Paving calculator for patios and slabs

A patio, from the stone up

The calculator works out four things from one area. The slabs: how many fit, counting each slab plus one joint, then your wastage on top. The sub-base: 100mm of Type 1, plus 10%, weighed at 2.2 t/m³ compacted. The bed: 40mm of mortar, converted to tonnes of sharp sand and bags of cement for your chosen mix. The joints: their total volume, turned into tubs of jointing compound.

Worked example: a patio 6m by 3.6m in 600 × 600mm slabs, 22mm thick, with 10mm joints and 10% wastage. Every other setting is left at its default.

• Area: 6.0 × 3.6 = 21.6m².
• Slabs: each takes 0.61 × 0.61 = 0.3721m², so 21.6 ÷ 0.3721 = 58.05 laid. Plus 10% = 63.85, so 64 slabs.
• Type 1: 21.6 × 0.100 = 2.16m³, plus 10% = 2.376m³, × 2.2 = 5.23 tonnes. That is 7 bags of 800kg, or 6 tonnes loose.
• Mortar bed: 21.6 × 0.040 = 0.864m³. Each m³ of bed takes 1,820kg of dry materials, so that is 1,572kg. At 4:1, cement is one part in five: 314.5kg, so 13 bags. Sharp sand is the other four parts: 1.26 tonnes.
• Jointing: the joints run about 3.25m per m² of patio. At 10mm wide and 22mm deep, that is 0.0155m³ across the patio, so 2 tubs of 15kg.

How many slabs per square metre

With 10mm joints, a square metre takes 2.69 slabs of 600 × 600mm, 1.80 of 600 × 900mm and 4.73 of 450 × 450mm. Without the joint you would expect 2.78, 1.85 and 4.94, so ignoring the gap over-orders by about 3 to 5%.

Check how the maker states its sizes before you trust any count. Marshalls' garden paving technical information says that for its natural stone, including Indian sandstone, sizes and coverage are those attained when laid with a 10mm joint. So a "600 × 600" sandstone slab is a little under 600mm, and 1m² really does take 2.78 of them. For some of its concrete ranges the sizes are actual and do not include the joint.

The calculator has a switch for this. Answer Yes to "Does the slab size already include the joint?" and it divides by the slab size alone. On the 6m by 3.6m patio that changes the order from 64 slabs to 66.

On wastage, 10% is our starting point, not a published standard. Lower it for a square patio that fits whole slabs. Raise it for curves, a diagonal pattern or a lot of cutting round drains and steps.

The mortar bed: mix and depth

Lay slabs on a full bed of mortar, between 30mm and 40mm thick, mixed from sharp sand and cement. The makers agree on the full bed. They differ on the mix, from 3:1 to 6:1.

Marshalls' patio laying guide makes an even 40mm layer of three parts sand to one of cement. Its sandstone guide uses four parts sand to one of cement at about 30mm, and its porcelain guide specifies a wet, workable 1:4 bed 30 to 40mm thick with a primer on the back of every slab. Bradstone's step-by-step patio guide uses 6 parts sharp sand to 1 part cement, under the whole slab and not just the corners.

The calculator defaults to 40mm at 4:1, and offers 3:1 and 6:1. To turn the wet bed into dry materials, it allows 1,820kg of dry mix for every m³ of bed, worked out from the yield on Hanson's sand and cement mortar data sheet. Tapping each slab down squeezes some of the bed out at the edges, so do not trim the order below this.

Five dabs of mortar under each corner and the middle is quicker, and it is what goes wrong. Marshalls says porcelain must never go on a five spot bed. Hollow slabs rock, crack and let water sit underneath.

Sub-base, digging and falls

Under the bed goes 100mm of compacted Type 1 on most domestic patios. That is the depth in Marshalls' porcelain and sandstone guides, and the default here. The Type 1 calculator explains the depth choices and why its tonnes differ from other sites. Bradstone's own guide uses a 75mm layer of concrete, 6 parts all-in ballast to 1 of cement, as the foundation instead.

Marshalls' patio guide digs out about 150mm across the whole area. Bradstone allows 150mm for the foundation and the paving. Next to a house, both makers set the finished paving 150mm or more under the damp proof course, so the dig there can reach 300mm below it.

Build in a fall so rain runs away from the house. Bradstone uses 1 in 60: a 3m wide patio drops 50mm across its width. Marshalls' porcelain guide asks for at least 1 in 80 along the patio and 1 in 40 across it. Set the fall in the sub-base, so the mortar bed stays an even thickness.

How much jointing compound

Jointing compound coverage depends on the joint, not the patio. Marshalls' jointing range guide gives a 15kg tub of WeatherPoint 365 about 7m², for 10 to 15mm joints, and says that figure is based on 450 × 450mm paving with 10mm joints 35mm deep.

That works out at about 0.0107m³ of joint per tub, and the calculator uses it. It multiplies the length of joint per m² by the joint width and the slab thickness, then rounds up to whole tubs. So 22mm sandstone in 600 × 600mm with 10mm joints gets nearly 15m² from a tub, about twice the headline figure. Wider joints, thicker slabs or smaller slabs use more.

Fill the joints to the full depth of the slab. Marshalls gives a minimum joint depth of 20mm for paving, and a minimum laying temperature of 5°C and rising for WeatherPoint 365. Porcelain is different: Marshalls specifies 3 to 5mm joints and its own jointing, and with its Symphony jointing, drainage ports through the bed. If you point with mortar instead, Bradstone uses 3 parts soft building sand to 1 of cement, pressed into the joints with a trowel and kept off the face of the slabs.

Before you order

The calculator gives quantities for a rectangle. For an L-shaped patio, run each rectangle separately and add them. For a curve, measure the rectangle round it and raise the wastage.

Quoting this for a client? The quote template takes the slabs, Type 1, sand, cement and labour line by line, and lets you say how long the price holds.

• Not included: edgings, kerbs, a geotextile membrane, slurry primer, drainage channels and the skip for the spoil.
• Primer: Marshalls asks for its paving primer on natural stone and porcelain. Budget for it.
• Batches: natural stone varies in colour. Buy the full quantity at once and mix slabs from several packs as you lay.
• No prices. Slab and sand prices vary by merchant and by region. Ask for a quote.

Common questions