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

I think you need 45.

Sorry everyone. That last message from me was a bit stroppy.

But this thread demonstrates that nobody can do this sum.

I think.

Or b and q have a calculator

12 ft= 365.76cm, divided by 45= 8.128
8 ft = 243.84cm, divided by 45= 5.42
8x5= 40

40... maybe?

Would it be easier to go back outside and measure your garden in mm?

450mm slabs are approximately 1ft 6 inch square.

If you assume 40 slabs (58) that would fill approximately 7ft 6 12ft.

Another 4 slabs to halve for the remaining area.
So 44 slabs.

Rose789’s calculations are the same as mine but you may need to round up to the nearest whole slab, so

9x6=54

40 slabs, assuming there's small gaps in between for grout/whatever.

However, my maths is appaling and I worked this out by drawing it on the computer... so please don't take a randoms word for it!

450mm is just under half a metre - seems like a normal size.

OP I think it’s 54. 12 ft is 366cm, divided by 45 is 8.13, so you need 9 slabs along the long side. 8ft is 244, divided by 45 is 5.42 so you need 6 slabs along the short side. 9x6 is 54.

Is it not 48? 450mm is just under 1.5ft. If you round it up to 1.5ft to allow for mortar then 1.5ft fits 8 times into 12 and 6 times into 8. 8x6 is 48. I could be wrong but I think you might need extra...

I converted the ft to cm and got 44 slabs.

I then tried it the other way and converted the mm to ft and got 48!

Aaaarrrgghhhhhhhh

I get 44 slabs (allowing for 0.25cm gap in between)

(add 10% for damage/adjustment/future repairs)

You need to convert your imperial measurements to metric - so your total area is 8.19msq = round up to 9msq for ease

*B&Q calculator say 50 slabs (maybe it doesn't allow for gaps)🤷🏻‍♀️

“ But this thread demonstrates that nobody can do this sum.”

No, it demonstrates that the sum depends on what you actually mean in the question. The people who are calculating the area of the space and dividing it by the area of the slabs are failing to take into account the fact that both the space and the slabs have specific dimensions

I feel the need to draw this out...

Assuming it's 4500mm slabs!

My reckoning.
12ft= 3600cm/3.6m so divided = 8 slabs across.
8ft= 2400cm/2.4m so 6 (5.3) slabs down. 6 x 8 = 48 whole slabs.

It is 8 slabs across and 5.3 slabs down. So 8x6 = 48 slabs. Don't really need spares as the off cuts will be the spare pieces.

Usually you can return extra slabs for a refund. So just buy 50, lay the patio with the spacing you want and then return any extra slabs.

This is the best I can manage but for goodness sake double and triple check my maths as I wouldn't rely on me :

One side of your space is 12ft. A ft is 30.48 cm. So the side is 366cm.
One side of your space is 8ft. So that side is 244 cm.
Your slab is 45cm. So you will need just over 8 to fill your 12ft edge.
You will need just over 5 to fill your shorter edge.
If you call that 9 x 6 it's 54 slabs.

But you'd have some wastage because you'll need to cut slabs to fit and you'd be able to use the other "half" (or smaller) to fit into subsequent rows. Depends how precise your cutting is! You could potentially reduce by 8.

Snap!

The consensus is that we have not bought enough.

Bollocks. It won't be difficult to get more, I suppose.

But I am surprised that the woman at Jewson's (where we bought them) got three different answers and eventually sold us 36 slabs.

I got three different answers too.

if you are allowing for space between slabs then obviously you will manage with fewer

12 feet = ~366cm
8 feet = ~244cm

Each slab is 45cm

366/45 = 8.1
244/45 = 5.4

So you need a grid of 9x6 = 54.

Plus a few spares.

There are a lot of different answers but have you noticed that none of them are 36??

I'm with the 12 feet is 3.6 m, meaning 8.1 slabs across. And 8 feet being 2.54m, meaning 5.4 slabs.

Now, quite how good you are at cutting slabs will depend on how many you need......

I think, including mortar, you will only need 8 for the 12ft length.
However, I'd probably break loads cutting them in half, and think you will need 6 to do the 8ft length.
6x8 is 48.