Lawn Seed Calculator

Lawn Seeding Guide

What should I know about Seeding Rates?

New lawn from seed: 35–50g per m² (higher rate gives faster establishment but not necessarily a better long-term lawn). Overseeding an existing lawn: 25–35g per m² — lower rate as existing grass fills gaps. Patch repairs: 50–70g per m² to ensure dense coverage in the repaired area. Shaded areas may need specific shade-tolerant seed mixes and can be sown slightly more densely as germination rates are lower.

What's the key thing to understand about Best Times to Sow?

Spring (April–May): soil is warming but moisture is often adequate. Germination takes 10–21 days in good conditions. Main risk: spring can be dry — watering commitment essential. Autumn (August–October): the preferred time for most UK lawn professionals. Soil is warm from summer, air is cooler, rainfall increases, and there is less competition from weed seeds. Germination is slightly slower but establishment is generally more reliable. Avoid sowing in summer heat or winter cold.

What's the key thing to understand about Seed Bed Preparation?

Good preparation makes the most difference to success. Dig or rotavate to 15–20cm, remove all perennial weeds including roots (bindweed, couch grass, thistle). Rake to a fine tilth. Apply pre-seed fertiliser (low nitrogen, high phosphorus) and rake in. Firm the surface by walking across it or using a roller — seed needs contact with firm soil. Final rake to create a criss-cross surface texture. Sow half the seed in one direction, the other half at 90° for even coverage.

What do I need to know about After Sowing?

Keep the seedbed consistently moist until germination — this may mean watering twice daily in dry weather. Never let the surface dry out during germination. Once established (5–6cm tall), first cut at the highest mower setting (5–6cm), removing no more than one-third of the blade height. Avoid walking on a newly seeded lawn for 6–8 weeks. Apply autumn lawn feed 6–8 weeks after sowing to strengthen roots before winter.

What's the key thing to understand about Grass Seed Mixes?

Not all grass seed is the same, and choosing the right mix for how a lawn will be used matters as much as getting the seeding rate right. General-purpose lawn mixes, often a blend of perennial ryegrass and fescues, are the most common choice for family gardens, offering a reasonable balance of durability and appearance. Hard-wearing or family-and-pet mixes contain a higher proportion of tough perennial ryegrass, which germinates quickly and recovers well from foot traffic or dog use, but tends to give a coarser, less fine-textured finish. Fine or ornamental lawn mixes, by contrast, are dominated by fescues and bents for a dense, soft finish similar to a bowling green, but this type of grass is far less tolerant of heavy use and wears thin quickly under regular foot traffic. Shade-tolerant mixes include a higher proportion of fine fescues, which cope better than ryegrass in areas getting less than around four hours of direct sun a day, useful under trees or on the shaded side of a house. Matching seed type to actual lawn use avoids the common disappointment of a fine ornamental mix sown on a garden used daily by children or pets thinning out within a season.

What do I need to know about Overseeding an Existing Lawn vs a New Lawn?

The seeding rate needed changes significantly depending on whether you're sowing entirely bare soil or thickening up an existing, patchy lawn. New lawns sown on bare, prepared soil need the full seeding rate to achieve good, even coverage and to out-compete weed seedlings while the grass establishes. Overseeding an existing lawn to thicken thin patches typically needs only around half that rate, because the existing grass already covers most of the ground and the new seed only needs to fill gaps rather than populate open soil from scratch. For overseeding to succeed, the existing lawn should be mown short, then scarified or raked firmly to expose soil in thin areas, since seed sitting on top of dense thatch or old grass rarely germinates well, and ideally top-dressed with a thin layer of seed-friendly compost or soil to give the new seed good contact with soil — good seed-to-soil contact is one of the biggest factors in germination success for either a new lawn or an overseeding job.

What Watering Schedule Suits New Grass Seed in the First Month?

New grass seed needs consistent moisture to germinate and establish, and getting the watering schedule wrong is one of the most common reasons newly seeded lawns fail or come up patchy. For roughly the first two weeks after sowing, while seeds are germinating, the soil surface needs to stay consistently damp but not waterlogged — this usually means light watering once or twice daily rather than one heavy soak, since a heavy watering can wash seed into clumps or expose it, while infrequent watering lets the surface dry out and kills germinating seedlings. Once grass has germinated and reached a few centimetres tall (typically two to four weeks after sowing, depending on weather), watering frequency can reduce but each watering should go deeper, encouraging roots to grow downward rather than staying shallow near a constantly-wet surface — this transition to deeper, less frequent watering is important for building a lawn that's resilient to dry spells later. Watering is best done early morning, which reduces evaporation loss compared with midday watering and allows the grass to dry out before evening, reducing the risk of fungal problems that can develop on grass that stays wet overnight. The exact schedule needs adjusting for weather — hot, windy or dry periods will dry the seed bed faster and may need more frequent light watering, while cool, damp conditions need less supplemental water.

Overseeding Rates vs New Lawn Rates

Seeding rate depends heavily on whether you're overseeding an established lawn to thicken it up, or sowing an entirely bare patch or new lawn from scratch — using a new-lawn rate on an overseeding job wastes seed and can smother existing grass, while using an overseeding rate on bare soil leaves a thin, patchy result. (For full seed-mix selection by lawn type and soil preparation steps, see this site's grass seed calculator — this page focuses specifically on getting the right rate for patch repair and overseeding an existing lawn.)

Applying seed too heavily doesn't speed up establishment — seedlings compete for light, water and nutrients, often producing a thinner, weaker result than a correctly rated application, alongside wasted seed cost.

Worked Example: Repairing a Worn Patch Behind a Garden Gate

A 3m × 1.5m patch (4.5m²) behind a garden gate has worn down to bare soil from repeated foot traffic. This is a bare-patch repair situation, calling for the higher end of the patch-repair rate — around 30g/m². Seed needed: 4.5m² × 30g/m² = 135g. Since seed is typically sold in 1kg, 2.5kg or larger bags, 135g is a small amount — most of a typical 1kg "patch repair" pouch (often sold specifically in small sizes for exactly this kind of job) would cover this patch with some left over for a second light pass two weeks later if germination is patchy. Preparation matters as much as the rate here: the compacted soil from foot traffic needs breaking up with a fork to at least 5-7cm depth before sowing, since seed sown onto compacted, poorly-aerated soil germinates unevenly regardless of how accurately the rate was measured. After sowing, lightly raking the seed into the top few millimetres of soil and keeping the patch consistently moist (not waterlogged) for the first 10-14 days is what determines germination success more than the seed quantity itself — a patch sown at the perfect rate but allowed to dry out for even a day or two during germination will patch unevenly, often needing the same reseeding process repeated.