Free Online Calculators
Calculate fertiliser amount for your lawn
This calculator works out how much fertiliser your lawn needs: enter the lawn area in m², pick a fertiliser type with its default rate or type the rate from the label in g/m². It shows the total in kg and the number of 25 kg bags. Example: 100 m² of universal fertiliser at 30 g/m² needs 3 kg, one bag.
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Measure your lawn's area, then work out how much nitrogen the grass should get over that area and divide by the nitrogen percentage shown on the bag. The result is the weight of product to spread; splitting it into two passes at right angles gives more even coverage.
Follow the application rate printed on the bag for your lawn's size, since the right amount depends entirely on the nitrogen content of that particular product. More is not better — excess nitrogen burns the blades, encourages disease and leaches into groundwater.
That depends on the analysis on the bag: a low-nitrogen organic product needs far more weight per given area than a concentrated synthetic one to deliver the same nutrients. Read the nitrogen percentage and calculate backwards from the nitrogen rate you want.
The coverage is printed on the bag and varies with the product's nutrient concentration, so two bags of the same weight can cover very different areas. Always use the manufacturer's stated coverage rather than assuming a standard figure.
It works, but it's not ideal for established lawns: grass mainly needs nitrogen, and a balanced blend supplies more phosphorus than most established turf can use. Balanced products suit new seedings or soils that a test has shown to be short on phosphorus and potassium.
Neither is inherently better; they are the same balanced ratio at different strengths, so the more concentrated one simply needs less product per area. Choose based on what your soil test calls for and what's cheaper per unit of nutrient.
There isn't one — the best choice depends on your grass type, soil test results and the season. A slow-release nitrogen product suits most established lawns, while new seedings and nutrient-deficient soils need something matched to the deficiency.
A lush, green lawn doesn't happen by accident. It takes the right care, the right timing, and crucially, the right amount of fertiliser. Apply too little and your grass stays thin and patchy. Apply too much and you risk burning your lawn or creating an environmental hazard. Our free Lawn Fertiliser Calculator takes the guesswork out of the equation, helping you work out exactly how much fertiliser your lawn needs based on its size and the product you're using.
Fertilising your lawn is one of the most effective ways to promote healthy growth, improve colour, and strengthen the turf against weeds and disease. But the benefits only come when you apply fertiliser correctly. Here's why accuracy matters:
Using the calculator is straightforward. You don't need any specialist knowledge – just a few basic measurements and the details from your fertiliser packaging.
It's that simple. The only conversion you may need is from your label to grams per square metre.
Walk into any garden centre and you'll find shelves packed with different fertiliser products. The key to choosing the right one lies in understanding the NPK ratio – the three numbers printed on every bag representing Nitrogen (N), Phosphorus (P), and Potassium (K).
Seasonal lawn fertilisers are formulated with these needs in mind. A spring feed might be labelled 20-5-10, while an autumn product could read 5-5-20. Always match your fertiliser choice to the time of year and your lawn's specific needs.
Timing is just as important as quantity. Fertilising at the wrong time of year can do more harm than good.
Always check the weather forecast before applying. Fertiliser should be applied to moist soil but not before heavy rain, which can wash it away before absorption.
Even with the perfect quantity calculated, uneven spreading leads to patchy results. Here are some practical tips for a professional finish:
For a rectangular or square lawn, simply multiply the length by the width in metres. For L-shaped or curved lawns, break the area into smaller geometric shapes, calculate each individually, and add the totals together. A measuring wheel or a long tape measure makes this much easier.
The calculator takes the rate in g/m² only, so convert first: 1 kg per 100 m² = 10 g/m², and 1 lb per 1,000 sq ft ≈ 4.9 g/m². Your lawn area also needs to be in square metres (1 sq ft ≈ 0.093 m²).
Yes, absolutely. Applying too much fertiliser – particularly products high in nitrogen – can cause fertiliser burn, which appears as yellow or brown streaks and patches. It can also lead to excessive top growth at the expense of root development, making your lawn less resilient over time.
Most lawns benefit from two to four applications per year. A typical schedule involves a spring feed, an optional early summer top-up, and an autumn feed. Avoid applying more frequently than the product label recommends, as this can lead to build-up and imbalances in the soil.
Only partly. The result is always a weight in kg and a number of 25 kg bags. For a liquid product, multiply your lawn area by the label rate yourself – for example 100 m² × 20 ml/m² = 2 litres of concentrate.