i have seen a lot of member postings in the past asking for estimate of construction cost which is basically a total of the cost of the construction materials + labor. so i think it's a good idea to share my experience with my cost estimation of materials with regards to our house construction before. hopefully, it will help owners in planning better for their project ... for any estimation, you need to have a house/bldg plan of your construction so that you can get an estimate of the materials needed. without a plan, it would be difficult for you to come up with a good and realistic estimate :-) he he he ...

let's start with concrete (cement + sand + gravel) as this is one of the major items in any construction ... lagi ko kasing na babasa ito sa mga postings :-) ...

2 things you need to know to compute the amount of concrete you need for a project ... one is the area/volume of the space in which the concrete is to be use and the ratio of the mixture of the concrete (cement + sand + gravel) ... if water is also critical, you can also include this in your estimate by getting the info on the amount of water (liters) per bag of cement ...

to get the area (e.g. of a slab, beam, column, footing, etc), all you need is it's dimension - L (lenght) * W (width) * D (depth). it's best to measure everything in metric (mm - milimiters), this way you can compute for everything, even the amount of concrete needed for 1 CHB :-) ...

let's use as an example a 20 sqm (4m x 5m) concrete slab with a depth of 100mm (4") ... from a recent posting i saw the other day ... so this will be 4000mm x 5000mm x 100mm in mm ...

1. to get the volume (e.g. cubic meters) of concrete needed for a 20 sqm slab ... the formula is below ...

{ [ ( L x W x D ) / 100000] / 10000 }

( L x W x D ) is the area ... divide the result (product) by 100000 to convert millimiter (mm) to meter (m) ... divide the result (product) by 10000 to convert meter (m) to cubic meter (m3) ... end result is you have a volume in cubic meters ... for our example, a 20 sqm slab with a depth of 100mm will need 2 cubic meters (m3) of concrete ... { [ ( 4000 x 5000 x 100 ) / 100000] / 10000 }

2. concrete requires 3 things - cement + sand + gravel or aggregates that is mix based on a specified ratio. in construction, concrete for structure is normally in the ratio of 1:2:4 - 1 part cement + 2 parts sand + 4 parts gravel or aggregate ... the end result is a concrete of 7 parts (volume) ... FYI, for mortar (walls) the ratio can be 1:6:0 and for plastering (palitada) it can be 1:3:0 ... you don't need gravel/aggregate for mortar or plastering which is why the 3rd ratio is 0 ...

so, to get the amount of cement needed for a 2 cubic meters of concrete the formula is ... (volume * ratio of cement) / mixture total ratio ... (2 * 1) / 7 = 2/7 = 0.2857 cubic meter of cement ...

so, to get the amount of sand needed for a 2 cubic meters of concrete the formula is ... (volume * ratio of sand) / mixture total ratio ... (2 * 2) / 7 = 4/7 = 0.5714 cubic meter of sand ...

so, to get the amount of gravel/aggregate needed for a 2 cubic meters of concrete the formula is ... (volume * ratio of gravel) / mixture total ratio ... (2 * 4) / 7 = 8/7 = 1.1429 cubic meter of gravel/aggregates ...

in summary ... 2 cubic meters of concrete with a ratio of 1:2:4 will require ...

0.2857 of cement

0.5714 of sand

1.1429 of gravel

--------

2.0000 cubic meters of concrete (total) ...

to convert the amount of cement into bags of cement ... just multiply the cement in cubic meters by 35.31 ... 0.2857 * 35.31 = 10.0886 bags of cement ... from my research in the internet, the normal estimate is that 1 cubic meter of cement = 35.31 bags of cement ...

by doing the above computations for each item (footings, columns, beams, walls, plastering inside and outside) that will require concrete in your project, you will be able to get a good estimate of the amount of cement, sand, and gravel that you will need for the whole project ... you can also add 5% for wastage in your totals of cement, sand, and gravel/aggregate ... a simple excel file using the above formula for each item in your construction project is not that hard to do :-) ...

next topic i'll post is how to estimate the rebars needed for a project which is a little bit more complicated as you have different sizes of rebar to use for a project and you also need to consider how to cut the rebars to minimize wastage ... hope this info will help owners to better plan for their construction project ... and for contractors, to have a better (lower) estimate in bidding for a proejct ... cheers

Boo!

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