NPK fertilizer numbers: what the three figures mean and how much to apply

The three numbers on a fertilizer bag, such as 20-5-10, are percentages by weight: 20% nitrogen (N), 5% phosphate (P₂O₅) and 10% potash (K₂O). To deliver a target amount of a nutrient, divide the target by that nutrient's percentage. One pound of nitrogen from a 20-5-10 product takes 5 lb of fertilizer. The fertilizer calculator does that sum for any grade, area and bag size, and this guide explains what sits behind it.

What the three numbers mean

The grade, often called the NPK ratio, always lists nitrogen, phosphate and potash in that order. A 25 lb bag of 20-5-10 contains 5 lb of nitrogen, 1.25 lb of phosphate and 2.5 lb of potash. The other 65% is not wasted: it is the rest of the nutrient compounds, plus carriers, fillers and sometimes other nutrients such as sulphur, magnesium or iron, which are listed separately on the label. Each of the big three has a broad job. Nitrogen drives leafy growth, phosphorus supports roots and young shoots, and potassium supports flowering, fruiting and hardiness.

Oxides versus elements

By convention, the nitrogen figure is the element itself, but phosphorus and potassium are expressed as the oxides P₂O₅ and K₂O, a holdover from old laboratory methods. To convert to the elements:

• Phosphorus = P₂O₅ × 0.436
• Potassium = K₂O × 0.830

So a 10-10-10 fertilizer contains 10% N but only about 4.4% elemental phosphorus and 8.3% elemental potassium. In the UK, labels may also show elemental values alongside the oxides, so check which figure a recommendation uses before comparing products. Soil-test reports and fertilizer recommendations usually use the oxide basis, matching the bag.

How much to apply

Recommendations are usually given as an amount of nutrient per area, and the bag grade turns that into an amount of product:

Product = target rate × area ÷ nutrient percentage

• A common lawn feed is about 1 lb of nitrogen per 1,000 sq ft. With 20-5-10 that is 1 ÷ 0.20 = 5 lb of product per 1,000 sq ft, so a 5,000 sq ft lawn takes 25 lb, exactly one 25 lb bag.
• The same feed also delivers 0.25 lb of phosphate and 0.5 lb of potash per 1,000 sq ft.
• In metric, 1 lb per 1,000 sq ft is about 4.9 g per m². Rounding to 5 g of nitrogen per m², 20-5-10 needs 25 g per m², or 5 kg for 200 m².
• A 10-10-10 product needs twice as much, 10 lb per 1,000 sq ft, for the same nitrogen.

The calculator also lets you target phosphate or potash instead, and shows how much of the other two come along. If you are unsure about the percentages, How to calculate percentages walks through the arithmetic, and the area calculator handles odd-shaped beds and lawns.

Choosing a ratio

The right grade depends on what the soil lacks, not on the plant's general reputation. Lawns mostly need nitrogen, which is why lawn feeds have a big first number and little or no phosphate. Balanced grades such as 10-10-10 suit general feeding where a soil test shows no particular shortage. High-potash feeds, such as tomato feeds, support flowering and fruiting. Many soils already hold plenty of phosphorus; the RHS notes that most UK garden soils feed plants adequately without regular fertiliser. A soil test, from a lab or a home kit, is the only reliable way to know what is missing. Organic options such as compost and manure have low NPK numbers and release nutrients slowly, but they improve soil structure in ways bagged fertilizer cannot; How to make compost: greens, browns, moisture and air covers making your own.

Applying it safely

• Follow the label. Product rates reflect how quickly the nutrients are released, and slow-release and quick-release products are not interchangeable.
• Do not overdo it. Too much nitrogen scorches grass, and excess fertilizer washes into streams, where it feeds algae.
• Check local rules. Some places restrict phosphorus on lawns or ban application near water or before heavy rain.
• Spread evenly. Split the amount into two passes at right angles, then water it in if the label says to.

When reseeding at the same time, the grass seed calculator works out seed for the same area, and the compost calculator helps balance a heap for slower, organic feeding.