Artificial Grass Calculator

Key Takeaways

• An artificial grass base is 100mm of compacted MOT Type 1 on firm ground, 150mm on soft ground, topped with 15 to 25mm of grano dust screeded level. The grass sits on top of that, so the dig is about 125mm deep.
• Artificial grass copies whatever is underneath it. A dip in the base is a dip in the lawn, for good.
• MOT Type 1 is spread about 30% deeper than its finished depth, so 130mm of loose stone compacts to 100mm. The weight does not change, only the depth.
• Weed membrane under artificial grass goes on top of the compacted MOT Type 1 and under the grano dust, in 1m strips with a 100mm overlap, pegged every 500mm along the edges and the joins.
• Artificial grass comes in 2m and 4m wide rolls, and the strips all run the same way. A 5m by 4m lawn is two strips of 2m roll with one join, or one strip of 4m roll with none.
• Kiln dried sand infill for artificial grass is about 5kg per m², so a 20m² lawn takes 100kg, which is four 25kg bags.

Most artificial grass calculators work out the grass and stop. This one works out the whole job: the MOT Type 1 sub-base, the grano dust laying course, the membrane and pegs, the grass rolls and joins, the tape, the pins and the sand. Every weight on this page comes from a bulk bag we filled and measured ourselves, and the calculator adds the 30% that Type 1 loses under the compactor, which the other calculators leave out. Pick the shape, type in your measurements, and you get a materials list you can take straight to the shop, print, or copy as a link and send to whoever is doing the barrowing.

What Goes Under Artificial Grass?

Under artificial grass there is a layer of compacted MOT Type 1, a membrane, and a thin laying course of grano dust or sharp sand screeded level, and the grass sits on top.

Dig out the turf and the topsoil down to firm ground. Topsoil is soft and full of roots and it keeps settling, so a base laid on it sinks with it. The dig for a lawn on firm ground is about 125mm: 100mm for the Type 1 and 25mm for the grano. On soft or clay ground it is 175mm.

How Much MOT Type 1 Do I Need Under Artificial Grass?

Artificial grass needs 100mm of compacted MOT Type 1 on firm ground and 150mm on soft ground, which is about 194kg per m² at 100mm once the compaction allowance is added. The sum the calculator does, on a 5m by 4m lawn at 100mm, goes like this:

1. Work out the area: 5m × 4m = 20m².
2. Multiply by the compacted depth in metres: 20m² × 0.1m = 2.00m³ once it is compacted.
3. Add 30% for compaction: 2.00m³ × 1.3 = 2.60m³ of loose stone to order.
4. Multiply by the loose weight: 2.60m³ × 1,490kg = 3,874kg, about 3.87 tonnes.
5. Divide by the bag: 3,874kg ÷ 850kg = 4.6, so five bulk bags, with about 376kg spare.

That 1,490kg per m³ is not a figure off another website. We filled an 850kg bulk bag of our MOT Type 1 and measured it at 0.57m³ loose. The 30% is the compaction allowance the trade uses. Most artificial grass calculators use a compacted weight of about 2 tonnes per m³ and skip the allowance, or use the compacted weight and then add an allowance on top, which counts the same thing twice. We ran a 5m by 4m lawn at 100mm through the calculators that come up on page one for this search. The answers ran from 2.5 tonnes to 4.4 tonnes, and ours says 3.87.

How Much Grano Dust Do I Need for Artificial Grass?

Grano dust for artificial grass goes on at 15 to 25mm, which at 25mm is about 37kg per m², so a 20m² lawn takes about 745kg, one bulk bag.

Grano dust (fine crushed granite, 0 to 4mm) is the laying course, the thin, smooth bed the grass actually lies on. It goes on top of the membrane, gets screeded dead level with a straight edge, and is run over once with the plate. It is not a second sub-base, so the calculator does not add a compaction allowance to it, and 25mm is as thick as it should go. The calculator starts on 25mm and lets you type anything from 15mm up.

Here is the thing that catches people. Grano does not set. It is not cement. Compacted, it goes firm and holds a level, but you can still dig it with a trowel, and that is normal. If it stays loose and footprints show in it, the usual causes are a layer that is too thick, or a Type 1 base underneath that was never properly compacted, so the grano is sinking into gaps. Thin layer, hard base, one pass with the plate.

Sharp sand does the same job and plenty of installers use it. The calculator will work it out for you if you pick it. We stock grano dust in bulk bags and in packs of 20kg mini bags, and that is what I used on every lawn I laid.

Where Does the Membrane Go Under Artificial Grass?

Weed membrane under artificial grass goes on top of the compacted MOT Type 1 and under the grano dust, so the fine laying course cannot work its way down into the stone.

Membrane is one of those jobs where the DIY guides disagree with each other. Some put it on the soil under the Type 1, some put it under the grass. For a lawn carrying feet rather than cars, it goes between the sub-base and the laying course. Nobody is parking on the lawn, so the job of the sheet here is to keep the grano and the Type 1 apart and give the grass a clean, firm bed.

How Much Artificial Grass Do I Need?

Artificial grass is sold off 2m and 4m wide rolls by the metre, so the amount you need is the number of strips across the lawn times the length of each strip, plus 5 to 15% for cutting.

The calculator does not just multiply length by width. It lays strips across your shape, tries both directions, and keeps the one with less waste. A 5m by 4m lawn is two strips of 2m roll, each 5m long, with one join down the middle: 10m of roll, plus 5% for trimming, so 10.5m of roll and 21m². Switch to a 4m roll and it is one strip, no join, and no tape. Fewer joins is nearly always worth paying a little more for, because the join is the thing that shows.

Why Does Artificial Grass Sink and Go Wavy?

Artificial grass sinks and goes wavy when there is no compacted sub-base under it, and no amount of extra sand infill will fix that afterwards. The lawn I was most often called out to look at had been laid straight onto soil, or onto a thin scatter of sand, by someone who promised the sand would "settle it". Soil settles unevenly, roots rot and leave voids, and within a year the grass has waves and dips you can feel. The fix is the one nobody wants to hear: lift the grass, build the base properly, lay it again.

Worked Example: A 5m by 4m Artificial Lawn

A 5m by 4m artificial lawn on firm ground needs five bulk bags of MOT Type 1, one bulk bag of grano dust, two rolls of membrane, 76 pegs, 21m² of grass and 100kg of sand.

Five bags of MOT Type 1 is about 31 loads of an 85L wheelbarrow. The calculator tells you that number too, and it is worth knowing before the bags land on the drive, because it is the difference between a Saturday and a weekend.

Next Steps

Put your measurements into the calculator and tap "Add to basket" on the MOT Type 1, grano dust, membrane and pegs, or "Take me there" to see each product first with your measurements already filled in. Copy the link to your answer and send it to whoever is helping, or save it as a PDF for the day.

If you would rather talk your artificial grass base through with one of our team, call Outdoor Aggregates on 0330 043 7585. The office is open 7am to 7pm, seven days a week.

Have you ever laid artificial grass yourself, and what did you put under it? Tell us in the comments how deep you went and whether it has stayed flat, we would love to hear.

FAQs

How much sub-base do I need for artificial grass?

Artificial grass needs 100mm of compacted MOT Type 1 sub-base on firm ground and 150mm on soft or clay ground, which is about 194kg per m² at 100mm. An 850kg bulk bag covers about 4.4m² at 100mm.

Do I need a sub-base for artificial grass on hard ground?

Artificial grass on hard ground still needs the full 100mm of MOT Type 1, because ground that feels firm in a dry summer softens in a wet winter and the grass is down for ten years. The base is what keeps it flat through both.

Can I lay artificial grass on an existing patio or concrete?

Artificial grass can go straight onto sound concrete or paving that drains, with no sub-base, as long as the surface is level and water does not sit on it. Cracked, uneven or puddling concrete needs the grano laying course, or lifting.

Is grano dust or sharp sand better under artificial grass?

Grano dust is the laying course I used under every artificial lawn, because it screeds level and firms up tighter than sand. Sharp sand does the same job and the calculator works out either.

How much does the base for artificial grass cost?

The base for a 5m by 4m artificial lawn is five bulk bags of MOT Type 1 at £570, one bulk bag of grano dust at £125, two rolls of membrane at £31 and a pack of 100 pegs at £15.95, all before delivery.

How much kiln dried sand does artificial grass need?

Artificial grass takes about 5kg of kiln dried sand per m², brushed into the pile in two or three light passes once the grass is down. Some grass is sold as non-infill and needs none, so check the maker's instructions.

How do I join two rolls of artificial grass?

Two rolls of artificial grass are joined by butting the edges together, never overlapping, with joining tape underneath and adhesive on the tape. Keep the pile leaning the same way on both pieces or the join shows as a colour change.