Can anyone do this sum because I have given up

[TheMustressMhor]

Okay - so we are putting concrete slabs on a small area of the garden.

This measures 12 ft x 8 ft.

The slabs we have chosen measure 450mm x 450 mm.

My brain has fallen out trying to work out how many slabs we will need.

Even the woman in the shop got three different answers and eventually sold us 36 slabs.

I think 36 slabs is wrong.

But the fact that I am incapable of working this out is driving me mad. My brain has fallen out and I appeal to anyone who has any sense to tell me how many slabs we need.

TIA

Are the slabs definitely 450mm x 450mm as this seems very small; are they not 4500 x 4500?
that would be 4.5 metres a slab
Definitely 450mm

Yep, the one think it's not is 36!

THING

44 slabs

It’s 54 plus 10% for breakages

Some questions

Will there be a gap between slabs?
How easy is it to break the slabs? - so if you need two half slabs will you be able to use the same one or do you need two?

Yes, I've noticed that none of the answers are 36, fairygoodmother.

When DH came out of the shop and told me he had been sold 36 slabs I said immediately that it wasn't enough.

Then my phone died so I could not access any assistance.

Depends if you can cut them, not easy! You 8 x 6, 48, if you don't cut them.

It's 48. And you will need to trim somewhere.

You lay 8 slabs along the 12' length. They will cover 18".
Then another 8 (16 in total) covering 36" (or 3ft)
Then another 8 (24 in total) covering 54" (or 4ft 6)
Then another 8 (32 in total) covering 72" (or 6ft)
Then another 8 (40 in total) covering 90" (or 7ft 6)
Then another 8 (48 in total) covering 108" (or 9ft)

You could try and do it with 40 and spread the slabs out, but that would leave a nasty gap, and you'll have enough weeds as it is.

I am going to ring Jewson's up and change the order.

I shall also ask if they will take back any unused slabs.

I would go out and measure the space again, in metric not imperial. Your sum will be easier to compute using the same system.

I am not personally going to even attempt the sum as my head would fall off (dyscalculia). My DH and DS are my Tech Department and Maths Department. I Do Not Do Maths in which case why did I even click on your thread, must be barking

OP could you share photo of your new patio when it's finished and number each slab so we all know the answer. I'm now invested in this as much as the parking thread

Working it out in mm
12ft = 3657.6mm
8ft = 2438.4 mm
That makes the area 8,918,691.84 mm squared

One slab is 450 mm x 450mm = 202,500 mm squared

Divide area by one slab = 44.04 slabs so best to get around 50 to cover splitting slabs to fit/breakages

My very first calculation was 48, interestingly.

Then the woman changed the subject slightly and said that they had some which she was getting rid of as clearance, so much cheaper than the ones we looked at first.

They are fine (the clearance ones) so we chose them.

And that's when she came up with various different answers.

I went back to the car and left DH to it. He came out eventually and said he had paid for 36.

That is when I said I would start a thread about it.

we may have had a bit of a row as well

One side is 12' or 3658mm, requiring 8 slabs assuming you can fill in 58mm with the gaps between each slab and don't want a 5cm bit of slab at the end. If you do, add another 1-2 slabs to your end total to break up and add here.

The other side is 8' or 2438mm, requiring about 5.5 slabs.

So you need 8x5 + 8 half-slabs (so 4, each to be broken in half) - so 44. Or 45/46 if you want the extra bits as in step 1.

Why don't you lay the 36 and see how many more you need?

The different answers are because people are rounding figures differently when they convert between mm and in; because some are dividing the area paved by the area of the slab and ignoring how the slabs will actually be laid out; making different assumptions as to whether you'll be able cut slabs into half neatly; and making different assumptions about the gaps between the slabs.

If you take your rough calculation that 450mm is 18 inches, then the 12ft length gives you exactly 8 "rows", and the 8ft length gives about 5.33 "columns". If you think you can cut slabs in half neatly you'll need 40 slabs to fill 5 "columns" plus another 4 each cut in half to fill the left over .33 column, ie 44 total. If you don't think you can cut slabs neatly, you'll need 8 slabs (discarding 2/3 of each) to fill the .33 column, ie 48 total.

It's weird the way you are doing it. Does it have to be that exact size? Will you be happy to cut the slabs? When we laid a path, we picked the slabs we wanted and built the path so it used full-sized slabs. You could also choose slabs that more evenly fit the space.

I will gladly share photos of the new patio InOtterNews and I may even bury DH under it

I'd run back for more then, in case they're discontinued.

It's utterly normal have rows about the number of slabs required, it's the maths wot does it

The only way 36 could be right is if you're having different slabs as edging to make a pattern and make it fit better and not have a weird 5-6cm gap.

40 full slabs, and 8 half slabs (well, slightly less)

You need a minimum of 44, depending on pattern.

We don't mind cutting the slabs and we intend to have slate chips round the edges and sand and cement between the slabs.

Damn it. I can't post pictures

So 12x8 are 96 square feet of garden
That is 8.9 square metres, let’s do 9 square metres
Each slab is 0.45mx0.45m=0.2025square metres

9 square metres / 0.2025 = 44 slabs

But your garden is not square, your garden is 12 feet x 8 feet therefore your garden is 3.7 metres x 2.5 metres.

Since each slab only covers 0.45 metres you need

3.7/ 0.45= 8.22 slabs for the long part
2.5/0.45= 5.6 slabs for the short

Let’s round up to 9 and 6
9x6=54

The reason why you get different answers if because people are not accounting for your garden being a rectangle and the slabs being square.

assuming 45cm = 1.5 feet
8 1.5 = 12 in one direction
5 1.5 = 7.5 in the other direction

So the 7.5 side deficit is .5 of a foot, so you need 8 * .5 which can be got from 3 slabs.

In total then 8 * 5 + 3 = 43