Paving Slab Calculator & Sub-Base

How the Paving Slab Calculator Works

The calculator works through four separate material estimates, each built from your area dimensions and the slab size you choose.

First, it calculates the total paved area by multiplying length by width. For the slab count, the key concept is effective slab area — the space each slab actually occupies once you include the joint gap on two sides. A 450mm slab with a 10mm joint has an effective dimension of 460mm (0.46m), giving an effective area of 0.2116 square metres rather than the bare 0.2025 square metres of the slab alone. The calculator divides your total area by this effective slab area, rounds up to whole slabs, and applies your waste percentage.

Sub-base volume comes from multiplying the paved area by the sub-base depth you specify (our step-by-step paving guide covers the full layer system in detail). The calculator converts this volume to weight using a density of 2.1 tonnes per cubic metre, which is the standard figure for compacted MOT Type 1 aggregate. This gives you a tonnage figure suitable for ordering from a builders' merchant or aggregate supplier.

Bedding sand follows the same area-times-depth approach but uses a density of 1,600 kilograms per cubic metre — the typical figure for sharp sand used in a bedding layer. The result is in kilograms, which you can convert to 25kg bags or order as bulk loose sand.

Jointing compound is calculated from the total linear length of joints across the paved area. The calculator works out how many joint lines run in each direction, multiplies by the joint cross-section (width times slab depth), and converts the total volume to weight. This accounts for every gap between slabs, including the perimeter joints where slabs meet the edging.

Every formula in this calculator has been independently verified by a trade professional with over 25 years in construction and an academic mathematician who checks unit conversions, rounding, and complex shape calculations.

Making Sense of the Output

The results give you a full shopping list of four materials, each measured in the unit your supplier expects.

Slabs needed is a whole-slab count with your chosen waste factor built in. You cannot buy half a slab, so the calculator always rounds up. A 10% waste allowance covers cutting at edges, the odd breakage during handling, and any damaged slabs in the delivery. If your patio has lots of angled cuts — around a curved border or drain cover, for example — you might increase this to 15%. The same principle applies to indoor floor tiling, where diagonal layouts push waste even higher.

Sub-base aggregate is shown in tonnes because that is how builders' merchants sell it. One bulk bag (also called a dumpy bag or tonne bag) typically holds between 0.8 and 1.0 tonnes, so a result of 2.78 tonnes means you need three bulk bags. Sub-base is heavy and awkward to move by hand, so plan your delivery point close to the work area.

Bedding sand is shown in kilograms. A standard 25kg bag of sharp sand covers roughly 0.6 square metres at 25mm depth. For larger patios, bulk sand (sold by the tonne) is cheaper and generates less plastic waste. A result of 845kg works out to about 34 bags or just under one bulk bag.

Jointing compound is also in kilograms. The figure might seem small compared to sand and aggregate, but jointing compound fills only the narrow gaps between slabs. Smaller slabs with wider joints need proportionally more jointing — the path example needs 31.7kg of jointing for 9 square metres because 300mm slabs create far more joint lines per square metre than 450mm slabs do, and the 15mm joints are wider than the patio's 10mm gaps.

Practical Tips for Hardscape

Getting the groundwork right is the difference between a patio that lasts twenty years and one that sinks after the first winter. These steps take time, but skipping them costs far more later.

Dig out to the correct total depth before anything else. Add up your sub-base depth, bedding sand depth, and slab thickness, then subtract from the finished level you want. Any excavated topsoil can be reused in borders or raised beds — the topsoil volume estimator helps you work out if you have enough for other areas of the garden. For a standard 100mm sub-base, 40mm sand bed, and 50mm slab, you are digging down 190mm below the final patio surface. Mark this depth on pegs around the perimeter and use a string line to keep the excavation even.

Compact the sub-base in layers, not all at once. Spread and compact 50mm at a time using a vibrating plate compactor (hire one for the day — hand tamping a full patio is brutal work). Each layer should feel rock-solid underfoot before the next goes down. Rushing this stage causes uneven settlement that shows up as rocking slabs within a few months.

Check the fall gradient with a spirit level and a shim. Patios must slope away from any building at a fall of between 1:60 and 1:80 — that means a drop of roughly 12mm to 17mm per metre of run. Set your string lines to this gradient before you start laying. Water pooling against a house wall causes damp problems that are expensive to fix.

Dry-lay the first row of slabs along your longest straight edge before committing any mortar or sand. This lets you check that the joint spacing works out evenly and that you will not end up with a thin sliver of cut slab at the far end. Adjusting the joint width by a millimetre or two across the full run can often eliminate an awkward cut entirely.

Factors That Change the Calculation

The default values suit a typical pedestrian patio, but several real-world conditions call for different numbers.

Vehicle traffic roughly doubles the sub-base requirement compared to a pedestrian patio. A driveway that will bear car weight needs at least 150mm of compacted sub-base — and 200mm or more if the ground is soft or poorly drained. The preset for driveways in this calculator already uses 150mm, but if you park anything heavier than a family car (a campervan, for instance), go deeper.

Clay soils hold water and shift with the seasons. If you can see grey, sticky clay when you dig the foundation, add 50mm to your planned sub-base depth and consider laying a geotextile membrane between the clay and the aggregate. If the ground is too problematic for paving altogether, synthetic turf is a low-maintenance alternative that tolerates poor subsoils. This stops fine clay particles migrating up into the sub-base and weakening it over time.

Slab size affects joint proportions and your total jointing quantity. Smaller slabs (300mm and below) produce many more joints per square metre, which means more jointing compound and more labour. Larger format slabs (600mm+) reduce joint lines but are heavier to handle and harder to keep level. Choose the size that balances appearance with practical handling for your project.

The choice between a sand bed and a mortar bed affects both technique and material quantities. A full mortar bed (typically a 10:1 or 6:1 sand-to-cement mix) provides stronger adhesion and is standard for driveways and areas with heavy use. A plain sand bed is quicker to lay and easier to lift individual slabs for future repairs, making it suitable for garden paths and lightly used patios. If you choose a mortar bed, you will need cement on top of the sand quantity this calculator provides — roughly one 25kg bag of cement per two to three square metres of patio at a 10:1 mix ratio.