Fertiliser Calculator

How Much Fertiliser Do I Need?

So your plants look hungry, the fertiliser aisle's confusing, and you don't want to burn anything. Here's the honest version.

By Gary Hodson · Ex-Army. I do the sums so you buy the right amount. · Rates cross-referenced against RHS guidance and named manufacturer labels · Last reviewed 4 October 2026

Fertiliser is one of those garden jobs where the bag's instructions can be worse than doing nothing. Too little and the borders stay pale all summer. Too much and the tips of the grass or the leaves of your tomatoes yellow off within a week. That's fertiliser burn, and it's far easier to avoid than to fix. Most labels assume you know what "per square metre" means on your actual plot, which most gardeners don't, which is why the calculator at the top of this page exists.

Enter your area in metres (length × width for a rectangle, split L-shapes into rectangles, round irregular plots to the nearest square metre), pick your product, and it returns the exact weight to apply and the number of packs to buy. The rest of this page is the context that makes the number trustworthy, particularly around timing, NPK ratios, and the one thing you really want to avoid, which is feeding anything that's already stressed by heat, cold, or drought.

What the NPK ratio on the bag actually means

Every bag of fertiliser carries three numbers, usually separated by dashes: 7-7-7 on Growmore, 10-10-10 on a general-purpose granular, 14-2-5 on a spring lawn food. The N is nitrogen (leaf and stem growth). The P is phosphorus (roots and establishment). The K is potassium (flowers, fruit, and cold hardiness). A balanced feed has roughly equal numbers and is safe for most borders. A high-N feed drives lush leafy growth (lawns in spring, salad crops, greenhouse tomatoes in early growth). A high-K feed is for fruit, flowers, or autumn lawn hardening. You don't need to memorise every ratio. Matching the N:K balance to the season and the plant does 90% of the work.³⁵

The other number that matters and isn't always on the front of the bag is the application rate in g/m². Standard granular feeds run 30–70 g/m² depending on product. Blood, fish and bone is typically 70–100 g/m². Chicken manure pellets go higher at 100–150 g/m².¹¹ Liquid concentrates are measured in millilitres per 10 L of water instead. The calculator handles all these units, but the rate is why two similar-looking bags can do very different jobs, so always check the rate before you spread anything.

When to feed, and the months to skip entirely

The UK feeding year has two windows. March to September is the main push: lawns, veg, flowering borders, and container plants all benefit from feeding every four to eight weeks during active growth. Inside that window, stop feeding two to three weeks before the end of each crop's season (late tomatoes, for example, don't need nitrogen in late August). September to October is the autumn window, with a last high-K lawn feed to toughen grass against frost, and nothing for perennials or trees already going dormant.

November to February: don't feed anything. Grass is dormant, soil biology is slow, and fertiliser applied to cold or waterlogged soil leaches straight through into the watercourse. That wastes your money and pollutes the water.¹ The single biggest timing mistake UK gardeners make is "giving the lawn a boost" in late February before soil temperatures are reliably above 5 °C. You're feeding nothing that can absorb it, and you risk burn once growth finally restarts.

Fertiliser burn — what actually causes it

Fertiliser burn is a salt problem, not a nutrient problem. The concentrated mineral salts in feed draw moisture out of plant roots when too much is applied, leaving scorched brown patches on lawns, yellowed tips on shrubs, or crispy edges on container plants. The three triggers are over-application (going over the rate on the pack), applying in drought (no soil moisture to dilute the salts), and applying to wet foliage (granules stick to leaves and scorch them directly). All three are avoidable: measure accurately with the calculator, feed before expected rain or water it in yourself within twelve hours, and don't feed bone-dry soil.

If you've already over-applied (brown stripes on the lawn, yellowing leaf tips on the borders), soak the area heavily for 24 to 48 hours to flush the excess salts through the root zone. Lightly scorched grass may recover once the salts are flushed out, but there's no set timescale. Dead patches (where the grass pulls away easily with no root resistance) need reseeding. Our grass seed calculator works out the seed quantity for bare spots.

Which type of fertiliser for which job

For a typical mixed UK garden, three products cover 90% of what you'll need. Growmore (7-7-7) is the reliable general-purpose granular: borders, veg, shrubs in spring, and a lighter mid-summer top-up. Blood, fish and bone is the slow-release organic alternative. It takes four to eight weeks to release, perfect for planting new roses and shrubs where you want gentle feeding without the burn risk. Chicken manure pellets are the cheapest nitrogen per kilo if you've got a lot of leafy crops to feed, but they smell fierce for 48 hours after application, so apply on a still day with rain forecast.⁵

Lawns need a specialised feed rather than general-purpose granular: our dedicated lawn feed calculator covers spring, summer, and autumn formulations separately because the NPK balance shifts by season. For greenhouse tomatoes, buy a tomato-specific liquid feed (high K, around 4-5-8 or similar) rather than using garden-grade fertiliser, because the nutrient ratio matters once flowering and fruiting start. Whatever you pick, use the calculator above to get the exact weight before you open the bag.

Fertiliser Application Rates

Different fertilisers need different application rates. Use this guide alongside the calculator to choose the right product and dose for your needs.

How to Apply Fertiliser Evenly

Getting an even spread matters as much as using the right amount. Patchy application leads to stripes on lawns and uneven growth in borders.

For lawns

A handheld or wheeled spreader gives the most even coverage. If spreading by hand, divide your total fertiliser into two halves: apply the first half walking north-to-south, then the second half walking east-to-west. This crosshatch pattern avoids missed patches and double-dosing.

For beds and borders

Weigh out the correct amount and scatter evenly by hand, then lightly fork or rake it into the top few centimetres of soil. Water well afterwards, because dry granules sitting against plant stems can cause burns.

Handful guide

A level adult handful of granular fertiliser weighs roughly 35–40 grams. Two handfuls is about 70g, enough for one square metre of flowers at the Growmore rate. Before planting, Westland's rate is 135g, so roughly four handfuls. The calculator shows a handful estimate in the results to help you spread without scales.⁵

Weed and Feed: How Much Do I Need?

Miracle-Gro EverGreen Complete 4-in-1 weed and feed goes on at 35g per square metre. Other brands vary, so pick yours from the list in the calculator above for the exact amount.⁷ Check the pack for mowing gaps: the EverGreen Complete label says don't mow for 2 days before or after.⁷

For timing and product comparisons, use our lawn feed calculator. It covers lawn feeding for every season in full.

Feeding your veg plot? Our vegetable sowing calendar shows exactly when each crop needs planting and feeding. Check the planting calendar for this month's timing.

Frequently Asked Questions

Types of Fertiliser for UK Gardens

Walk into any garden centre and the fertiliser aisle can feel overwhelming. The labelling is needlessly confusing. Here is the plain-English breakdown.

Understanding NPK Ratios

Every fertiliser bag shows three numbers separated by dashes, for example 7-7-7 or 20-5-10. These are the NPK ratio, and they tell you the percentage by weight of the three main plant nutrients:

N (Nitrogen) drives leaf and stem growth. A high first number means lush, green foliage. This is what makes your lawn thick and dark green. P (Phosphorus) promotes strong root development and helps plants establish after transplanting. K (Potassium) strengthens cell walls, improves disease resistance, and boosts flower and fruit production. A high third number is what you want for tomatoes, roses, and autumn lawn hardening.

A balanced fertiliser like Growmore (7-7-7) gives equal parts of each nutrient: a safe, general-purpose choice when you are not sure what your soil needs. A spring lawn feed might be 20-5-10 (heavy on nitrogen for growth), while an autumn lawn feed might be 5-5-15 (heavy on potassium to toughen roots before winter).

Fertiliser Comparison Table

Organic vs Synthetic — Which Should I Use?

Organic fertilisers (blood, fish & bone, chicken manure, bonemeal) release nutrients slowly as soil microbes break them down. They improve soil structure over time and are less likely to cause fertiliser burn. The downside is they are slower to act (you will not see results for 2-4 weeks) and they are bulkier per nutrient delivered. They suit borders and veg patches where you want long-term soil health.¹

Synthetic (manufactured) fertilisers deliver precise NPK ratios and can act within days. They are ideal for lawns where you want a quick green-up, and for container plants that need regular feeding. The risk is over-application: too much synthetic fertiliser scorches roots and can leach into groundwater. A common approach is synthetic lawn feed in spring for a quick result, and organic feeds such as chicken manure pellets on beds.

For most UK gardens, a combination of both works best. Use our compost calculator to work out how much organic matter to add alongside your fertiliser for the best soil improvement.

How to Fertilise Your Garden — A Practical Guide

Get the feeding wrong and you get scorched lawns, leggy tomatoes and yellowing roses. Here is how to feed each part of a UK garden, with the measurements.

Feeding Your Lawn

The Miracle-Gro EverGreen Complete label gives 35g per square metre.⁷ Autumn feeds vary: the Miracle-Gro autumn label caps the dose at 35g/m²,⁸ while the Harrowden tub we link uses 20g/m².¹⁰ Go by your own pack. For when to feed, how often and how much for your lawn size, use our lawn feed calculator.

Feeding Borders and Flower Beds

For established borders, apply Growmore (7-7-7) at 135g/m² around the base of the plants. Westland says you can repeat it every 4-6 weeks from February to the end of October. Scatter the granules around the base of plants (not directly against stems) and fork lightly into the top 2-3cm of soil. Water well afterwards.⁵

For roses, use a dedicated rose feed with higher potassium (something like 5-5-10) at 35g per plant, applied in April and again after the first flush of flowers in July. Scratch it into the soil around the drip line, about 30cm from the stem.

Newly planted borders benefit from bonemeal (3.5-15-0) at 70g/m² worked into the planting hole. The high phosphorus promotes root establishment through the first season. Mix it with the backfill soil rather than sprinkling it on top.¹

Feeding Your Vegetable Patch

Vegetables are hungry plants. Before sowing or planting out in spring, work Growmore into the soil at 135g/m². That's roughly four level handfuls per square metre. Once crops are up, Westland drops the rate to 70g/m².⁵

Once crops are growing, side-dress with a balanced feed every 4-6 weeks.⁵ For tomatoes and peppers, switch to a high-potassium liquid feed (Levington's Tomorite label says 20ml in 4.5 litres of water, which is about 4.4ml per litre)¹² once the first fruits set. The RHS says fruiting crops in pots need a liquid feed every two weeks, or weekly if you use a homemade feed.²

Check our planting calendar for the exact sowing and feeding windows for 30+ UK vegetables.

Feeding Container Plants

Containers are a special case because nutrients wash through with every watering. Compost in pots runs out of feed after about 6 weeks. After that, you need to supplement with either slow-release granules mixed into the top layer (one application in April lasts until August) or liquid feed every 7-10 days through the growing season.²

For hanging baskets and window boxes, one option is a liquid feed at half the recommended strength but twice as often, which gives a steadier supply without the feast-or-famine cycle. At the Tomorite label rate of 20ml in 4.5 litres, a 1-litre bottle makes 50 cans of feed (1,000 ÷ 20 = 50), about 225 litres in all.¹²

UK Seasonal Fertilising Calendar

How Much Does Fertiliser Cost in the UK?

Prices shift with the season and the shop, so we don't print a price list here. Compare packs on cost per square metre, not price per box. Divide the pack price by the area it covers at your rate. Bigger packs usually win on that sum.

If you are also improving your soil structure, consider adding compost alongside your fertiliser. Our compost calculator works out how much you need and what it will cost. For new lawns, pair feeding with the right amount of grass seed to get the best establishment. Building new beds or topping up borders? Use our soil calculator to work out exactly how much topsoil you need before you start feeding.

5 Common Fertiliser Mistakes (and How to Avoid Them)

Each of these costs time, money or a dead patch of lawn. Here is how to avoid them.

1. Over-feeding Your Lawn

A classic mistake is doubling up on fertiliser because "more must be better." It is not. Applying 70g/m² of a spring lawn feed when the recommended rate is 35g/m² will scorch the grass within 48 hours. You will end up with yellow-brown stripes that take weeks to recover. Always measure your lawn area first (our calculator does this for you) and weigh the fertiliser rather than guessing by eye. A level adult handful is approximately 35-40g, which is roughly the right amount for one square metre of lawn feed.⁵⁷

2. Using the Wrong NPK for the Season

Applying a high-nitrogen spring feed (20-5-10) in October is counterproductive. Nitrogen pushes soft, lush growth that frost will damage overnight. In autumn, you need the opposite: a feed high in potassium (5-5-15) to harden the grass and strengthen roots for winter. Marking spring and autumn feeds with different coloured tape stops you grabbing the wrong bag.⁶

3. Feeding During Drought or Extreme Heat

Granular fertiliser needs moisture to dissolve and reach the roots. Spreading it on bone-dry soil in a July heatwave means it just sits there, concentrated, waiting to burn any plant tissue it touches when the next rain arrives. If you must feed during a dry spell, water the area thoroughly the evening before, apply the fertiliser in the morning, and water again immediately afterwards. Better yet, wait until the weather breaks.⁵

4. Ignoring Soil pH

If your soil pH is below 5.5 or above 7.5, most nutrients become locked up regardless of how much fertiliser you apply. Before spending money on feed, test your soil pH with a simple kit (around £6 from any garden centre). Most UK garden soils sit between 6.0 and 7.0, which is fine. If yours is acidic (below 6.0), the RHS says lime usually goes on in winter, and the amount depends on your pH test and soil type. Don't mix lime with fertiliser or manure.⁹ If your soil needs improvement beyond pH, check our soil calculator for guidance on adding topsoil.

5. Not Watering In After Application

This one catches people every year. You scatter your lawn feed on a sunny afternoon, feeling productive, then forget to water. The granules sit on the grass blades in direct sunlight and the concentrated salts burn the leaf tissue. The result is brown spots exactly where each granule landed. Always water within 24 hours of applying granular fertiliser if rain is not expected. A phone reminder helps.

More Fertiliser Questions Answered

Choosing Fertiliser: A Buyer's Checklist

The fertiliser aisle is mostly marketing. Before you put anything in the trolley, here is what actually matters (what the RHS recommends checking), followed by the feeds we point readers towards.

• Match the NPK numbers to the job. The three numbers on every bag (nitrogen, phosphorus and potassium) are shown by law,³ and they tell you what the feed is for: nitrogen drives leafy growth and lawns, phosphorus builds roots, and potassium pushes flowers and fruit.¹ That is why a high-potassium "potash" feed is the one for tomatoes and fruit trees, which need plenty of potassium to crop.⁴
• Decide organic or synthetic, and fast or slow. Organic feeds (pelleted manure, blood, fish and bone, seaweed) are usually less concentrated and slower acting, which makes them gentler to use and typically gives them a smaller carbon footprint; synthetic feeds are more concentrated and faster, but must be applied to the instructions to avoid plant damage. Slow-release granules suit feeding at planting; a liquid feed gives a quick mid-season boost and is well suited to plants in containers.¹
• Be honest about whether you need it at all. The RHS is plain about this: most UK soils already feed plants perfectly well without routine fertiliser.¹ It is mainly fruit, vegetables and anything growing in a container that genuinely needs feeding,² so buy for those jobs rather than dosing the whole garden out of habit.
• Buy for the right season. The feeding window is roughly spring to early autumn: slow-release and organic dry feeds go on around February to March, faster-acting granular feeds a little later, and liquid feeds from April to September.¹ Avoid feeding after September or through winter. Very few plants need it, and the soft growth a late feed forces is easily damaged by cold.²
• Follow the rate: more is not better. Stick to the dose on the pack, keep granular feed off the foliage (fertiliser on the leaves is what causes scorch), and remember that overfeeding (even with organic feeds) leaches out and pollutes waterways.²¹ Use the calculator above to get the exact weight for your area, so you buy the right pack size and apply the right amount.

Sources

Figures and guidance on this page were checked against the sources below. Each entry shows the date we last checked it. Manufacturer instructions supplied with a product always take precedence.

1. Royal Horticultural Society — Fertilisers: types and uses. rhs.org.uk/garden-jobs/fertilisers. Accessed 4 October 2026. ↩
2. Royal Horticultural Society — How to feed plants. rhs.org.uk/garden-jobs/nutrition-feeding-plants. Accessed 4 October 2026. ↩
3. Royal Horticultural Society — Fertiliser labels explained. rhs.org.uk/garden-jobs/fertiliser-labels-explained. Accessed 27 June 2026. ↩
4. Royal Horticultural Society — Fruit trees: feeding and mulching. rhs.org.uk/fruit/fruit-trees/feeding-and-mulching. Accessed 27 June 2026. ↩
5. Westland Garden Health — Westland Growmore plant food (rates and analysis). gardenhealth.com/westland-growmore-plant-food. “1 handful of Growmore is about 35g.” Accessed 4 October 2026.
6. Royal Horticultural Society — Monthly gardening tips: October – Lawns. rhs.org.uk/advice/in-month/october/lawns. Accessed 12 September 2026.
7. Miracle-Gro — EverGreen Complete 4-in-1 pack label (PDF). lovethegarden.com pack label. “Maximum individual dose: 35g per square metre.” Accessed 4 October 2026.
8. Miracle-Gro — Autumn Lawn Care pack label (PDF). lovethegarden.com pack label. “Use between September and mid November.” Accessed 4 October 2026.
9. Royal Horticultural Society — Lime and liming. rhs.org.uk/soil-composts-mulches/lime-liming. Accessed 4 October 2026.
10. Homebase — Harrowden Rejuvenator Autumn & Winter Lawn Feed 2kg (product listing). homebase.co.uk product page. “Each tub covers 100m² at the recommended dose of 20g/m².” Accessed 4 October 2026.
11. Royal Horticultural Society — Chicken manure. rhs.org.uk/soil-composts-mulches/chicken-manure. “An average dose for top-dressing is 150g per sq m (4½oz per sq yd) for most crops”; 100g per sq m for less hungry crops. Accessed 4 October 2026.
12. Levington — Tomorite Concentrate 1L pack label (PDF). lovethegarden.com pack label. “DOSING RATE: Tomato 20ml for 4.5 litres (1 gallon) of water.” Accessed 4 October 2026.

Last updated 4 October 2026 by Gary Hodson, GardenCalc.