Autumn Lawn Feed Calculator UK | Lawn Hardener & Winter Feed
UK Autumn Lawn Care Tool

Autumn Lawn Feed
Calculator UK

Calculate exactly how much low‑nitrogen, high‑potassium autumn lawn hardener you need in kg — to toughen your UK lawn for winter and resist frost, disease and wear.

3‑12‑12 Autumn Feed 6‑5‑10 + Iron Low‑N Winter Hardener Disease Resistance
Calculate Autumn Feed Autumn Feed Guide
Tool

Autumn Lawn Feed Quantity Calculator

Most UK autumn/winter lawn hardeners are applied at 25–40 g/m². Enter your lawn area and label rate to see the total kilograms needed — plus how many bags to buy.

Autumn / Winter Lawn Hardener Calculator

For granular autumn feeds — low N, high K, with or without iron

If you enter length AND width, area is calculated automatically.
Autumn hardeners typically 25–40 g/m². Always use the rate on your product label.
Enter a bag price to get an estimated total cost.
Autumn Feed Required
Area
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m²
Rate
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g/m²
Total Product
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g
Total Kilos
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kg
Chosen bag size
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10 kg bags
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20 kg bags
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Estimated Cost (optional)
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UK Autumn Rates

Typical Autumn Lawn Feed Types & Application Rates

These rows show how common UK autumn/winter lawn hardeners differ in NPK and typical g/m² rates.

Feed Type Typical NPK Key Focus Rate (g/m²) 20 kg Coverage (m²)
Classic Autumn Hardener 3‑12‑12 Root strength, winter hardiness 25–35 g/m² ≈ 571–800 m²
Autumn Tonic + Iron 6‑5‑10 + Fe Green colour, moss suppression 25–35 g/m² ≈ 571–800 m²
High‑K Winter Prep 4‑2‑16 Maximum potassium hardening 30–40 g/m² ≈ 500–666 m²
Autumn + Worm Suppressant Low N + Fe granules Casting reduction + hardening 25–30 g/m² ≈ 666–800 m²

Swipe to see the full table on small screens.

Feed Types

Common UK Autumn Lawn Feed Styles

Three main types of autumn lawn feed cover most UK garden and amenity turf requirements.

Classic

3‑12‑12 Autumn Hardener

UseStandard UK lawns
Rate25–35 g/m²
SeasonAugust–October
BenefitRoot & winter strength

The traditional UK autumn lawn formula — very low nitrogen to avoid soft growth, high phosphorus and potassium to harden the plant before frost.

MOST POPULAR
Iron + Autumn

6‑5‑10 + Iron Tonic

UseColour + moss control
Rate25–35 g/m²
SeasonSeptember–November
BenefitGreens turf, hardens leaf

The iron darkens and hardens the grass leaf while the low‑N formula avoids pushing soft, disease‑prone growth into winter.

High‑K

4‑2‑16 Winter Prep

UseMaximum winter prep
Rate30–40 g/m²
SeasonSeptember–October
BenefitFrost & disease resistance

Very high potassium for maximum cell‑wall strength and frost hardiness going into a cold UK winter.

Method

How to Apply Autumn Lawn Feed

Four simple steps to get the most from your autumn lawn hardener on a UK garden lawn.

1

Time it right

Apply from late August through to October while soil temperatures are above about 8°C and grass is still actively growing. Avoid applying once the ground is frozen.

2

Mow first

Cut the lawn to its normal height a day or two before feeding. Shorter grass allows granules to fall to the soil surface and begin working faster.

3

Spread evenly

Use a calibrated rotary or drop spreader set to the correct g/m² rate. Walk in two directions — half the dose each way — to avoid stripes and missed patches.

4

Water in

Apply before expected rain, or water lightly after spreading to wash granules off leaf blades and into the root zone. This reduces scorch and speeds up nutrient release.

NPK Explained

Why Low‑N, High‑K Matters in Autumn

The switch from spring/summer feeding to autumn feeding is one of the most important things you can do for a UK lawn.

Nitrogen in autumn — why less is more

High nitrogen drives soft, sappy leaf growth — exactly what you do NOT want going into autumn and winter. Soft growth is highly susceptible to fungal diseases like Fusarium patch and Red Thread, both of which peak in cool, wet UK autumns. Autumn feeds keep nitrogen very low, typically 3–6%, to prevent this.

Potassium — the winter hardener

Potassium (K) is the key nutrient in autumn feeds. It thickens cell walls, improves water regulation in the plant and significantly increases frost tolerance. A high‑K autumn feed (e.g. 3‑12‑12 or 4‑2‑16) builds up plant reserves before growth stops for the season.

What to look for on the bag

  • N figure below 6 — for minimal soft growth stimulus.
  • K figure equal to or higher than P — for maximum hardening.
  • Iron (Fe) listed if you want greening and moss suppression.
  • Application rate printed clearly — use this in the calculator above.

Example: 180 m² Autumn Programme

Lawn size: 180 m² | Product: 3‑12‑12 at 30 g/m²

  • • 180 × 30 g = 5,400 g = 5.4 kg total
  • • One 10 kg bag covers the job with plenty spare
  • • At 35 g/m², same bag covers ≈ 285 m²

Don't use spring feed in autumn

Applying a high‑nitrogen 10‑4‑4 spring fertiliser in August or September is one of the most common UK lawn mistakes. It pushes soft growth that is immediately attacked by fungal disease and killed by first frost. Always switch to an autumn‑labelled low‑N product from August onwards.

FAQ

Autumn Lawn Feed — 15 Frequently Asked Questions

Expert answers on timing, NPK ratios, rates, disease prevention and everything else about autumn lawn feeding in the UK.

The ideal window for autumn lawn feed in the UK is late August through to the end of October. Apply while soil temperatures are above about 8°C so that the grass roots can still absorb and use the nutrients, especially potassium, before growth stops completely. Feeding in November or later is generally too late for the plant to benefit and simply wastes product.
A good UK autumn lawn feed should have a low nitrogen (N) figure — typically 3–6% — and a higher potassium (K) figure, such as a 3‑12‑12 or 6‑5‑10 analysis. The potassium hardens the grass plant, improves frost resistance and strengthens cell walls. Avoid any fertiliser with a high first number (nitrogen) at this time of year.
Most granular autumn and winter lawn hardeners are applied at 25–35 g/m², with some high‑K products going up to 40 g/m². The exact rate is always printed on the product label — use that figure with the calculator at the top of this page to work out your total kilograms needed. Do not guess the rate; applying too much can scorch the turf.
No. Using a high‑nitrogen spring feed (such as a 10‑4‑4) in autumn is one of the most damaging things you can do to a UK lawn. It stimulates soft, sappy leaf growth going into the cooler months, which is immediately vulnerable to Fusarium patch, Red Thread and other fungal diseases that peak in damp autumn conditions. Always switch to an autumn‑labelled, low‑N product from August onwards.
Potassium is often called the "hardening" nutrient for good reason. It regulates water movement within the grass plant, thickens cell walls, improves root development and significantly increases the plant's ability to withstand cold temperatures and frost. A lawn well‑fed with potassium going into winter will also recover faster in spring.
Iron is an excellent autumn addition. It quickly darkens and greens the turf without pushing growth, and it hardens the grass leaf, making it more resistant to fungal attack. Many autumn feeds include iron as standard (e.g. 6‑5‑10 + Fe). Granular iron products can also help suppress moss, which becomes more active in cool, wet autumn conditions.
Multiply your lawn area (in m²) by the label rate (in g/m²) to find the total grams needed. Divide by 1,000 to convert to kilograms, then divide by your bag size to find the number of bags required — always rounding up. For example, a 200 m² lawn at 30 g/m² needs 6,000 g (6 kg), so one 10 kg bag is sufficient. Use the calculator on this page to do this automatically.
Yes, indirectly and significantly. Fusarium patch (Microdochium nivale) is the most common UK lawn disease and it attacks soft, nitrogen‑rich grass tissue in cool, damp conditions. By switching to a low‑nitrogen, high‑potassium autumn feed, you harden the grass, reduce the amount of vulnerable soft growth, and make the lawn far less susceptible. A good autumn programme is one of the most effective disease prevention steps you can take.
Ideally, apply just before light rain is forecast or on a cool, overcast day with rain expected within 24–48 hours. Light rain helps wash granules off leaf blades and into the root zone without scorching. Avoid applying to a waterlogged lawn — if the soil cannot absorb water, nutrients will simply wash away. Also avoid applying during heavy downpours which cause surface run‑off and waste.
Scarify first, then feed. Scarifying removes thatch, moss and surface debris, opening up the soil surface so that fertiliser granules fall directly into the root zone where they can be absorbed. If you feed before scarifying, you risk raking much of your fertiliser back out along with the thatch. Allow a few days after scarifying before applying autumn feed to let the lawn settle.
You can apply them at the same time or within a few days of each other, but it is better to use a specific pre‑seed or renovation fertiliser (higher in phosphorus) alongside new seed rather than a standard autumn hardener. Phosphorus supports root establishment in new seedlings. Once the new seed has germinated and established after 4–6 weeks, switch to a standard autumn/winter low‑N feed to harden it before winter.
For most UK domestic lawns, one well‑timed autumn application is sufficient. Apply in September for most of the UK, or late August if you are in Scotland or a higher altitude area. Applying twice — in early September and again in mid‑October — can be beneficial on lawns that were heavily used in summer, but make sure the second application is done while the grass is still growing and before soil temperatures drop too low.
No — and that is intentional. A correctly formulated autumn feed contains very little nitrogen, so it does not stimulate significant leaf growth. You may see a slight greening from any iron content, but the main purpose is root development and cell hardening rather than visible top growth. If your lawn suddenly grows rapidly after applying an autumn feed, the product likely contains more nitrogen than it should for autumn use.
If the product contains iron, you may notice a slight greening of the grass within 7–14 days. The root‑strengthening effects of potassium are not visible above ground but work over several weeks as the nutrients are taken up through the roots. Slow‑release granular products continue releasing nutrients over 8–12 weeks, providing a steady supply through the rest of the growing season and into early winter.
Wait until the new lawn has been mown at least two or three times before applying any granular feed. For a newly seeded autumn lawn, use a pre‑seed or post‑seed fertiliser with higher phosphorus content first to support root establishment. Once the lawn is established and has had its third or fourth cut, you can apply a light autumn hardener at a reduced rate (half the normal label rate) to toughen the young turf before winter without overwhelming the immature root system.

Ready to harden your lawn for winter?

Use the calculator to find your exact autumn feed quantity, then choose a low‑nitrogen, high‑potassium product to protect your UK lawn through frost and cold weather.

Shop Autumn Lawn Feed