NPK stands for nitrogen (N), phosphorus (P) and potassium (K), three nutrients used by grass. The numbers on a fertiliser label show its declared nutrient analysis in that order. They help you compare products, but the application rate is a separate instruction: use the amount stated on the label for your lawn.
What do the three nutrients do?
Nitrogen supports leaf and stem growth. Phosphorus supports roots and shoots, while potassium contributes to plant functions including cold hardiness. Grass also needs other nutrients; the NPK numbers do not describe everything in a product or the existing soil.
Reading the numbers on a UK label
A UK fertiliser label commonly expresses the phosphorus and potassium figures as phosphate (P₂O₅) and potash (K₂O) equivalents. These are not the same as the percentages of elemental phosphorus and potassium. The RHS guide to fertiliser labels explains the distinction and shows an example of the elemental values alongside the oxide figures.
For example, a hypothetical 10-5-5 oxide-based analysis declares 10% nitrogen, 5% phosphate equivalent and 5% potash equivalent by weight. It does not mean the product contains 5% elemental phosphorus and 5% elemental potassium. Check which convention is used before comparing numbers from different labels or websites. A larger number does not by itself tell you whether a feed is right for your lawn, how quickly it releases nutrients or how much to spread.
Choosing a feed for the job
Look at the complete label: the intended season, application rate, watering advice and recommended interval between treatments. Lawn feeds formulated for autumn can differ from those intended for spring and summer. If you are sowing seed, check that the specific product is suitable for new grass and follow its instructions.
More nitrogen is not automatically better. Applying too much fertiliser can damage plants, and incorrect use can pollute water. Measure the lawn, spread at the specified rate and keep fertiliser away from drains and watercourses. Follow the label for storage and when people and pets may return.
Does a thin lawn always need feed?
No. Shade, wear, drought, waterlogging, close mowing and compacted ground can all leave grass looking thin. Look at the pattern and growing conditions before choosing a treatment. A lawn that is heavily shaded or regularly walked over may need a change in conditions as well as repair.
For more on nutrient use and application, see the RHS fertiliser guide. If you are planning to repair bare areas, our grass seed guide will help you choose a mixture.