How the Aggregate Base Calculator Works

The calculation is volume times density. Measure length and width of your area in metres and decide your compacted depth in millimetres. The calculator multiplies these to find volume, then converts to weight using bulk density of your chosen material.

MOT Type 1 sits at around 2.1 tonnes per cubic metre. Sharp sand is lighter at roughly 1.6 tonnes per cubic metre and recycled 6F2 hardcore lighter still. Because aggregate is sold by weight in the UK, getting density right is what turns a tape measure reading into an accurate order.

How Deep Should an Aggregate Base Be

Depth depends entirely on what sits on top:

• Footpaths and garden paths: 50 to 75mm
• Patios and paved seating areas: 100mm
• Driveways carrying cars: 150mm
• Areas taking vans or heavier vehicles: 200mm or more

These are compacted depths. On soft or clay heavy ground go deeper and consider a geotextile membrane beneath the base to stop the aggregate punching down into the subgrade over time.

Do Not Forget the Compaction Allowance

This is where most DIY orders go wrong. MOT Type 1 is graded from 40mm stone down to dust and when a wacker plate runs over it the fines pack into voids between the larger stones. The material loses around 15 to 20 percent of its loose volume in the process.

That means loose pile you order must be bigger than finished depth you need. For a 150mm compacted driveway base you effectively need around 180mm of loose material. The calculator builds this allowance in, but if you are pricing up from a supplier’s coverage chart check whether their figures are loose or compacted before you commit.

Compact in layers no deeper than 75mm at a time. Trying to compact a full 150mm lift in one pass leaves the bottom half loose and the surface will settle and rut later.

MOT Type 1 vs Type 2 vs Type 3

MOT Type 1 is industry standard sub base, produced to Clause 803 of Specification for Highway Works. Its full grading from 40mm to dust compacts into a dense load bearing layer, which makes it default choice under block paving, tarmac, concrete slabs and patios.

Type 2 contains more fines and is a cheaper general fill. It compacts well but drains poorly, so keep it away from anything needing permeability.

Type 3 is an open graded material with fines reduced. Water passes through it freely, which makes it the required choice under permeable block paving and resin bound surfaces where sustainable drainage rules apply. If your new driveway drains to road rather than within your property, planning rules in England push you towards a permeable build up, so check before ordering Type 1 by default.

Recycled 6F2 crushed concrete is worth a look too. It typically costs 20 to 30 percent less than virgin stone and performs well as bulk fill, though quality varies between suppliers so ask for the specification.

Worked Example

A 6m by 4m driveway at 150mm compacted depth:

• Volume: 6 x 4 x 0.15 = 3.6 cubic metres
• Weight: 3.6 x 2.1 = 7.56 tonnes
• Plus 15 percent compaction allowance: roughly 8.7 tonnes
• In 850kg bulk bags: 11 bags, rounded up

At that quantity a loose tipper load will almost always beat bulk bag prices, often by a wide margin.

Bulk Bags or Loose Load

Bulk bags hold around 800 to 850kg and suit smaller jobs or sites with no space for a tipped pile. Above roughly 5 tonnes loose delivery becomes significantly cheaper per tonne and many suppliers offer free delivery over a minimum order. Grab lorries can also place loose material over a wall or fence, which saves hours of borrowing. Get quotes both ways once you know your tonnage.