My brain hurts
Trying to workout how many bags (25kg) i need to concrete an area 7' square by 4"deep using the 1 2 3 mix. 1 part cement, 2 parts sand and 3 parts stones
Very grateful for your combined brain power.  Common sense isn't very common.
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I can’t see how you can get an answer without knowing the volume of a 25kg bag of concrete. The volume of your hole is 16.33 cubic feet so you need 1/6 of that in cement, i.e. 2.71 cu feet. But how to relate that to weight?

Good point, unfortunately the bags only have weight written on them, i could measure the bag size. Common sense isn't very common.
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2007 Land Rover Freelander2 I6 hse Auto

I use these sites - there are others

https://www.practicaldiy.com/general-build...or-met.php

https://www.practicaldiy.com/general-build...-met-2.php Jules

Loads of online calculators to work out how much you need, in small bags of aggregate or in bulk. Bought some cement the other day ....... It's gone up by almost 50% !!!!

https://wikipedikia.org/how-many-25kg-bags...-concrete/

I find pre mixed aggregate ( sand and gravel much easier to use ). Use at 5 to 1 for general diy concrete jobs.

Hope this helps

Last edited by Grimdog on 22nd May 2022 12:40 pm. Edited 2 times in total

My son is an Agronomist and often has to work out soil mass and compacted volume and he does the following. If you have a suitable container fill it with water and then put the bag in. According to Archimedes it will displace its own volume of water. Either collect the water or find the decrease in weight of the container plus remaining water. 3 x FL1 2 manual + 1 auto
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8 is more than enough.

axle wrote:

My brain hurts
Trying to workout how many bags (25kg) i need to concrete an area 7' square by 4"deep using the 1 2 3 mix. 1 part cement, 2 parts sand and 3 parts stones
Very grateful for your combined brain power.

A lot.
For that amount you'd be better off having the correct amount delivered.

When I made a new oil tank base, I did it all with standard DIY bags of cement and aggregate. I used this because it was difficult to get ready mixed concrete delivered in such a small quantity.

My base is 5' by 5' and up to 12" deep on the lowest side. The base needed to hold a tank which would exceed 1000kg in weight constantly, so I also included some rebar.

I can't remember exactly how many bags of each I used now, but it's a lot. I used the formula of 100 25kg bags per meter cubed, which isn't far off from correct.
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Agree with Nodge . Get a price for half a cubic metre of ready mix . If you have to mix 1100 kgs by a small mixer or worse by hand your finished job is going to be a bit dodgy as you’ll be mixing and pouring , mixing and pouring . It’s about 5 full loads out of a Belle Minimix , the typical hire concrete mixer , then you’ve to clean the thing etc .
Also as you are casting a thin slab , you might want to consider the supplier putting some fibre in for a few quid more .

We had half a cube delivered for fixing some steel posts for a crush on my godson’s farm . I can’t remember exactly but It wasn’t silly money for how much labour it saved us. The driver even brought his own barrow , and the price included half an hour on site to let us get it to the job . 2009 GS Auto Zermatt Silver - Sold June 21 after 10 years of ownership

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axle wrote:

My brain hurts
Trying to workout how many bags (25kg) i need to concrete an area 7' square by 4"deep using the 1 2 3 mix. 1 part cement, 2 parts sand and 3 parts stones
Very grateful for your combined brain power.

Thank you very much guys.
Finally got my head round it.
A lot more work than i thought so will look into ready mixed.
Again thanks for the help.
 Common sense isn't very common.
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2007 Land Rover Freelander2 I6 hse Auto

Here's another way..keep it all metric.
Your project needs 0.21 cu.m of concrete which weighs approx. 1.7 times = 0.357kg
For that you need 15 x 25kg bags of ALL-IN CONCRETEING BALLAST. Don't bother with separate sand and ballast. (If you do buy separately, make sure it's SHARP sand and not soft sand).
And buy 3 x 25kg of cement. Don't bother about its volume in the ratio, cement doesn't bulk out the volume which is why it's often mixed by weight rather than volume.
Having said all that I strongly recommend buying ready mixed by the suppliers of site mixed concrete rather than Ready Mixed.
They charge double the cost per m3 but only for what's mixed on site. Ready mixed delivered in those big spinning trucks on the other hand is cheaper per m3 but you only need a fraction of their 6m3 volume and you will pay a fortune for the empty part of the truck.