MOT Type 1 Sub-Base Calculator

Key Takeaways

• MOT Type 1 to order is the area times the compacted depth, plus 30% for compaction. A 5m by 4m patio at 100mm needs 3.87 tonnes, which is five 850kg bulk bags.
• An 850kg bulk bag of MOT Type 1 covers about 4.4m² at 100mm compacted, and about 2.9m² at 150mm.
• MOT Type 1 is spread about 30% deeper than its finished depth, so 130mm of loose stone compacts to 100mm.
• The best compacted depth of an MOT Type 1 sub-base is 100mm for paths, patios, shed bases and artificial grass, and 150mm for a driveway.
• MOT Type 1 on soft or clay ground needs 200mm or more. Always laid in layers of 150mm or less and compacted between each run.

The trouble is that some calculators guess at the weight of the stone. So we stopped guessing. We filled an 850kg bulk bag of our MOT Type 1 and measured it: 0.57m³ loose. Every figure on this page, and in the calculator above, comes from that bag and a 30% compaction allowance. Pick the surface type, type in your measurements, and the calculator gives you the loose volume to order, the weight, how deep to spread it before compacting, how many barrow loads that is, and the bags that cover it. Anything over 10 tonnes is a big order - call us and we can price it as a loose tipped load, usually at a significant discount.

How Much MOT Type 1 Do I Need?

The MOT Type 1 you need is the area times the compacted depth, plus 30% for compaction, which gives the loose volume to order. Here is the sum the calculator does, on a 5m by 4m patio at 100mm:

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.

Here's another example:

• A 7ft by 4ft shed base at 100mm is about 2.6m² and needs about 500kg. One bulk bag covers it with plenty left over. If that sounds like a lot of stone for a shed, it is the right amount. A thin shed base is the one that ends up with a door that sticks.

Measure the area you have dug out, not the area of the slabs or the shed. The base wants to run a little past the edges of whatever sits on it. For an odd shape, pick circle, triangle or L shape in the calculator. For anything more awkward, split it into rectangles and add them up.

How Much Does a Bulk Bag of MOT Type 1 Cover?

The coverage already allows for the 30% the stone loses under the compactor, so these are the areas you will finish with. A tonne of Type 1 covers about 5.2m² at 100mm and about 3.4m² at 150mm. Those numbers check out against the trade, within about 3%.

How Much Does MOT Type 1 Compact?

MOT Type 1 compacts by about 30%, so 130mm of loose stone goes down to about 100mm under a plate compactor. That 30% is a middle figure, not a promise. Three things move it:

1. The rock. A hard granite Type 1 compacts less, nearer 20%. A softer sandstone one compacts more, nearer 40%.
2. The blend. Type 1 is 40mm down to dust, and it only locks solid when the fines fill the gaps between the bigger stones. Too few fines and it stays open. Too many and it goes soft.
3. The moisture. Damp Type 1 compacts properly. Bone dry or soaking wet, it never gets there.

I always spread to the loose depth in the calculator, compacted, then checked the level with a string line before adding more. Topping up a few low spots is easy. Scraping off an extra 20mm across a whole patio is a miserable afternoon.

Keep each layer to 150mm compacted or less. A domestic plate will not reach the bottom of anything thicker, so the top goes hard and the bottom stays loose. That is why the calculator splits a 200mm base into two layers. Give each layer at least eight passes with the plate, until it stops moving and the plate starts to bounce.

Why Do MOT Type 1 Calculators Give Different Answers?

MOT Type 1 calculators give different answers because each assumes a different weight for the stone, and only some add a compaction allowance.

We ran the same 5m by 4m patio at 100mm through a dozen UK sub-base and Type 1 calculators. The answers ran from 3.45 to 5.04 tonnes. That is the difference between four bulk bags and six.

The top two rows are where the money goes. A weight of 2.1 tonnes per m³ is what Type 1 weighs once it has been compacted. Compaction does not change the weight of the stone, it only squeezes it into less space. So a calculator that uses the compacted weight and then adds 20 or 25% for compaction is counting the same thing twice.

The fix is to use one or the other, never both. Either the compacted volume times the compacted weight, or the loose volume times the loose weight. We use the loose figures, because loose is how the stone arrives in the bag.

How Deep Should an MOT Type 1 Sub-Base Be?

The depth that matters is the compacted one. Every figure in this table is the finished depth, after the plate, and the calculator adds the 30% for you.

Soft ground is where most sub-bases go wrong. If the base feels spongy after you have compacted it, the Type 1 is rarely the problem. The ground underneath is too wet or too soft to hold it up. Dig out the soft spots until you reach firm soil, fill them with Type 1, and on heavy clay put a geotextile down first so the stone and the clay cannot mix.

Where the membrane goes depends on the job. Under a driveway it sits at the bottom of the dig, under the Type 1, because the weight of a car pushes the base down into the soil. Under a loose gravel path it goes on top of the compacted Type 1, to stop the gravel and the base mixing. Self-binding gravel goes straight onto the Type 1 with nothing between them. We stock landscape fabric for the job, and my guide on how to lay a sub-base properly goes through setting out, digging and compacting.

Bulk Bags or a Loose Load?

MOT Type 1 in bulk bags suits most garden jobs up to about 10 tonnes, and a loose tipped load suits bigger drives. A bulk bag is dropped where the lorry can reach, usually the drive, and you barrow it from there. One 850kg bag of Type 1 is about seven loads of an 85L wheelbarrow, so the 5m by 4m patio above is about 31 trips. The calculator works out the barrow loads for you. Knowing that number before the bags arrive tells you whether you need a second pair of hands for the day.

Over 10 tonnes, we suggest a loose load instead. A tipped load is cheaper per tonne at that size, but it lands in one heap, so it needs somewhere to go that you do not mind covering in dust. Call us for the best bulk discounted price..

What Should MOT Type 1 Look Like When It Arrives?

MOT Type 1 should look like a mix of angular stones up to 40mm with plenty of dust and fines through it, and nothing else. Do not panic if the top of the bag looks all big stones. The fines settle to the bottom on the lorry, so the bag sorts itself on the journey. Turn it over as you spread it, and the mix comes back.

What is not normal is rubbish. Bits of plastic, wood, glass, old gloves or lumps of brick bigger than 40mm do not belong in Type 1, recycled or otherwise. If a bag from us arrives like that, take a photo and call us before you spread it.

If the base still looks open and full of gaps after compacting, it is short of fines. Brush a little fine material into the surface and run the plate over it again until it closes up tight.

Type 1, Type 3, Crushed Concrete or Scalpings: Which Do You Need?

MOT Type 1 is crushed rock graded from 40mm down to dust. The fines are what make it lock solid, which is why it is the sub-base I recommend under paths, patios, shed bases, artificial grass and drives.

• Type 3 is the opposite: it is made without the fines, so water runs straight through it. It belongs under permeable paving, where the water is meant to soak away through the base. Ordinary Type 1 is the right base for everything else.
• Crushed concrete does the same job as Type 1 when it is clean and well graded. The risk is rubbish mixed in, as above.
• Scalpings vary a lot from one quarry to the next. Some make a good sub-base, some are mostly dust. You only know by looking at the actual stone.
• Ballast is not a sub-base. It is a mix of rounded gravel and sand, and the rounded stones roll rather than lock, so it never compacts to a firm base. Ballast is for concrete: mixed with cement for footings, steps and setting fence posts.
• Grano dust is not a sub-base either. Grano dust is a fine laying course, 15 to 25mm deep, spread on top of a compacted Type 1 base, most often under artificial grass.

Next Steps

Put your measurements into the calculator, check the loose depth and the barrow loads, and tap "Take me to MOT Type 1" to order your MOT Type 1 with your measurements already filled in.

If you would rather talk your sub-base through with us, call Outdoor Aggregates on 0330 043 7585. The office is open 7am to 7pm, seven days a week.

Have you ever laid a sub-base for a patio, path or drive, and how deep did you go? Tell us in the comments how many bags it took in the end, we would love to hear.

Frequently Asked Questions

How much MOT Type 1 do I need per m²?

MOT Type 1 at 100mm compacted takes about 194kg per m², allowing 30% for compaction. At 150mm for a driveway, it takes about 291kg per m².

How much does a tonne of MOT Type 1 cover?

A tonne of MOT Type 1 covers about 5.2m² at 100mm compacted, and about 3.4m² at 150mm. That works out from a measured 850kg bag filling 0.57m³ loose.

How much does MOT Type 1 cost for a patio?

MOT Type 1 for a 5m by 4m patio at 100mm takes five 850kg bulk bags, which is £570 before delivery. One bulk bag is £114.

Do I need a wacker plate for MOT Type 1?

MOT Type 1 needs a plate compactor (a wacker plate) for anything bigger than a small slab base. A hand tamper is exhausting and never gets the stone as tight. Hiring a plate for a day costs less than redoing a sunken patio.

Can I use gravel instead of MOT Type 1?

Gravel does not make a sub-base, because it has no fines to lock it together, so it never compacts solid. Gravel goes on top of the Type 1 as the surface, with the sub-base doing the work underneath.

Can I use ballast as a sub-base?

Ballast should not be used as a sub-base, because its rounded stones roll against each other and never compact to a firm layer. Use MOT Type 1 for the base and keep ballast for mixing concrete.

How many wheelbarrows are in a bulk bag of MOT Type 1?

An 850kg bulk bag of MOT Type 1 fills about seven loads of a standard 85L wheelbarrow. A cubic metre of loose Type 1 is about 12 loads.