Turf and Grass Seed Calculator for Your Lawn

How the calculation works

Turf and grass seed: how to work out the right quantity

Anyone turning a rough garden into a finished lawn faces the same question: how many turf rolls to order, or how much seed to buy? Order too little and a second trip to the garden centre becomes necessary, and turf from the same grower rarely matches exactly from one delivery to the next. Order too much and the surplus is money wasted. Turf also has a short window for laying once it has been cut, so getting the figure right first time saves both time and frustration.

The formula: area plus margin

The starting point is always the area to cover, in square metres. For a rectangle, multiply the length by the width. For irregular shapes, such as gardens with curves, beds to leave out or obstacles to work around, the simplest method is to sketch the area on squared paper and count the squares, or to divide the garden into rectangles and add their areas together.

The wastage margin allows for cutting. Straight edges against a fence or wall produce very little waste. Corners, curves and access points produce a lot more, because each cut leaves an offcut that cannot easily be used elsewhere. For a simple rectangular or L-shaped garden, a margin of 10% is usually enough. For gardens with curves or many obstacles, increase it to 15 to 20%.

The number of rolls is found by dividing the total area, including the margin, by the area of one roll, and rounding up. Turf is sold in whole units, so a fraction of a roll is always rounded up to a complete one. For seed, the total area is multiplied by the sowing rate. The calculator uses a fixed rate of 35 g/m², which is built into the formula and does not change with your soil or seed mix. If the seed pack gives a different rate, the pack rate takes precedence, and the seed quantity should be recalculated by hand as area multiplied by the pack rate.

What is the area of a roll of turf?

There is no single standard size. The area of each roll depends on the supplier and on the cutting equipment used at the grower, so it should always be confirmed on the product data sheet or with the supplier. The calculator uses 1 m² as its default, which corresponds to a roll of 0.4 m by 2.5 m (about 1.3 ft by 8.2 ft). Some suppliers sell smaller slabs, and for large jobs wider rolls are laid by machine.

The difference has a direct effect on the number of units. For the same 44 m², that is 44 rolls of 1 m², but 88 slabs if the slabs were 0.5 m² each. Before comparing offers, convert them all to price per square metre, and enter the real roll area into the calculator so that the quantity matches the product you will actually receive.

Turf or seed?

Turf is the choice for anyone who wants an immediate result. Once laid and watered, a turf lawn can usually be walked on within a few weeks, although it should be kept off until the roots have knitted into the soil. Turf is generally sold through large DIY retailers, garden centres and specialist turf growers, and deliveries are often made on pallets. A common mixture is a blend of tall fescue (Festuca arundinacea) and perennial ryegrass (Lolium perenne), which is widely used across the UK. Suppliers quote a price per square metre, and it is worth checking the current figure before you commit.

Seed is considerably cheaper in materials, but it takes longer. Good-quality mixtures containing fine fescues, tall fescue and perennial ryegrass are sold in bags of various sizes, so check the weight printed on the label. At the 35 g/m² rate, a 44 m² area needs 1.54 kg of seed, and an 88 m² area needs 3.08 kg. The material cost of seed is much lower than that of turf for the same area, but the lawn needs several weeks to establish, and the young grass must be kept evenly moist during that time.

Common mistakes that cost money

The most frequent mistake is to add no margin at all. A badly measured flower bed, a tree left in the middle of the lawn or a paved path can throw the count out, and the shortfall only becomes obvious once the first rolls are down.

The second mistake is comparing turf prices per roll without checking the area of each roll. Because formats vary between suppliers, a cheaper roll can work out more expensive per square metre.

The third mistake, specific to seed, is sowing too thickly. A heavier rate does not give a better lawn. Crowded seedlings compete with one another and establish poorly, so the extra seed is wasted. A thin patch is better repaired with a light overseed at the right time of year than with a heavier first sowing.

Preparing the ground: the step that decides the result

Whether you lay turf or sow seed, the outcome depends largely on the soil underneath. Cultivate and loosen the ground to a full spade depth, then level it and remove stones and roots. On heavy clay soils, or on ground compacted by building work, incorporating coarse sand and a layer of good-quality topsoil before laying the turf makes a real difference. Topsoil should meet the British Standard for topsoil specification, BS 3882, before it is spread.

Timing matters too. In the UK, autumn and spring are generally the best seasons to lay turf or sow seed, because temperatures are milder and natural rainfall reduces the need for watering. Summer installation is possible, but only with a reliable watering plan, because newly laid turf dries out quickly in hot, dry weather. Work laid during a cold spell may take longer to root, so the autumn and spring windows are the safer choice for most gardens.

Cases the calculator does not cover

The calculator works on flat or near-flat areas. On steep slopes, the actual surface is larger than the plan area, so the margin should be raised to allow for the extra material. Overseeding an existing lawn is a different job from establishing a new one, usually with a lower sowing rate, so the 35 g/m² figure should not be used for thin patches on an established lawn. The calculator also does not work out topsoil volumes, drainage, irrigation or edging, which should be planned separately.

When to hire a professional

For smaller gardens on level ground, laying turf is a job a careful DIYer can manage, provided the soil is prepared well. For larger areas, slopes, difficult soils or gardens with an integrated irrigation system, a landscaper or turf installer is the safer choice, because errors are hard to correct once the turf is down. Labour costs vary with the complexity of the ground and with the region, so it is worth getting several quotes before deciding between doing the work yourself and hiring a professional.