Fertiliser Calculator

Fertiliser application rates are specified on the product packaging in grams per square metre (g/m²). Common UK lawn fertilisers recommend 35–70 g/m² depending on the product and season. Spring/summer feeds are nitrogen-rich (for growth), while autumn feeds are higher in potassium (for root strength and disease resistance).

This calculator takes your lawn or garden area and the product's recommended application rate to calculate the total weight needed. Applying too much fertiliser risks scorching the grass or causing excessive soft growth that is vulnerable to disease. Always weigh the product rather than estimating by hand.

Enter the area and application rate (from the product label). The calculator shows total weight, number of boxes or bags needed and coverage. For liquid feeds, it converts to litres of diluted solution. Water the lawn before applying granular fertiliser and again lightly afterwards to help it reach the roots.

How much fertiliser do you need? Calculation: area × application rate. Lawn fertiliser: 30-50 g/m² spring; 25-35 g/m² autumn. Vegetable garden general: 60-80 g/m² for hungry crops (brassicas, leafy greens); 25-40 g/m² for legumes and root crops. Border fertiliser (Growmore or fish-blood-bone): 70-140 g/m². Sample 100 m² lawn × 40 g/m² spring feed = 4 kg fertiliser. Buying: 20 kg bag covers 400-700 m² lawn = £15-£40/bag general purpose; £25-£60/bag organic.

UK fertiliser types — NPK ratios decoded. NPK = Nitrogen-Phosphorus-Potassium percentages. High-N (24-4-12) for lawns: promotes leaf growth, green colour. Balanced (10-10-10 or Growmore 7-7-7): general garden use. High-K (4-4-22): tomatoes, peppers, fruit (improves flowers + fruit). High-P (10-30-10): root vegetables, root development. Liquid feeds: faster-acting but more frequent application. Slow-release (Osmocote): one feed lasts 4-6 months. Always read product label for actual rates — varies by formulation.

UK organic fertiliser options. Fish, blood and bone: 5-5-5 balanced organic, slow-release — £6-£15/3kg. Chicken manure pellets (5-3-2): nitrogen-heavy, lawn and vegetables — £8-£20/8kg. Seaweed extract: trace minerals, biostimulant — £10-£25/L. Bone meal (4-21-0): phosphorus, root development for autumn planting. Compost as fertiliser: ~1-1-1 NPK, applied 50-100mm mulch. Comfrey tea: DIY high-K liquid feed. Pelleted chicken manure smell: temporary, dissipates 24-48 hours.

When to apply fertiliser — UK seasonal guide. March-April: spring feed (high-N for lawns, balanced for borders). May-June: side-dress vegetables. July-August: high-K for fruit/tomatoes; avoid high-N late summer (encourages soft growth, frost-vulnerable). September-October: autumn lawn feed (high-K + low-N, builds winter resilience). November-February: dormant — no fertiliser needed (wasted, runs off). Always water in after applying granular fertiliser. Never apply to wet leaves (scorch risk) or frozen ground.

Over-fertilising — common mistakes. Brown patches on lawn: usually nitrogen burn from spilled or over-applied lawn feed. Lush growth but no flowers/fruit: too much nitrogen, not enough P+K. Algae bloom in pond: fertiliser runoff. Soil nitrate buildup: excessive synthetic feeding without organic matter. UK Nitrate Vulnerable Zones (NVZs) cover ~58% of England: restrict farm fertiliser timing/quantity. Domestic gardens: no regulation but ethical considerations. Soil test every 2-3 years (£20-£40 via RHS) — most UK gardens overfed, especially in nitrogen.