How to work out the area of a ring (oh-er)

[Dr Murdoch]

I've got a planter that circles a tree, about a 500mm deep. I'm just trying to figure out how much gravel I need to cover the surface of the soil,

Outer circle has about a 5m circumference, inner about 2.5m.

Thanks

[Shakermaker]

Area of a circle is pi * radius squared

And if you just want to fill the outer ring then I would go

5m diameter is 2.5m radius

2.5 squared * pi = 19.63m squared

But then subtract the area of the inner circle

1.25 squared * pi = 4.90 m squared


19.63m - 4.90m = 14.73 metre squared

[Mandat]

Dr Murdoch said:

I've got a planter that circles a tree, about a 500mm deep. I'm just trying to figure out how much gravel I need to cover the surface of the soil,

Outer circle has about a 5m circumference, inner about 2.5m.

Thanks

π r^2 is what you need.

ETA: Shakermaker has done the sums for you above, although be aware that the result is 14.7 square metres, and not 14.7 metres squared. The difference is quite big.

Edited by Mandat on Friday 20th March 09:53

[The Mad Monk]

Shakermaker said:

Area of a circle is pi * radius squared

And if you just want to fill the outer ring then I would go

5m diameter is 2.5m radius

2.5 squared * pi = 19.63m squared

But then subtract the area of the inner circle

1.25 squared * pi = 4.90 m squared


19.63m - 4.90m = 14.73 metre squared

I may be wrong - I often am, but I think you are misusing the word squared. I think it should be square metres, not metres squared.

https://simple.wikipedia.org/wiki/Square_metre

[hutchst]

He has also conflated circumference with diameter

[CaptainSlow]

hutchst said:

He has also conflated circumference with diameter

Indeed, I'd only give it an A at GCSE. Sorry no A******* for him.

[Mandat]

hutchst said:

He has also conflated circumference with diameter

Good spot.

OP needs to be clearer on what he has actually measured.

[Dr Murdoch]

Mandat said:

Good spot.

OP needs to be clearer on what he has actually measured.

Hopefully this clears it up?

[Dr Murdoch]

hutchst said:

He has also conflated circumference with diameter

I haven't.

One has a measuring wheel....

Bit tricky to measure the diameter as there is a ruddy trunk in the way...

[droopsnoot]

So as circumference is pi * diameter, 5m = 3.14159 * 1.59m, so the outer diameter is around 1.6m. The inner diameter on the same basis would be around 0.8m.

Inner circle area is now 0.5 square meters, outer circle is just over 2 square meters, so the difference is around 1.5 square meters.

[boyse7en]

In that case,

C=Pi x D

Big circle

5 = 3.14 x D
D= 1.6m


Small circle
2.5= Pi x D
D= 0.8m

Then area:

Big circle = 2m squared
Small circle = 0.5m squared

So 1.5m squared of gravel

[Parsnip]

https://www.wolframalpha.com/input/?i=area+of+a+ci...

Wolfram alpha is great for stuff like this.

[Dr Murdoch]

Thanks for the helpful replies, legends!

[GAjon]

By the time you’ve worked out the calculation you need to calculate what you need.
You could have gone to the merchant, bought one bag of stone, put it in seen how far it filled then got the balance to suit.

[Dr Murdoch]

GAjon said:

By the time you’ve worked out the calculation you need to calculate what you need.
You could have gone to the merchant, bought one bag of stone, put it in seen how far it filled then got the balance to suit.

And wheres the fun in that!

[Shakermaker]

CaptainSlow said:

hutchst said:

He has also conflated circumference with diameter

Indeed, I'd only give it an A at GCSE. Sorry no A******* for him.

Damn!

And you're right, that is exactly what I did get at GCSE

[GAjon]

Dr Murdoch said:

And wheres the fun in that!

True, and it’ll also deprive you of the hours of fun pouring over your calculations whilst looking at 4 one tone bags of decorative gravel you’ve got left over!

[Piersman2]

GAjon said:

Dr Murdoch said:

And wheres the fun in that!

True, and it’ll also deprive you of the hours of fun pouring over your calculations whilst looking at 4 one tone bags of decorative gravel you’ve got left over!

Colour I presume as bags of stones don't make much noise?

[Greenmantle]

<pedantic mode on>

aggregate comes in cubic metres!

</pedantic mode off>

[spikeyhead]

Greenmantle said:

<pedantic mode on>

aggregate comes in cubic metres!

</pedantic mode off>

when I were a lad it came in cubic yards.