How do I work out how much sub-base, sand and slabs a patio needs?

Ordering paving materials goes wrong in three predictable places: forgetting that sub-base compacts down by around a quarter, forgetting cuts and breakages on the slabs, and forgetting the jointing entirely until the day. All three are arithmetic you can do in ten minutes with a tape, and getting them right saves a second delivery charge, which is often more than the material itself.

Step by step

1. Measure the area properly, in rectangles.Break an awkward shape into rectangles and add them up. Round each dimension up to the nearest 100mm. Do not measure the paving you want — measure the excavation, which is bigger, because the sub-base extends beyond the paved edge.
2. Set the build-up depth for the use.A foot path is a shallower build-up than a patio, and a drive is deeper again. Excavation depth is finished level, minus slab thickness, minus bedding depth, minus compacted sub-base depth. Work it out on paper before digging — this is the number everyone gets wrong on site.
3. Calculate the sub-base by volume, then add for compaction.Area in square metres times compacted depth in metres gives cubic metres. MOT type 1 is roughly 2.1 tonnes per cubic metre. Then add about 25% to the volume, because loose delivered stone compacts down by roughly that much. Ordering the compacted volume leaves you short every time.
4. Calculate the bedding layer.Area times bed depth in metres gives cubic metres of mortar. For a 4:1 sand to cement mix, the sand volume is close to the total volume — sand and cement do not add up, the cement fills the voids. Sharp sand is roughly 1.6 tonnes per cubic metre. Work out the cement from the ratio and round up to whole bags.
5. Add the slabs with a proper allowance.Area divided by the coverage per pack, plus 10% for cuts, and a little more for a shape with many angles or a patterned laying course. Order it all in one go: slabs vary between batches and matching later is difficult.
6. Do not forget the jointing.Total joint length times width times depth gives a small but real volume, and jointing compounds are sold by coverage per square metre of paving for a stated joint width and depth. If your joints are wider or deeper than the stated basis, you need more. Read the tub.
7. Add the edge restraints and the haunching concrete.Perimeter length in metres for the edging, plus concrete for the bed and haunch — commonly around 0.03 to 0.05 cubic metres per metre run for a domestic edging. It is a small quantity that is always forgotten.
8. Round to how it is actually sold.Bulk bags are typically around 800kg and are sold as a nominal tonne — check with your supplier. Loose tipped material is sold by the tonne with a minimum load. Working out that you need 2.3 tonnes is only useful once you know it will arrive as three bags.
9. Sanity check against the spoil.The volume you dig out is roughly the volume you are putting back plus the slab thickness. Excavated soil bulks up by around 20 to 30% when loosened, so the skip or the heap will be bigger than the hole.

Tips

• Order sub-base and sand together to share a delivery. Two separate deliveries of half a tonne each usually costs more than one of a tonne and a half.
• Buy one extra bag of cement. Running out of cement mid-bed on a Sunday stops the whole job.
• Keep the packaging labels and the batch numbers from the slabs until the job is done and you are happy.
• If the shape is complicated, draw it on squared paper at scale and lay out the slab sizes. It shows you the cuts, which shows you the real wastage percentage.
• Weight matters for access. Ask where a bulk bag can actually be craned off the lorry to before you order six of them.

Common mistakes

• Ordering the compacted volume of sub-base — Delivered stone is loose and compacts by roughly a quarter. Ordering exactly the finished volume leaves you a quarter short with the plate compactor on hire.
• Measuring the paved area rather than the excavation — The sub-base has to extend beyond the paving to support the edge, and the edging needs its own trench. Both add area.
• Ordering slabs to the exact area — Every cut wastes part of a slab, and a small percentage arrive damaged or with a flaw you will not want on the finished patio. 10% is the normal working allowance.
• Forgetting the jointing until the slabs are down — Jointing compound coverage depends on the joint width and depth, both of which are now fixed. It is the one material you cannot adjust the design to suit at that point.

Questions people ask