MOT Type 1 Calculator

What this calculator does

MOT Type 1 is a crushed, graded aggregate widely used in the UK as the load-spreading layer beneath driveways, patios, paths and shed bases. This page turns your plan measurements and finished depth into the volume you will have once it is compacted, the larger loose volume you need delivered, an estimated weight in tonnes or US tons, and a bulk-bag count. It works equally for other granular sub-bases, such as gravel base or basecourse, if you enter that material's density.

How to measure

1. Measure the length and width of the excavated area, not the finished paving edge.
2. Split L-shapes and irregular areas into rectangles, run each one, and add the results.
3. Enter the compacted depth you want to end up with, separate from bedding and paving layers.
4. Check the depth in several places; a shallow corner or a soft spot changes the total.

Compacted volume versus delivered volume

A digger bucket of loose stone takes up more space than the same stone after a plate compactor has gone over it. The calculator therefore keeps two volumes apart. The compacted volume is simply area × finished depth. The delivery volume multiplies that by your compaction factor and then adds waste. Weight is calculated from the density type you pick, so a compacted density is matched with the compacted volume and a loose density with the loose volume. That way the compaction allowance is never counted twice.

Metric example

A 5 m × 4 m driveway at 150 mm compacted depth covers 20 m² and needs 3 m³ compacted. With a 1.2 compaction factor that is 3.6 m³ loose. Using a 2,100 kg/m³ compacted density estimate and no waste, the weight is 6.3 tonnes, or 7.41 bags of 850 kg, so you would buy 8 bags.

Imperial example

A 20 ft × 10 ft patio at 4 in compacted depth covers 200 sq ft and needs 66.67 ft³, or 2.47 yd³, compacted. At 131 lb/ft³ that is about 8,733 lb, or 4.37 US tons, before any waste.

Buying bulk bags

A bulk bag is sold by weight, not volume, and merchants differ: 800 kg, 850 kg and one-tonne bags are all common, and fill weights vary. Enter the capacity shown by your supplier. The exact bag figure is kept for reference, and the purchase count is always rounded up. For larger jobs, a loose tipper load priced per tonne may be cheaper; ask your supplier for the density they use when quoting.

Common mistakes

• Entering the loose depth instead of the finished compacted depth.
• Adding compaction twice, once as a factor and again through the waste percentage.
• Using a compacted density with the loose volume, which overstates the weight.
• Assuming every bulk bag holds one tonne.

When to get professional input

Sub-base depth and specification depend on the ground, drainage, frost and the loads the surface will carry. For driveways used by heavy vehicles, soft or clay ground, or work subject to planning or building rules, speak to an installer, engineer or your local authority before ordering.

The starting density, compaction factor and bag size on this page are editable estimates, not standard values for MOT Type 1.

Laying a concrete shed slab on the sub-base? The shed base calculator estimates both, kept separate. For the sand bed above a paving sub-base, use the paving sand calculator.