What is the best mortar mix for paving slabs?

Common site mixes are 1:5 or 1:6 cement:sharp sand by volume. Use 1:5 for stronger bedding and set 30–50 mm thick depending on the slab and sub-base.

How much mortar do I need per m²?

Mortar per m² ≈ bed thickness (in metres). For 40 mm: 0.04 m³ per m². Split by mix parts to get cement bags and sand volume.

How many 25 kg cement bags per m² at 40 mm bed?

About 0.38 bags per m² at 40 mm for a 1:5 mix (cement ≈ 9.6 kg). Round up and add 5–10% waste.

Should I use a full bed or spots?

Use a full bed. Dot-and-dab bedding is not recommended for external paving; a continuous bed supports the slab and improves durability.

Do I need a primer on the back of slabs?

Many porcelain and some sandstone slabs require a slurry primer for bond. Check your slab manufacturer’s guidance.

SMARTCALC — Paving Bed

Yellow Edition

Paving Bed Calculator | Estimate Mortar and Sand for Slabs & Block Paving

Work out bedding mortar volume and split into cement bags & sand. Add waste and pick your mix.

Enter valid numbers greater than zero.

Input Mode

Use a shape + bed thickness or enter a known paving area.

Diagramnot to scale

Length / Base / Diameter

Width / Height

Known Area

m²

If you already know total paving area in m².

Bed Thickness

Typical bedding: 30–50 mm. Add waste for falls/irregular slabs.

Mortar Mix & Settings

cement

sand

waste %

bag kg

Common paving mixes: 1:5 or 1:6 (cement:sand). Increase waste on rough sub-bases.

Material Densities

cement kg/m³

sand kg/m³

mortar kg/m³

Mortar density is used for sense-check; split is by mix ratio parts.

Results

Area0.00 m²

Bed Volume (wet)0.000 m³

+ Waste0.000 m³

Cement0 kg (0 bags)

Sand0.000 m³ (0.00 t)

Invalid values entered.

Tip: typical bedding 30–50 mm; add waste for falls & uneven sub-bases.

Mix Breakdown

Cement (bags) Sand (m³) Bed Vol (m³)

Estimate Mortar & Sand for Slab or Paver | Bed Depth, Mix & Coverage (UK 2025)

Estimate Mortar & Sand for Slab or Paver

Use this guide to estimate bedding mortar for porcelain, sandstone, or block paving. It pairs with the Paving Bed Calculator for instant totals in cement bags, sand volume/tonnes, and a quick visual breakdown.

Roofing nails and fixings beside roof tiles during installation.

Formulas (Simple & Reliable)

Mortar Volume (per m²) = Bed Thickness (m) Mortar Volume (job) = Area (m²) × Bed Thickness (m) Split by Mix (C:S = 1:5) → Cement Volume = 1/6 × Mortar, Sand Volume = 5/6 × Mortar Cement (kg) = Cement Volume × 1440 kg/m³ Sand (m³) = Sand Volume Sand (tonnes) = Sand Volume × 1.60 t/m³

Defaults: Cement density 1440 kg/m³, sharp sand 1600 kg/m³, common mixes 1:5 or 1:6 (cement:sand by volume).

Coverage per m² (by Bed Thickness)

Quick reference for a 1:5 bedding mortar:

Bed Thickness	Mortar per m² (m³)	Cement (kg) per m²	25 kg Bags per m²	Sand (m³) per m²	Sand (t) per m²
30 mm (0.03 m)	0.030	7.2	0.29	0.025	0.040
40 mm (0.04 m)	0.040	9.6	0.38	0.033	0.053
50 mm (0.05 m)	0.050	12.0	0.48	0.042	0.067

Tip: Add 5–10% waste for falls, irregular sub-bases and trimming. Increase to 15% for riven or mixed-thickness stone.

Worked Example

Patio: 20 m² at 40 mm bed, mix 1:5.

Mortar Volume = 20 × 0.04 = 0.80 m³ Cement Volume = 1/6 × 0.80 = 0.133 m³ → × 1440 = 191.5 kg → ~7.7 bags (25 kg) Sand Volume = 5/6 × 0.80 = 0.667 m³ → × 1.60 = 1.07 t +10% Waste → Mortar 0.88 m³, Cement ~8.5 bags, Sand ~0.73 m³ (~1.17 t)

UK Price Guide (2025)

Item	Typical Price (ex. VAT)	Notes
Cement 25 kg	£5.50 – £8.50	Bulk deals lower cost.
Sharp Sand (bulk bag ~0.6 m³)	£40 – £70	~0.9–1.0 t; density varies by quarry.
Sharp Sand (loose per tonne)	£25 – £45	Delivery + minimum order may apply.
Slurry Primer (porcelain)	£18 – £35 per tub	Boosts bond; follow data sheet.
Wacker Plate Hire	£25 – £45 / day	Weekend bundles available.

Prices vary by region and supplier. Always confirm before ordering.

Tools & Materials (Affiliate)

Disclosure: Some links may be affiliate links that help support SmartBuildCalc at no extra cost to you.

Related Calculators

Frequently Asked Questions

What mortar mix for porcelain slabs?

Use a full bed with 1:5 or 1:6 cement:sharp sand and a suitable porcelain primer slurry on the slab backs.

How thick should the bedding be?

30–50 mm is typical. Go thicker to correct levels on uneven sub-bases. Always compact the sub-base first.

Can I add plasticiser?

Yes — it improves workability. Dose as per the product label; don’t exceed manufacturer guidance.

Do I need a geotextile membrane?

Recommended over weak or clay sub-grade to separate soil from the sub-base and reduce pumping.

Note: Figures are estimates for planning. Site conditions and slab manufacturer guidance should always take priority.

Paving Bed Calculator | Estimate Mortar and Sand for Slabs & Block Paving

Estimate Mortar & Sand for Slab or Paver

Formulas (Simple & Reliable)

Coverage per m² (by Bed Thickness)

Worked Example

UK Price Guide (2025)

Tools & Materials (Affiliate)

Related Calculators

Frequently Asked Questions

What mortar mix for porcelain slabs?

How thick should the bedding be?

Can I add plasticiser?

Do I need a geotextile membrane?

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