How much cement, sand and aggregate concrete takes
How to work out cement bags, sand and aggregate for concrete from its size and grade, what M20 and M25 mean, why the 1.54 dry-volume factor is used, and when a nominal mix is not good enough.
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In short
- Wet volume × 1.54 gives the dry volume of materials, which is then shared by the mix ratio. One cubic metre of M20 takes about 8 bags of cement.
- M20 means a strength of 20 N/mm² at 28 days. Its nominal mix is 1 part cement, 1.5 parts sand and 3 parts aggregate.
- A 50 kg bag of cement is 0.0347 m³, because cement weighs about 1,440 kg/m³.
- A 20 × 15 ft slab, 5 inches thick, is 3.54 m³ and takes about 29 bags in M20.
- Nominal mixes are estimates. Structural concrete above about M20 is normally designed by an engineer and tested.
Before a concrete pour, someone has to order cement, sand and aggregate, and get all three to site at the right time. Too little cement stops the pour halfway, which is about the worst thing that can happen to a slab. Too much ties up money and risks bags setting in storage. The usual site method gives a good estimate from nothing but the dimensions and the grade, and this page explains how it works and where it stops being enough.
The method
There are three steps. First find the volume of wet concrete. For a slab, that is length × width × thickness.
wet volume = length × width × thickness
Second, allow for the air between the dry grains. When water goes in, finer particles fill the gaps between coarser ones, so a cubic metre of finished concrete takes more than a cubic metre of loose material. The usual factor is 1.54.
dry volume = wet volume × 1.54
Third, share the dry volume by the mix. In a mix of 1 : 1.5 : 3, there are 5.5 parts in all, so cement is 1 part in 5.5:
cement (m³) = dry volume × 1 ÷ 5.5
cement bags = cement (m³) ÷ 0.0347
The 0.0347 is the volume of a 50 kg bag, since cement weighs about 1,440 kg/m³.
For one cubic metre of M20, the dry volume is 1.54 m³, cement is 0.28 m³, which is 8.06 bags. Sand is 0.42 m³ and aggregate 0.84 m³.
What the grades mean
The M stands for mix, and the number is the characteristic compressive strength in N/mm² at 28 days. The usual nominal mixes are:
| Grade | Mix (cement : sand : aggregate) | Cement bags per m³ | Sand per m³ | Aggregate per m³ |
|---|---|---|---|---|
| M10 | 1 : 3 : 6 | 4.44 | 16.3 cu ft | 32.6 cu ft |
| M15 | 1 : 2 : 4 | 6.34 | 15.5 cu ft | 31.1 cu ft |
| M20 | 1 : 1.5 : 3 | 8.06 | 14.8 cu ft | 29.7 cu ft |
| M25 | 1 : 1 : 2 | 11.09 | 13.6 cu ft | 27.2 cu ft |
M10 and M15 are used for plain work such as levelling courses under footings and floors. M20 is the common grade for slabs, beams and columns in small buildings. Beyond that, concrete is normally made to a designed mix, as the next section explains.
Worked examples
A roof slab. A slab 20 ft by 15 ft and 5 inches thick has a volume of 3.54 m³, or 125 cu ft. In M20 it takes 28.54 bags of cement, so buy 29, along with 52.5 cu ft of sand and 105 cu ft of aggregate. In practice, a few spare bags are worth having, because a slab must be poured in one go.
A yard floor. A 10 × 10 ft floor, 4 inches thick, is 0.94 m³. In M15, it takes about 6 bags of cement, 14.7 cu ft of sand and 29.3 cu ft of aggregate. The same floor in M25 would take 10.47 bags.
A thousand square feet. A 5 inch slab over 1,000 sq ft is 11.8 m³. In M20, that is about 95 bags of cement, 175 cu ft of sand and 350 cu ft of aggregate.
Sand and aggregate are often sold by the brass in western India. One brass is 100 cu ft, or 2.83 m³. A cubic metre of M20 uses about 0.44 brass of sand and aggregate together.
Why 1.54, and is it exact?
The 1.54 is a working figure, not a law of physics. It reflects the voids in loose dry material and allows for a little loss. You will also see 1.52 and 1.57 in different books. With different sand and aggregate, the true figure moves a bit.
For ordering, this hardly matters, as the difference between 1.52 and 1.57 is about 3 percent, which is less than the sensible margin you would keep anyway. If the job is big enough for that to matter, it is big enough to have a mix design.
Concrete mix can also be worked out by weight, with a design that starts from the water-cement ratio. That method gives different cement figures from the nominal one, which is why one calculator may say 7 bags and another 8 for the same grade. Neither is wrong, because they start from different assumptions.
Where a nominal mix is not enough
A nominal mix assumes average materials, measured by volume. Real strength depends on many things the ratio does not capture:
- Water. Extra water makes concrete easier to place and much weaker. It is the most common cause of poor concrete on site.
- Measurement. Batching with head-pans is rough. Weight batching is more reliable.
- Sand moisture. Damp sand bulks up, so a pan of it contains less sand than a pan of dry sand.
- Compaction and curing. Air left in the concrete, or concrete allowed to dry too soon, loses strength. Curing with water for the required days matters as much as the mix.
For this reason, grades above about M20 are normally produced as a designed mix, tested with cube samples. Ready-mix concrete suppliers provide this, and it can be the better choice for a slab if a transit mixer can reach the site.
Adding wastage and other parts of the job
This calculator gives the quantities for the volume you enter. Allow a few percent on top for spillage and for an uneven base. Formwork that bulges can also add a surprising amount of concrete. Round up on cement bags in particular, as running short mid-pour is worse than having a spare bag.
Concrete in a slab is only part of the structure. The reinforcement is covered by the steel weight calculator, and the walls by the brick calculator. For shapes other than a flat slab, such as a round column, get the volume from the volume calculator and use that figure with the per-cubic-metre table above.
Materials per 100 square feet
For a quick budget, this table gives M20 quantities for each 100 sq ft of slab. Scale it to your own area.
| Thickness | Wet volume | Cement bags | Sand | Aggregate |
|---|---|---|---|---|
| 4 inches | 0.94 m³ | 7.6 | 14.0 cu ft | 28.0 cu ft |
| 5 inches | 1.18 m³ | 9.5 | 17.5 cu ft | 35.0 cu ft |
| 6 inches | 1.42 m³ | 11.4 | 21.0 cu ft | 42.0 cu ft |
One extra inch of thickness across a slab adds a fifth or more to every material, which is why levels are worth checking before the pour.
Storing and handling cement
Cement reacts with moisture from the air as well as from rain, so storage matters. Stack bags on a raised platform, away from damp walls, and cover them. Use the oldest bags first, and check the week of manufacture printed on them. Cement that has formed hard lumps which do not crumble between the fingers should not go into structural work.
Have the sand and aggregate on site before you start, and check that the sand is clean. Silt reduces the strength of the concrete, and a simple test is to shake some sand in a jar of water and see how thick a layer of silt settles on top.
What this calculator assumes
- The concrete is a flat rectangle, with thickness in inches when measuring in feet and centimetres when measuring in metres.
- The dry volume factor is 1.54, and a 50 kg bag of cement is 0.0347 m³.
- The mix is nominal, by volume, with the ratios in the table above.
- Quantities exclude wastage, formwork losses and water.
- It is an estimate for ordering materials. An engineer should set the grade, thickness and reinforcement for any structural work.