Patio Calculator: How to Calculate Patio Slabs, Mortar, Sub-Base and Total Cost for Your Garden Project

Building a patio is one of the most popular garden projects in the UK — and one of the easiest to get wrong. Order too few slabs and your project stalls. Order too many and you've wasted hundreds of pounds. Get the sub-base wrong and your patio will sink, crack, or flood within a year.

Our free patio calculator takes the guesswork out of the entire project. Enter your patio dimensions and slab size, and it calculates exactly how many slabs, how much mortar, how much MOT Type 1 sub-base, and the estimated total cost.

This guide walks you through every calculation, material choice, and common mistake — whether you're DIYing or getting quotes from landscapers.

What You Need to Calculate

A patio project involves five key calculations:

1. Patio area — total square metres
2. Number of slabs — including wastage allowance
3. Sub-base material — MOT Type 1 hardcore
4. Mortar/adhesive — for laying the slabs
5. Total cost — materials + optional labour

Let's work through each one.

Step 1: Calculate Your Patio Area

Rectangular Patios

The simplest calculation:

Area = Length × Width

For example, a 4m × 3m patio = 12 square metres.

L-Shaped Patios

Split the L into two rectangles, calculate each area, and add them together.

Circular Patios

Area = π × radius²

A circular patio with a 2m radius = 3.14159 × 4 = 12.57 square metres.

Irregular Shapes

For irregular shapes, divide the area into rectangles and triangles, calculate each, and add them up. Or use our patio calculator which handles the maths for you.

Step 2: Calculate Number of Slabs

The Formula

Number of slabs = Patio area ÷ Individual slab area

Then add 5–10% for wastage (cuts, breakages, and future replacements).

Common UK Patio Slab Sizes

Example Calculation

For a 12 m² patio using 600 × 600mm slabs:

• Slabs needed: 12 ÷ 0.36 = 33.3 → round up to 34
• Add 10% wastage: 34 × 1.10 = 37.4 → order 38 slabs

Step 3: Calculate Sub-Base (MOT Type 1)

The sub-base is the foundation that prevents your patio from sinking. MOT Type 1 (crushed limestone/granite) is the UK standard for patio foundations.

The Formula

Volume = Area × Depth

Standard sub-base depth is 100mm (0.1m) for pedestrian patios, or 150mm (0.15m) if vehicles will drive over it.

Converting to Tonnes

MOT Type 1 weighs approximately 2.1 tonnes per cubic metre (compacted).

Tonnes needed = Volume in m³ × 2.1

Example

For a 12 m² patio with 100mm sub-base:

• Volume: 12 × 0.1 = 1.2 m³
• Tonnes: 1.2 × 2.1 = 2.52 tonnes
• Add 10% for compaction: 2.77 tonnes → order 3 tonnes

MOT Type 1 Prices

Typical UK prices for MOT Type 1:

• Bulk bag (850kg): £50–80 delivered
• Loose load (per tonne): £25–40 + delivery
• Small bags (25kg): £4–6 each (expensive for large areas)

Step 4: Calculate Mortar

You have two options for laying slabs: full mortar bed or five-spot method.

Full Mortar Bed (Recommended)

A full mortar bed provides the best support and prevents slabs rocking. You need approximately 0.05 m³ of mortar per square metre of patio (for a 50mm bed).

Mortar Mix Ratio

Standard patio mortar mix: 4 parts sharp sand : 1 part cement

For 12 m² of patio:

• Mortar volume: 12 × 0.05 = 0.6 m³
• Sharp sand: 0.6 × 0.8 = 0.48 m³ ≈ 0.77 tonnes (sand weighs ~1.6t/m³)
• Cement: 0.6 × 0.2 = 0.12 m³ ≈ 5–6 bags (25kg bags)

Jointing

For pointing the gaps between slabs, you'll need additional mortar or a proprietary jointing compound. Budget approximately 1 bag of jointing compound per 5–8 m².

Step 5: Calculate Total Cost

Here's a realistic cost breakdown for a 12 m² patio in the UK (2024 prices):

Use our patio calculator to get an accurate estimate for your specific dimensions and slab choice.

How to Lay a Patio: Step-by-Step

Here's the correct process for a long-lasting patio:

1. Mark Out and Excavate

• Mark the patio area with string lines and pegs
• Excavate to a depth of approximately 200mm (100mm sub-base + 50mm mortar + 50mm slab thickness)
• Ensure a fall of 1:60 to 1:80 away from the house (about 15mm per metre) for drainage
• The finished patio must be at least 150mm below the damp proof course (DPC)

2. Lay the Sub-Base

• Lay weed membrane over the excavated area
• Add MOT Type 1 in layers of 50mm
• Compact each layer with a plate compactor (hire for ~£30/day)
• Check levels with a spirit level and straight edge

3. Lay the Slabs

• Mix mortar: 4 parts sharp sand to 1 part cement, with enough water to make a firm but workable mix
• Spread a full mortar bed about 50mm thick
• Place each slab and tap down with a rubber mallet
• Use spacers for consistent 10mm joints
• Check each slab is level and maintains the drainage fall

4. Point the Joints

• Wait 24–48 hours for the mortar bed to set
• Fill joints with pointing mortar or jointing compound
• Smooth with a pointing tool
• Clean any mortar off the slab faces immediately

The Drainage Fall Calculator

Getting the fall right is critical — too little and water pools on your patio, too much and it looks visibly sloped.

Recommended fall: 1:60 to 1:80 (away from the house)

This means for every metre of patio width, the far edge should be 12–17mm lower than the house edge.

Common Patio Mistakes to Avoid

1. Skipping the Sub-Base

Laying slabs directly on soil is the number one cause of patio failure. The ground moves, slabs sink unevenly, and weeds push through. Always use a proper compacted sub-base.

2. Insufficient Depth

The finished patio must be below the DPC (damp proof course) of your house — usually the horizontal line of mortar or membrane visible near the base of your walls. Building above this can cause damp problems.

3. No Drainage Fall

A perfectly level patio will pool water. Always slope away from the house at 1:60 to 1:80.

4. Not Ordering Enough Materials

Always add 10% for wastage on slabs (cuts and breakages) and sub-base (compaction). Running out mid-project means delays and potentially different batches that don't colour-match.

5. Laying in Wet Weather

Mortar doesn't set properly in rain. Check the forecast and cover freshly laid slabs if rain is expected within 24 hours.

6. Wrong Mortar Mix

Too much cement makes the mortar brittle and prone to cracking. Too little and it won't hold. Stick to the 4:1 ratio.

Patio Slab Types and Costs

Do You Need Planning Permission?

In most cases, no. Patios are usually permitted development. However, you DO need planning permission if:

• Your property is listed
• You're in a conservation area and the patio is at the front
• The patio covers more than 50% of your garden
• You're raising the ground level significantly

Since 2008, if your patio is impermeable (doesn't let water drain through) and covers more than 5 m², you may need to ensure water drains to a permeable area (lawn, border) rather than onto the road. This is to prevent flooding.

Try Our Free Patio Calculator

Planning a patio project? Our free patio calculator works out exactly how many slabs, how much sub-base, mortar quantities, and estimated total cost — all from your patio dimensions. Save time, save money, and avoid the most common ordering mistakes.

Frequently Asked Questions

How much does a patio cost per square metre in the UK?

Materials only: £30–80 per m² depending on slab type. With professional labour: £80–150 per m². A typical 12 m² patio costs £400–650 for materials or £1,000–1,800 fully installed.

How deep should a patio sub-base be?

100mm of compacted MOT Type 1 for a pedestrian patio. 150mm if vehicles will drive over it.

How many 600×600 slabs do I need per square metre?

2.78 slabs per m² (each slab covers 0.36 m²). Add 10% for wastage.

What is the best mortar mix for patio slabs?

4 parts sharp sand to 1 part cement. Add water until the mix is firm but workable — it should hold its shape when squeezed.

How long before I can walk on a new patio?

Wait at least 24 hours before light foot traffic, and 48–72 hours before placing furniture. Allow a full week before heavy use.