Fertiliser Calculator

Calculate fertiliser needed for lawns, gardens and allotments based on area and application rate.

Last updated: April 2026 · Source: RHS — Fertilisers

Per Application

5.6 kg

Total (2x)

11.2 kg

25kg Bags

1

Area

80 m²

Disclaimer

This calculator is provided for informational purposes only and should not be considered as financial or tax advice. All calculations are performed locally in your browser — no personal data is collected or sent to our servers. Rates and thresholds are sourced from HMRC and GOV.UK and are updated for the current tax year. Always verify results with HMRC or consult a qualified professional before making financial decisions.

How It Works

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.

Example: Lawn 80 m², spring feed at 50 g/m²

  1. Total fertiliser: 80 × 50 = 4,000 g = 4 kg
  2. Standard box covers 100 m²: 1 box is sufficient
  3. Apply with a spreader for even distribution
  4. Water in lightly if no rain within 24 hours

Source: RHS — Fertilisers

Frequently Asked Questions

What does the Fertiliser Calculator do?
Calculate fertiliser needed for lawns, gardens and allotments based on area and application rate. All calculations are performed in your browser using official UK rates and thresholds.
Are these based on Defra rates?
Where applicable, this calculator uses rates and data from Defra, the Rural Payments Agency and industry standard references for UK agriculture.
Does this account for regional variations?
UK farming conditions vary by region, soil type and climate. This calculator provides national average figures — adjust for your specific location and circumstances.