Artificial Grass Calculator & Infill

What Goes Into the Numbers

The calculator starts with the total area of your garden and adds a waste percentage to cover trimming and fitting around edges. Both the grass itself and the weed membrane beneath it need this same adjusted area, because the membrane must extend right to the perimeter under every piece of grass.

Seam planning is the second half of the calculation, and it depends on your roll width. The calculator works out how many roll widths fit across the garden using the formula: widths needed = ceil(garden width ÷ roll width). If the garden is exactly as wide as the roll, or narrower, you get a single piece with zero seams. A 4-metre-wide garden paired with a 4-metre roll is the perfect example — one piece covers the full width, so no adhesive and no joining tape are needed.

When the garden is wider than one roll, seams become unavoidable. The seam length is calculated as (widths − 1) × garden length. An 8-metre-long garden requiring two roll widths produces one seam of 8 metres. Each seam needs joining tape underneath and adhesive on top to bond the two edges, with the calculator allocating 1 tube of adhesive per 5 metres of seam.

Sand infill is calculated separately from the seam logic. The formula is straightforward: total area (including waste) × your chosen infill rate in kilograms per square metre. At the standard 4 kg/m², a 24 m² lawn needs roughly 106 kg of kiln-dried sand. The infill sits between the grass fibres to weigh the surface down, keep the pile upright, and help rainwater drain through the backing. The width-based fitting approach is similar to how vinyl sheet quantities are calculated for interior floors.

Making Sense of the Output

Your results list six materials, each serving a specific role in the installation layers.

The grassArea figure is the quantity of artificial grass you need to order, expressed in square metres. It includes your waste allowance, so it will always be larger than the raw area. Order to this figure, not the bare measurement, because you will lose material to edge trimming and fitting around curves or obstacles.

AdhesiveTubes tells you how many tubes of seam adhesive to buy. This number comes directly from your seam length — 1 tube covers roughly 5 metres of join. If your result shows zero, that means your roll width covers the garden in a single piece with no joins at all. Zero-seam installations are ideal because they eliminate visible joins entirely and remove the most skill-intensive part of the fitting process.

The sandInfillKg value is the total weight of kiln-dried sand you need for infill. Sand is sold in 25 kg bags at most builders’ merchants, so divide the figure by 25 to find how many bags to buy. Round up — leftover sand stores well and you can top up infill after settling.

MembraneArea matches the grassArea figure, because the weed membrane must cover the full prepared area beneath the grass. Membrane rolls are typically 1 metre or 2 metres wide, so you will need to overlap strips by at least 100 mm to prevent weeds pushing through the gaps.

JoiningTapeM is the linear metres of self-adhesive joining tape to place under each seam before applying the adhesive on top. Like the adhesive, this reads zero if you have no seams. L-shaped gardens almost always need seams because the shape change forces at least two pieces of grass to meet at different angles or widths.

Getting It Right: Hardscape Advice

Getting a professional-looking finish from artificial grass depends on a few details during fitting that the numbers alone do not capture.

Lay every piece with the pile leaning in the same direction, ideally towards the house. When you look out of a window or patio door, grass with the pile angled towards you appears lighter and more natural. If two pieces have the pile facing opposite ways, the colour difference at the seam is obvious even with a perfect join. Check the pile direction by running your hand along the surface — it feels smooth in the direction of lean and rough against it.

Brush sand infill in using a stiff-bristled outdoor broom, working in multiple passes rather than dumping the full quantity in one go. Spread roughly a third of the sand, brush it into the fibres with short strokes, then repeat twice more. This method avoids burying the grass blades and gives a more even distribution. After the first rainfall, check for any low spots where sand has migrated and top up those areas.

Fit your grass on a dry day with no rain forecast for 24 hours. Adhesive needs a dry surface to bond properly, and wet membrane is slippery to work on. If you are joining two pieces, weight the seam down with sandbags or heavy planks overnight while the adhesive cures.

Drainage matters just as much below the grass as the grass itself. Before laying the membrane, make sure your sub-base is compacted and graded with a slight fall (roughly 1 in 80) away from the house or towards a drain. Without this gradient, water pools under the grass and creates soft, uneven patches. If your soil is heavy clay, consider a 50 mm layer of sharp sand over the compacted sub-base to improve water flow through the system. Our step-by-step fitting guide covers the full sub-base preparation process.

Factors That Change the Calculation

The default settings suit a straightforward rectangular garden on prepared ground, but several situations call for different numbers.

Odd-shaped gardens with curved borders or multiple angles produce more offcuts than a simple rectangle. Increase the waste factor to 15–20% if your garden has rounded edges, island flower beds, or paths cutting through the lawn area. Each curve means a cut, and each cut means material you cannot reuse. If you plan to surround the grass with bark mulch beds, the edging strip between them prevents infill sand from migrating.

Choosing a wider roll can reduce or eliminate seams entirely. If your garden is 5 metres wide, a 4-metre roll forces one seam, but some suppliers stock 5-metre rolls that would cover the width in a single piece. The wider roll costs more per square metre, but you save on adhesive, joining tape, and the labour of making an invisible seam. For gardens up to 4 metres wide, a standard 4-metre roll avoids seams altogether.

Pet owners should increase the infill rate from the standard 4 kg/m² to 6–8 kg/m². Heavier infill improves drainage through the grass backing, which helps flush pet waste through rather than sitting on the surface. It also adds weight that reduces movement and wrinkling in high-traffic areas where dogs run or turn frequently.

If you are laying grass onto an existing hard surface such as a concrete patio or decking, you can skip the weed membrane entirely. Weeds cannot push through a solid base, so the membrane serves no purpose. Set the membrane result to zero in your shopping list and save that cost. On a timber deck, use a breathable underlay instead of membrane to prevent moisture trapping between the grass and wood. For areas where you would rather have real lawn from seed, the maintenance commitment is higher but the feel underfoot is softer.