Paving Slab Calculator

Key Takeaways

• Paving slabs are counted by dividing the area by the face of one slab, then adding 10% for cuts and breakages, rounded up to whole slabs.
• A 600 x 600mm paving slab covers 0.36m², so a square metre takes 2.78 slabs and 20m² takes 62 with 10% extra.
• Curves, diagonal layouts and lots of cut-outs push the extra for paving slabs up to 15%, because more slabs are cut and more offcuts are wasted.
• Project packs of mixed-size slabs are ordered by area: the patio's area plus 10%, divided by the area one pack covers, rounded up.
• Ordering every paving slab for a job in one go keeps them from one batch, because a second order rarely matches the first in colour.

Running short of slabs is the most annoying mistake on a patio, and it is nearly always made at the ordering stage. The last few rows go down a week late, from a different batch, a shade lighter than the rest, and you look at that line for the next fifteen years. I would far rather a client ended up with three slabs propped behind the shed. The calculator above does the counting. The sections below cover the bits that catch people out: how many slabs fit in a square metre, how much extra to allow, how project packs work, and how to measure a patio that is not a neat rectangle.

How Many Paving Slabs Do I Need?

The number of paving slabs you need is the area in square metres divided by the area of one slab, plus 10% for cuts and breakages. For a patio 5m long and 4m wide in 600 x 600mm slabs, it works out like this:

1. Area to pave: 5m x 4m = 20m².
2. One slab: 0.6m x 0.6m = 0.36m².
3. Slabs to cover the area exactly: 20 ÷ 0.36 = 55.6.
4. Add 10%: 55.6 x 1.1 = 61.1, so order 62.

What about the gaps between the slabs?

Slabs are not laid touching. There is a gap of about 10mm between each one, called the joint, and it is filled with mortar once the slabs are down. So each 600mm slab really takes up 610mm of ground.

The calculator ignores the gaps and counts the slabs only. That gives you a slightly higher number than you strictly need. On the 5m x 4m patio above, allowing for the gaps would bring 62 slabs down to 60. Those two spares are a cushion, and cuts round drains and edges usually use them up. Running short costs you far more than two extra slabs. Measure the area where the slabs will sit, from edge to edge.

If you are putting a border of a different slab around the outside, the calculator does not count that part. Measure the area inside the border and enter that. For the border, measure its total length in metres, divide by the length of one border slab, and add 10%.

Can I size my patio so I don't need to cut any slabs?

Yes, and it saves time, waste and a lot of dust. Pick how many slabs you want along each side. Then work out the size from the slabs and the gaps between them, not from the slabs alone. For 600 x 600mm slabs with 10mm joints, a patio eight slabs long and six slabs wide is:

• Length: 8 slabs x 600mm, plus 7 joints x 10mm = 4,870mm (4.87m)
• Width: 6 slabs x 600mm, plus 5 joints x 10mm = 3,650mm (3.65m)

There's always one joint fewer than there are slabs. Without the joints, you'd get 4.8m x 3.6m, and every row would end in a cut. The calculator does this sum for you. Enter a rectangle and a slab size, and it shows the nearest size that takes whole slabs. Press "Use this size" to fill it in. Slabs are rarely exactly the size on the label. Natural stone can vary by a few millimetres. Before you set out the edging, lay one row loose along each side and measure it. With no cuts, you only need extra for breakages, so 5% is usually enough.

How Many Slabs Are in a Square Metre?

The number of paving slabs in a square metre is 1 divided by the area of one slab. A 600 x 600mm slab is 0.36m², so a square metre takes 2.78 of them.

The sizes are written the way they are sold, in millimetres. A slab size that divides neatly into the length and width of your patio leaves fewer cuts, so the same 10% goes further. A patio sized to fit whole slabs leaves no cuts at all. The section on gaps between the slabs shows how to work that size out. Slabs of 600 x 600mm and bigger are a two-person lift. I never lift those on their own, and on a big patio that changes how long the job takes as much as the slab count does.

How Many Extra Paving Slabs Should I Order?

An extra 10% of paving slabs covers the cuts and breakages on most patios. That is what the calculator starts on, and what I allowed on a straightforward rectangular job. Move it up or down depending on the layout:

1. 5% when the area is a rectangle, the slab size has been chosen to fit it, and there are almost no cuts.
2. 10% for most patios and paths, with cuts along one or two edges and round the odd drain or step.
3. 15% for curves, circles, a diagonal layout, lots of cut-outs, or porcelain, which is harder to cut cleanly and less forgiving when one cracks.

Order everything in one go. Slabs vary in colour from one production run to the next, and a top-up order a month later can come from a different batch. When the slabs arrive, take them from several packs as you lay rather than working through one pack at a time, so any variation spreads across the whole patio instead of sitting in one patch.

Keep the spares once the job is finished. A slab cracked by a dropped barbecue in five years' time is a fairly easy swap with a matching spare, and a painstaking hunt through every supplier in the country without one.

How Many Project Packs Do I Need?

The number of project packs you need is the area to pave plus 10%, divided by the area one pack covers, rounded up to a whole pack. A project pack is a mixed set of slab sizes sold to cover a set area, often around 15 to 20m², and laid in a random pattern. You cannot count it by the slab, because there are four or five sizes in each pack. Count it by area instead:

1. Area to pave: 20m².
2. Add 10%: 22m².
3. One pack covers 15.3m², so 22 ÷ 15.3 = 1.44, which means two packs.

The pack's coverage is printed on the product page and on the pack label. Use that figure, not your own sum of the sizes inside, because the maker has already allowed for the joints in the pattern. The trap catches people buying single-size packs to mix their own pattern. Packs of different sizes tend to hold the same area, not the same number of slabs, so a pack of small slabs holds far more slabs than a pack of large ones. If a pattern calls for twice as many small slabs as large ones by count, that is not two packs of small to one of large. Work out the area each size covers in your pattern first, then order packs by area.

How Do I Measure an L-Shaped or Odd-Shaped Patio?

An L-shaped patio is measured as two rectangles, one length times width for each part, added together. Measure them so they do not overlap, or the corner gets counted twice. The calculator takes four shapes:

1. A rectangle is the length times the width.
2. An L shape is two rectangles, entered as a first and a second length and width.
3. A circle needs only the distance straight across the middle, and the area is worked out for you.
4. A triangle takes one side as the base, and the height straight across from it to the far corner.

For anything stranger, split it into those shapes on a sketch, run each one through the calculator, and add the slab counts together. A round or curved area means plenty of cut slabs round the edge, so pick 15% extra for it. If you're old school and measure in feet, press "Switch to Imperial". The area is shown in square metres and square feet, because slabs and project packs are sold by the square metre.

How Do I Check a Landscaper's Slab Count?

A landscaper's slab count can be checked by multiplying the number of slabs by the area of one slab and comparing it with the area being paved, plus about 10%. The classic version of this puzzle goes: a quote for a 20m² patio lists 95 slabs of 450 x 450mm. One slab is 0.2025m², so 95 slabs cover 19.24m², which is less than the patio itself before a single cut. By the sum above, 20m² of that size needs 99 slabs to cover it and 109 with 10% extra.

That does not always mean the quote is wrong. Part of the area might be a border in a different stone, or the landscaper might be reusing slabs you already have. Ask what the count covers, and the answer usually turns up quickly.

Next Steps

If you are building the whole patio, the Patio Calculator works out the MOT Type 1, the sand and cement bed, the membrane and how deep to dig, and the button under your answer above takes your measurements straight there. If you only need the base, the MOT Type 1 Sub-Base Calculator does that on its own. For laying the slabs themselves, my guide to laying paving slabs on a dry mix covers the bed, the ratio and getting each slab to sit level.

If you would rather talk the sub-base and bedding materials for your slabs through with a person, call Outdoor Aggregates on 0330 043 7585. The office is open 7am to 7pm, seven days a week.

Have you ever counted out slabs for a patio, only to find you were a few short or had a pile left over? Which size did you lay, and how much extra did you end up needing? We would love to hear how it went in the comments.

FAQs

How many 600x600 slabs are in a square metre?

A square metre takes 2.78 slabs of 600 x 600mm, because each slab covers 0.36m². Round up and add 10% for cuts: 20m² takes 62 slabs.

How many 900x600 slabs do I need?

A 900 x 600mm slab covers 0.54m², so you need 1.85 slabs for every square metre. A 20m² patio takes 37 to cover the area and 41 with 10% extra for cuts.

How many paving slabs do I need for 40 square metres?

Forty square metres of paving takes 123 slabs of 600 x 600mm, 82 of 900 x 600mm, or 218 of 450 x 450mm, each with 10% extra for cuts and breakages included.

How much sand and cement do I need for a 30m² patio?

A 30m² patio on a 40mm bed at 6 to 1 takes about 1,646kg of sharp sand, 66 bags of 25kg, and 223kg of cement, 9 bags of 25kg. The Patio Calculator works it out for other bed depths and mixes.

What is the best sand and cement mix for laying paving slabs?

Sharp sand and cement at 6 to 1 suits slabs bedded one at a time, and 8 to 10 to 1 suits a screeded semi-dry bed. Never use building sand under slabs, because it holds water and lets the slabs settle.

Do you take the joints off when counting paving slabs?

Paving slab counts are worked out on the face of the slab, with the joints left in as part of the 10% extra. Taking a 10mm joint off saves about 2 slabs on a 20m² patio, and cuts round edges use those up.