Grano Dust Calculator

Key Takeaways

• A cubic metre of loose grano dust weighs about 1,490kg, the same as gravel, measured from an 850kg bulk bag that holds about 0.57m³.
• A 5m x 4m area at 25mm needs about 745kg of grano dust, so one 850kg bulk bag covers it. A single 850kg bulk bag of grano dust covers about 23m² at 25mm and about 14m² at 40mm.
• Grano dust under artificial grass is laid 15mm to 25mm deep, screeded level over compacted MOT Type 1.
• A patio slab bed of grano dust and cement mixed 6 to 1 is about 40mm deep, and six parts in seven of that bed are grano dust.
• A block paving bed of grano dust is 40mm deep, screeded level over a compacted MOT Type 1 sub-base.
• Grano dust is a laying course on top of a sub-base, so it gets no extra for compaction, only an allowance for levelling and waste.

How Much Grano Dust Do You Need?

Grano dust is worked out the same way as any loose aggregate: area times depth gives the volume, and volume times 1,490kg gives the weight. For a 5m x 4m artificial grass base at 25mm, that is 20m² x 0.025m = 0.5m³, and 0.5m³ x 1,490kg = 745kg. Add the 10% allowance the calculator starts on and the order comes to 820kg, which one 850kg bulk bag covers with 30kg to spare. A bulk bag is £125 from us.

The bit that catches people is the depth they type in. Grano dust is a thin layer, so a few millimetres either way moves the answer a long way. 20mm across that same 20m² is about 600kg, and 40mm is about 1,190kg. Measure the gap you actually have between the top of the compacted sub-base and where the finished surface needs to sit, and use that.

How Deep Should Grano Dust Be?

Grano dust depth depends on what goes on top of it, and there are three jobs it does well. The calculator has a button for each, and you can type any other depth yourself.

For artificial grass the calculator starts on 25mm, the top of the range, and I would only go thinner where the levels force you to. Getting the MOT Type 1 right underneath matters more than anything you do on top of it, and our MOT Type 1 sub-base calculator works that layer out. If you are pricing the whole lawn, the artificial grass calculator does the base, the grano dust, the membrane and the grass in one go.

On a patio slab bed the cement is the part people forget to order. Tap the patio button and the answer is the grano dust only, worked as six parts in seven of the bed, so a 40mm bed over 20m² needs about 1,020kg of grano dust plus the cement on top. Currently, we do not sell cement, so that comes from a builders' merchant.

How Far Does a Bulk Bag of Grano Dust Go?

A bulk bag of grano dust covers about 23m² at 25mm, and less the deeper you go. These figures come straight from the 850kg bag holding about 0.57m³, with no allowance added.

On a patio slab bed, where only six parts in seven are grano dust, one bulk bag covers about 16.5m² of 40mm bed. The mini bags exist for gardens a lorry cannot get near. Nobody enjoys barrowing a bulk bag through a terraced house, and 20kg bags can be carried through one at a time. The packs come in 20 and 40 bags, £115 and £148, and the calculator will mix packs and bulk bags to find the cheapest way to cover your area. Untick Bulk Bags if you cannot take one.

Grano Dust or MOT Type 1?

Grano dust and MOT Type 1 do two different jobs, and one cannot stand in for the other. MOT Type 1 is the sub-base: stone from 40mm down to dust, compacted in layers to carry the load. Grano dust is fine crushed granite, 0 to 4mm, laid as a thin levelling layer on top of it.

I got asked whether a deeper layer of grano dust could replace the Type 1 more than almost anything else on this subject, usually by someone hoping to save a bulk bag and a day of digging. In my opinion, no. A sub-base carries the load through a blend of big stones and fines locked together, and grano dust is only the fines. Put 100mm of compacted MOT Type 1 under a path or patio and 150mm under a drive, then put the grano dust on top.

Grano Dust or Sharp Sand?

Grano dust and sharp sand can both be a laying course, and on a lot of jobs either will do. Sharp sand (also called grit sand, with grains up to about 3 to 4mm) is the traditional bed for block paving and the usual partner for cement in a slab bed. Grano dust is a crushed stone dust from the same family, and it does the same job. Under artificial grass, grano dust is the laying course I recommend, with sharp sand as the alternative. Building sand (soft sand) is the one to avoid under anything. It holds too much water, and a bed of it settles and ruts.

Does Grano Dust Need Extra for Compaction?

Grano dust does not need a compaction allowance, because it goes down as a thin layer at the depth you want it to finish at. That is the difference from MOT Type 1, which is spread about 30% deeper than it ends up and then compacted down. What grano dust does need is a little extra for the base not being perfectly flat and for what gets left in the barrow. That is what the allowance in the calculator is for. It starts on 10%, which suits most jobs. Drop it to 5% if your sub-base is dead flat, or go to 15% on a base you know has dips in it.

Next Steps

Type your area and depth into the calculator at the top and it will give you the weight, the bags and the cheapest way to buy them. Our Grano Dust comes in 850kg bulk bags and in packs of 20kg mini bags, and both can go straight into your basket from the calculator.

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

FAQs

How much does 1 tonne of grano dust cover?

A tonne of grano dust covers about 27m² at 25mm and about 17m² at 40mm. A tonne of loose grano dust fills about 0.67m³, so divide that by your depth in metres for any other depth.

How much grano dust do I need per square metre?

Grano dust needs about 37kg per m² at 25mm and about 60kg per m² at 40mm. Multiply by your area and add the 10% allowance for levelling and waste.

Is grano dust the same as granite dust?

Grano dust and granite dust are two names for the same material: granite crushed down to a fine dust. The names vary from one supplier to the next, so check the grade on the listing. Ours is 0 to 4mm.

How many 20kg bags of grano dust do I need?

Grano dust at 25mm needs about two 20kg bags for every square metre, because one bag covers about 0.5m². For anything over a few square metres, a pack of 20 or 40 mini bags works out cheaper than single bags.

Can I use grano dust under block paving?

Grano dust works as a block paving bed, laid 40mm deep and screeded level over compacted MOT Type 1. The blocks then go straight onto it and are compacted down with a plate.