Fertilizer bags say "covers 5,000 square feet." Grass seed is rated per 1,000 square feet. Mulch is sold by the cubic yard. If you measured your garden in meters, or your lawn is an irregular shape, getting from your measurement to the right product quantity takes a few steps — but it's not complicated once you know the approach.

Step 1: Measure Your Area

For a simple rectangular lawn or garden bed, the area is length × width. A lawn that's 20 meters long and 12 meters wide is 240 square meters. A garden bed that's 15 feet × 8 feet is 120 square feet.

For irregular shapes, break the area into rectangles, triangles, and circles:

Triangles: area = ½ × base × height. A triangular corner bed that's 4 meters wide at the base and 3 meters deep has an area of ½ × 4 × 3 = 6 m².

Circles: area = π × radius². A circular flower bed with a 2-meter radius has an area of 3.14159 × 4 = 12.6 m².

Irregular shapes: Divide into rough rectangles and triangles, calculate each, and add them up. For most garden purposes, being within 5–10% is accurate enough to buy the right amount of product.

For large properties, walking the perimeter with a measuring wheel is faster than tape-measuring. GPS apps on a smartphone can also estimate area by walking the boundary — useful for lawns over 500 square meters.

Step 2: Convert to the Units on the Product

Most UK and European garden products are rated per 100 m² or per hectare. Most US and Canadian products are rated per 1,000 square feet or per acre. If your measurement is in different units, convert before calculating how much to buy.

Use the area converter to convert between square meters, square feet, acres, and hectares. The most common conversions for garden use:

Example: You have a lawn of 350 m² and a fertilizer bag covers 3,500 square feet. Convert 350 m² to square feet: 350 × 10.764 = 3,767 sq ft. You need slightly more than one bag.

How Much Fertilizer Do You Need?

Fertilizer application rates are typically given in one of three ways:

Per 1,000 square feet (US): Common for granular lawn fertilizer. Multiply your lawn area in sq ft by the rate, then divide by 1,000.

Per 100 m² (UK/Europe): Common for granular and liquid fertilizers. Multiply your area in m² by the rate, then divide by 100.

Per hectare (agricultural): Standard for large-scale application. Divide your area in hectares by 1 and multiply by the rate.

Example calculation: Your lawn is 280 m². A lawn fertilizer recommends 35g per m². Total needed: 280 × 35 = 9,800g = 9.8 kg. A 10 kg bag will just cover it.

Applying Too Much Fertilizer

Over-application is a genuine problem, not just a waste of money. Excess nitrogen causes lawn burn (grass turns yellow or brown), promotes excessive top growth at the expense of root development, and contributes to fertilizer runoff into waterways. Calculating the correct amount — and spreading it evenly — produces better results than guessing.

Grass Seed Coverage Rates

Grass seed coverage depends on whether you're overseeding an existing lawn or starting from bare soil:

• Overseeding (thickening an existing lawn): Typically 15–35g per m², or 0.5–1.0 lb per 1,000 sq ft
• New lawn from bare soil: Typically 25–50g per m², or 1.0–1.5 lb per 1,000 sq ft

Bare soil seeding needs more seed because germination rate is lower and seedlings face more competition from weeds before they establish.

Example: Seeding 150 m² of bare soil at 35g/m²: 150 × 35 = 5,250g = 5.25 kg of seed. Round up to 6 kg to allow for edges and irregular coverage.

Mulch and Compost: Calculating Volume

Mulch, compost, and topsoil are sold by volume (cubic yards in the US, cubic metres in the UK/Europe), not by area. To calculate how much you need, you have to factor in the depth of application:

volume = area × depth

For mulch, a standard application depth is 7–10 cm (3–4 inches) for weed suppression and moisture retention. For compost worked into a bed, 5–7 cm is typical.

Example (metric): Garden bed of 8 m² with 8 cm mulch depth: ` volume = 8 × 0.08 = 0.64 m³ ` A bulk bag typically contains 0.75–1 m³, so one bag covers this bed with some left over.

Example (US): Garden bed of 80 sq ft with 3-inch mulch depth: ` volume = 80 × (3/12) = 20 cubic feet 1 cubic yard = 27 cubic feet So you need 20/27 ≈ 0.74 cubic yards ` A standard cubic yard bag of mulch would cover this with some left over.

Weed Killer and Lawn Treatment Products

Most lawn weedkillers and treatments are applied as liquid concentrates diluted in water, with coverage rates given per litre of diluted solution or per square metre.

The calculation is the same as fertilizer: multiply your area by the application rate to get the total amount needed.

For contact weedkillers (which kill what they touch), accuracy is important — you don't want to apply to areas you're not targeting. For selective lawn weedkillers (which kill broadleaf weeds but not grass), even coverage matters but slight overlap on borders is fine.

Practical Tips for Accurate Coverage

Measure twice, buy once. A few minutes of careful measurement before you go to the garden centre saves a second trip. For irregular lawns, break the area into simple shapes and add them up rather than estimating.

Add a 10% buffer. For any area that has curves, edges, or irregular corners, buy slightly more than the calculated amount. Coverage rates assume even application on flat ground — slopes, edges, and uneven surfaces use product less efficiently.

Keep the product packaging. Note the actual coverage you got from each bag or bottle. Over time, this gives you reliable calibration for your specific spreader or sprayer and your local conditions.

Use the area converter whenever the product units don't match your measurement units. Converting 320 m² to square feet before calculating a US-rated product takes 10 seconds and prevents buying the wrong quantity.