Hong Life CalculatorsUnderstanding the lawn seed, turf and fertiliser calculator — concept and principle
This calculator estimates how much grass seed, how many rolls of turf and how much fertiliser you need for a lawn of a given area. Enter the lawn area, the seed sowing rate, the size of one turf roll and the fertiliser application rate from the product label. It helps you compare seeding with turfing and buy the right pack sizes.
How it is calculated
- Seed (kg) = area (m²) × seed rate (g/m²) ÷ 1,000.
- Turf rolls = ⌈area × 1.05 ÷ roll size (m²)⌉, including 5% for cutting and edges.
- Fertiliser (kg) = area × fertiliser rate (g/m²) ÷ 1,000.
Dividing by 1,000 converts grams to kilograms.
The principle
Seed and fertiliser are spread at a fixed rate per square metre, so the total is just rate times area. The rate controls density: too little seed gives a thin, weedy lawn, while too much makes seedlings compete and can encourage disease. Turf is a solid surface laid edge to edge, so the count is area divided by roll size, plus a small allowance because rolls are cut along curved edges and staggered like brickwork.
Worked example
Default inputs: area 100 m², seed 35 g/m², turf rolls of 1 m², fertiliser 35 g/m².
- Seed = 100 × 35 ÷ 1,000 = 3.5 kg.
- Turf rolls = 100 × 1.05 ÷ 1 = 105 rolls.
- Fertiliser = 100 × 35 ÷ 1,000 = 3.5 kg.
| Lawn area (m²) | Seed (kg) |
|---|---|
| 25 | 0.875 |
| 50 | 1.75 |
| 75 | 2.63 |
| 100 | 3.5 |
| 125 | 4.38 |
| 150 | 5.25 |
| 200 | 7 |
How to read the result
For a new lawn, seed rates of about 35–50 g/m² are common; overseeding a thin lawn typically uses less. Shade mixes and some species have different rates, so follow the packet. Turf rolls vary by supplier, often around 1 m² in the UK and Europe, so always enter the actual roll size. Fertiliser rates differ widely between products and seasons; slow-release and high-nitrogen products are not interchangeable. Buying seed in larger packs is usually cheaper per kilogram, and a little spare seed is useful for patching later.
Common mistakes
- Measuring the garden instead of the actual lawn area, including beds and paths.
- Using one fertiliser rate for every product.
- Ordering turf without the extra for cutting around curves.
- Sowing at double rate to "make sure", which often weakens the lawn.
Frequently asked questions
How much grass seed do I need per square metre?
About 35–50 g/m² for a new lawn and roughly half that for overseeding, depending on the mix.
Is seed or turf better?
Seed is cheaper and offers more species choice but takes weeks to establish; turf gives an instant lawn at a higher cost.
When should I lay turf?
In cool, moist conditions, typically autumn or spring, and it must be watered until it roots.
Sources: Royal Horticultural Society (RHS) — guidance on creating lawns from seed and turf