# Forum Home Renovation Sub Flooring  Concrete Calculations

## dub3media

Hi Guys just an apprentice here but was wondering if anyone had any formulas for working out concrete by hand (mixer) for post holes and slabs. 
How many 40kg bags are to a m3 if ratio is 6:1
How much Builders mix would I need for a m3 of concrete 
My boss once told me that if you took a 300mm x 300mm post hole x 8m that would be 1m3 of concrete, anyone know this or something similiar? 
thanks guys look forward to hear some tips

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## malb

> Hi Guys just an apprentice here but was wondering if anyone had any formulas for working out concrete by hand (mixer) for post holes and slabs. 
> How many 40kg bags are to a m3 if ratio is 6:1
> How much Builders mix would I need for a m3 of concrete 
> My boss once told me that if you took a 300mm x 300mm post hole x 8m that would be 1m3 of concrete, anyone know this or something similiar? 
> thanks guys look forward to hear some tips

  Concrete is a mix of sand, aggregate, cement and water. The water is partly absorbed by the sand/cement to bind the mix, and the balance is evaporated, causing some shrinkage. Hence the water content is important to the overall mix. 
A single ratio like 6:1 is fairly meaningless for concrete as it does not relate the quanities of the three solid components, something like 6:2:1 would be more meaningfull. The single ratio would be more relevant to a mortar (sand/cement or lime).
The other factor with ratios like that is whether they relate to volume or weight of components. In order to calculate quantities for a particular volume of set concrete, you would need to know the volume of cement contained in a 40Kg bag. 
Your boss might think that 300mm (.3m) x 300mm x 8m is a cubic metre but he would use this for estimating, not ordering, as if you multiply 0.3m x 0.3m x 8m, the answer is 0.72m3. 
To calculate the volume in a job, break it up into sections of similar size, multiply length x width x depth (all in m) for each section, and add up all the sections for a total volume.  
For example, a slab 6m x 4m x 100mm, with footings all round the edges 600mm deep overall x 450mm wide 
Footings =(6m+3.1m+6m+3.1m) x (.6m-.1m) x .45m
=18.2m x .5m x .45m = 4.1m3
Slab = 6m x 4m x .1m = 2.4m 
Total =Footings + Slab = 4.1 + 2.4 = 6.5m3
In the footings calculation, two corrections were made. The two 3.1m dimensions are the actual distance between the 6m footings (4m total width - 2 x .45 footing width). The .5m depth is the total depth (.6m) - the slab depth (.1m). These corrections are made to avoid inclding these sections in the calculations twice.

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## Brickie

Concrete

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## Blocklayer

> For example, a slab 6m x 4m x 100mm, with footings all round the edges 600mm deep overall x 450mm wide

  If you just want to work out concrete volumes in simple shaped slabs with or without perimeter footings and internal thickenings, try this  http://www.blocklayer.com/ConcreteCalculator.aspx  
or mobile  http://blocklayer.mobi/Slab.aspx 
Just click the correct shapes' radio button, enter the measurements and hit 'Calculate' 
.

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