How to Calculate Paving Slabs for a Patio
Laying a new patio or garden path is a rewarding DIY project, but buying paving slabs can get expensive quickly. Ordering too few means a paused project and mismatched batch colours when you finally buy more. Ordering too many wastes money.
Our Paving Slab Calculator takes the dimensions of your planned patio and instantly tells you exactly how many base slabs you need, calculating in the crucial mortar joints and industry-standard waste allowances.
Steps to Calculate Your Patio
- Measure Area: Record the length and width of your patio area in metres to find the total square meterage.
- Choose Material: Select Standard (Concrete/Stone) or Porcelain to determine the correct joint gap (10mm vs 3-5mm).
- Add Waste: Inclusion of a waste allowance (10% standard, 15% porcelain) is essential for edge cuts and breakages.
- Run Calculation: Divide your total patio area by the mathematical footprint of a single slab (Slab Size + Joint Gap).
How Many Paving Slabs per m²?
For a standard UK patio, you require approximately 2.78 slabs per square metre when using 600x600mm flags, or 4.94 slabs when using 450x450mm units. Use the table below for quick reference:
| Slab Size (mm) | Slabs per m² (approx) | Total for 20m² Patio |
|---|---|---|
| 600 x 600 | 2.78 | 56 |
| 450 x 450 | 4.94 | 99 |
| 300 x 300 | 11.11 | 223 |
| 900 x 600 | 1.85 | 37 |
Why Include Mortar Joints?
When measuring how many standard paving slabs (like 600x600mm) will fit into a space, beginners often forget the mortar joints. A standard patio uses a 10mm to 15mm gap between every single slab for pointing. However, porcelain paving typically requires a much tighter 3mm to 5mm joint to achieve a modern, seamless look.
Our calculator automatically expands the mathematical footprint of each slab to include your chosen joint gap (whether 3mm, 10mm, or custom), ensuring a highly accurate count.
Calculating Waste Options
You can rarely tile a space perfectly without making cuts. You need to factor in a waste allowance:
- 15% Allowance (Porcelain): Porcelain is brittle and difficult to cut; professionals always allow 15% to cover chipped edges and intricate cuts around obstacles.
- 10% Allowance (Recommended): Ideal for irregularly shaped patios, circular features, or areas requiring lots of edge cutting around drains.
- 5% Allowance: Perfect for a simple square or rectangular patio where very few slabs need to be cut.
Professional Specification: BS 7533 & Falls
To comply with BS 7533-101:2021, modular paving should always be laid on a full wet bed rather than "Dot and Dab". This prevents voids that trap moisture and lead to frost heave or loose slabs in UK winters.
Calculating the Fall: Every patio should have a minimum slope of 1:60 (approx. 17mm per linear metre) to ensure water drains away from any building structures and prevents standing water.
A Note on Project Packs
If you are buying "Project Pack" sizes (packs that include 4 or 5 different mixed sizes of slabs designed to lay in a random pattern), you cannot calculate them slab-by-slab. You must simply calculate the total square metreage of your patio and buy enough project packs to cover that m² area (including waste).