NPK Calculator UK —
Lawn Fertiliser Guide

NPK stands for Nitrogen (N), Phosphorus (P) and Potassium (K) — the three primary macronutrients that all plants require in the largest quantities. On UK lawn fertiliser packaging, NPK is expressed as a ratio of percentages by weight. Nitrogen (N) is presented as elemental nitrogen. Phosphorus (P) is expressed as phosphorus pentoxide (P₂O₅) — not as elemental phosphorus — which means the actual elemental phosphorus content is approximately 43.6% of the stated P₂O₅ figure. Potassium (K) is expressed as potassium oxide (K₂O) — the actual elemental potassium is approximately 83% of the stated K₂O figure. This matters when comparing products or calculating exact nutrient delivery for specialist applications, but for standard DIY lawn care the stated ratio on the bag is sufficient.

For a UK lawn in spring (March to May), you need a high-nitrogen fertiliser — the ideal NPK ratio is approximately 12:4:4 to 18:3:5, depending on your grass type and lawn condition. The high nitrogen content (12–18%) drives the rapid green-up and leaf growth that transforms a winter-dormant UK lawn. Phosphorus should be moderate (3–5%) and potassium low to moderate (4–6%). Products matching this profile include Scotts EverGreen Spring (NPK approximately 12:4:4), ICL Greenmaster Spring Pro (15:3:4) and many own-brand spring formulas. Avoid using your autumn feed in spring — an autumn low-N, high-K formula will not provide sufficient nitrogen to stimulate spring recovery and green-up.

For a UK autumn lawn feed (September to October), you need a low-nitrogen, high-potassium formula — the ideal NPK ratio is approximately 4:5:10 to 5:5:12. The very low nitrogen (3–6%) prevents the production of soft, frost-vulnerable growth as winter approaches. The high potassium (8–12%) is the key component — it strengthens grass cell walls, improves cold tolerance and significantly improves resistance to common UK winter fungal diseases such as fusarium patch. Products matching this profile include Scotts EverGreen Autumn (4:5:10), Johnsons Autumn and Winter (5:5:10) and most own-brand autumn formulas. Applying a spring high-nitrogen feed in autumn is one of the most common and damaging UK lawn care mistakes — the resulting soft growth can be severely damaged by the first hard frosts.

To calculate the grams of nitrogen delivered per m² from a granular lawn fertiliser, use this formula: N delivered (g/m²) = Application rate (g/m²) × N% ÷ 100. For example, if you apply a 12:4:4 fertiliser at 35g/m²: N = 35 × 12 ÷ 100 = 4.2g N per m² per application. For your total lawn, multiply by the area in m². The recommended range for a spring application on a UK lawn is 3–6g N per m² per application — below 3g/m² is unlikely to produce a visible response, and above 7g/m² risks nitrogen scorch, particularly in dry weather. For a 100m² lawn at 4.2g/m², the total nitrogen applied per spring treatment is 420g. Our calculator above performs this calculation automatically for all three nutrients.

No — higher NPK numbers are not always better and can actually be harmful if applied incorrectly. The correct NPK ratio depends on: (1) the season, (2) your grass type, (3) your soil's existing nutrient levels and (4) the lawn's current condition. A very high-nitrogen product (18:3:4) is ideal in March but damaging in September. Excessive phosphorus in UK soils (which are often already phosphorus-rich) contributes to waterway pollution and is regulated. Very high application rates of any nutrient can cause scorch, salt stress and root damage. The goal is not maximum nutrient delivery but the right nutrient at the right time in the right amount. A balanced programme following correct seasonal NPK ratios consistently over 2–3 years produces a far better lawn than sporadic applications of the highest-N product available.

Fast-release nitrogen (ammonium nitrate, urea, ammonium sulphate) dissolves rapidly in soil water and is taken up by grass within 5–7 days, producing quick visible greening. However, it also leaches through the soil quickly, especially in UK rainfall conditions, and can cause nitrogen scorch if applied at high rates or on dry grass. Slow-release nitrogen uses polymer or sulphur coatings around granules that dissolve gradually over 8–16 weeks — releasing a steady, consistent supply of nitrogen that better matches the grass's growth rate and greatly reduces leaching loss. Products like Sierrablen and ICL Greenmaster use slow-release coatings — this is one of the main reasons professional contractors prefer them. For UK homeowners, slow-release products applied in spring can eliminate the need for a separate summer nitrogen feed, making them more cost-effective despite a higher pack price.

Technically yes — the NPK ratio of a blend of two fertilisers is the weighted average of both products based on their mixing proportion. For example, mixing equal weights of a 12:4:4 and a 4:5:10 gives approximately 8:4.5:7 — a reasonable summer balance. However, mixing retail lawn fertilisers is generally not recommended for DIY use because: (1) if one product contains a herbicide (weed killer) or moss killer ingredient and the other contains a different active ingredient, you risk chemical incompatibility or unintended damage to the lawn; (2) achieving a precise mixing ratio by hand is difficult; and (3) most retail granular products differ in granule size and density, making uniform application after mixing very difficult. Mixing is only recommended if you are using simple single-nutrient fertilisers (e.g. pure ammonium sulphate + potassium sulphate) and have the equipment to apply them accurately.

If your lawn fails to respond to correctly applied fertiliser, there are several common UK-specific causes: (1) Soil pH is outside the 6.0–7.0 optimum range — below pH 5.5 or above pH 7.5, nitrogen, phosphorus and potassium become chemically unavailable to grass roots regardless of how much you apply. Test your soil pH first before increasing fertiliser applications. (2) Soil compaction — heavily compacted soils prevent root development and water/nutrient penetration; hollow tine aeration is needed before fertiliser can work effectively. (3) Thatch layer — a thick thatch layer (above 10–15mm) acts as a physical barrier preventing granules from reaching the soil. (4) Dry soil — granular fertilisers require moisture to dissolve and enter the soil. Applied to very dry grass in drought, they sit on the surface, can scorch and are ineffective. (5) Waterlogged conditions — nutrients are lost rapidly from waterlogged, anaerobic soils through denitrification.