I learned about wastage the hard way. I was relaying the patio at our previous house - about 18m2 of 600mm flags - and I calculated the slabs by dividing the total area by the slab area, added a cautious 5% wastage and ordered accordingly. About three-quarters of the way through, a slab cracked when I was cutting it. Then I miscalculated a corner and wasted another. I finished the job with an awkward gap where I needed one more complete slab that I did not have.
The slab supplier had a minimum order of 10 slabs and was 40 minutes away. What should have been a weekend job became a three-day project.
Always add 10% wastage minimum. Always.
The formula
The slight complexity in slab calculation is that you cannot just divide the patio area by the slab face area - you also need to account for the joint gaps between slabs.
Effective slab area = (Slab length + joint gap) x (Slab width + joint gap)
Number of slabs = Patio area / Effective slab area
Then round up and add your wastage percentage.
Worked example
Patio 5m x 4m (20m2), using 600x600mm slabs with 10mm joints:
- Effective slab area = (0.60 + 0.01) x (0.60 + 0.01) = 0.61 x 0.61 = 0.3721 m2
- Slabs needed = 20 / 0.3721 = 53.7
- Add 10% wastage = 59 slabs (rounded up to nearest whole)
If you skip the joint gap entirely: 20 / 0.36 = 55.6, or 56 slabs. That is three slabs short before you have even started cutting anything. On a job where slabs come in packs of 6, that shortfall might not become apparent until the third trip to the merchant.
Use the Patio Slab Calculator to do this calculation quickly for any slab size.
Common slab sizes and coverage
| Slab size | Face area | Slabs per m2 (approx) | Slabs for 15m2 | Slabs for 20m2 |
|---|---|---|---|---|
| 300x300mm | 0.09 m2 | 11.1 | 17 | 23 |
| 450x450mm | 0.2025 m2 | 4.9 | 8 | 11 |
| 600x600mm | 0.36 m2 | 2.8 | 47 | 62 |
| 600x900mm | 0.54 m2 | 1.85 | 30 | 40 |
| 600x1200mm | 0.72 m2 | 1.39 | 23 | 31 |
| 900x900mm | 0.81 m2 | 1.23 | 20 | 28 |
Figures include 10mm joint gap and 10% wastage.
How many packs do I need?
Most paving slabs are sold in packs or on pallets with a fixed number of slabs per pack. Once you know the slab count, divide by slabs-per-pack and round up to the next complete pack.
The Patio Slab Calculator includes a pack size input so you get the pack count directly.
An important note: always order complete packs and keep the surplus. Spare slabs from the original batch are invaluable when one gets cracked during or after the job, or when you want to extend the patio in a couple of years. Getting an exact colour match from a different production batch is notoriously difficult.
How much wastage to add?
| Layout type | Wastage to add |
|---|---|
| Simple rectangle, no obstacles | 10% |
| Rectangle with one or two cut-outs | 10-12% |
| Diagonal laying pattern | 15% |
| Irregular shape, lots of edge cuts | 15% |
| Natural stone (variable sizing) | 15-20% |
On a very simple patio with no obstacles, 10% is the right minimum. Every cut slab produces an off-cut, and most off-cuts cannot be used elsewhere. Some slabs will crack during cutting. A couple will arrive damaged in the pack. 10% covers all of this with a small margin.
I have never been in a situation where I genuinely wished I had ordered fewer slabs - the spare ones always get used somewhere. I have been in the situation of running short, and it is genuinely painful.
What joint gap should I use?
| Slab type | Recommended joint gap |
|---|---|
| Concrete paving slabs | 8-12mm |
| Natural sandstone | 10-15mm |
| Natural slate or granite | 5-10mm |
| Porcelain tiles | 5-8mm |
Wider joints are more forgiving of minor size variations between slabs (which is common with natural stone) and give room for pointing mortar. Tighter joints need very precise, consistent slabs and more skill to achieve a neat result.
The sub-base: do not skip it
A proper patio is not just slabs. Before any slabs go down, you need:
- Excavation - typically 200-250mm below finished surface level
- Compacted sub-base - 75-100mm of MOT Type 1 (Sub Base Calculator)
- Mortar or sharp sand bed - 25-50mm
- Slabs on top
The sub-base is usually the biggest quantity and cost of a patio project. Use the MOT Type 1 Calculator alongside the slab calculator to plan both together.
My tips for ordering and laying patio slabs
Over a few patio projects, including one complete re-lay, here is what I have learned:
Measure twice, order once. Before you do any calculation, measure the patio area at least twice and double-check. A 10cm error in one dimension adds up fast across the whole area.
Order all slabs from the same batch if possible. Colour variation between batches is common, particularly with concrete paving. If you buy extra at a later date, the colour match is often noticeably different. Ask the supplier to hold stock from the same batch if you might need to order more.
Stack slabs flat and protect them. Concrete slabs are heavy and crack surprisingly easily on edge. Stack them flat, on a firm surface, covered with plastic to prevent surface staining from rain. Standing them on their edge against a wall is asking for trouble.
Lay a test area first. Before you commit to bedding slabs in mortar, lay a few out dry to check spacing, check your joint widths look right, and make sure you are happy with the pattern. Much easier to adjust before the mortar is mixed.
Cut slabs with a disc cutter, not a bolster. A 230mm angle grinder with a diamond disc (or a proper slab splitter for straight cuts) gives far cleaner results than trying to split slabs with a bolster chisel. The first time you try to split a 600mm concrete flag by hand you will understand why.
Use the Patio Slab Calculator to get your numbers right before placing any order.