating and rooting, and the germinating zone is the top centimetre of soil, which dries in hours in warm weather. That means watering every day it does not rain for the first two to three weeks. Missing two hot days at the wrong moment kills the lot.
This is also why timing beats effort. Sowing in early autumn — warm soil, reliable rain, no summer heat — succeeds with far less watering than sowing in late spring. If the choice is available, autumn is the easier season by a wide margin.
Common questions
How much grass seed do I need for 1,000 square feet?
About 7.2 lb for a new lawn at 35 g/m², roughly 5.2 lb for overseeding, and around 10.3 lb for patch repair, which needs a heavier rate to establish quickly.
How much fertiliser for 3 g/m² of nitrogen?
With a 20-5-10 product, 15 g/m² — five times the nitrogen figure, because only 20% of the bag is nitrogen. Check the analysis on your own bag, as products range from about 10% to 30%.
When is the best time to sow grass seed?
Early autumn. The soil is still warm, rain is more reliable and there is no summer heat, so it establishes with far less watering than a late spring sowing.