Cement Calculator

This cement calculator answers the question of how much cement powder is consumed by the mixing components used. By entering volume and mixing ratio, the required quantities of bags of cement, sand, aggregates and water as well as the total cost of materials are calculated.

Most of the tools that show up on search results are concrete volume calculators. These only calculate up to a cubic meter or work with ready-made concrete in bags which is an entirely different product.

If you're using on-site cement bags, sand piles and aggregates to create a mixture, then you need exactly those detailed results.

Cement is not concrete.

Cement is a grey powder, it's an adhesive and is rarely used as a standalone building material.

Concrete is made of cement, sand, aggregates (such as gravel or crushed stone), and water. Mortar is made of cement, sand, and water; it does not contain any aggregates and is used to bind bricks or concrete blocks together.

Typically cement makes up only 10% to 15% of the total volume of concrete. This complicates ordering cement as even a small change in mix ratio can lead to a significant change in quantity required.

How much cement is required?

With only four steps, just the first step is required to obtain an estimate.

Start with the wet volume, which is the space that the finished concrete mix will take up. Multiply this by a factor to convert from dry to wet volume. There's air between the particles of the dry aggregates and this space will be filled in later with water. Then add a buffer for losses. Finally divide the result according to the proportion of each material in the mix ratio.

Here, s and g stand for the proportions of sand and aggregates respectively per unit cement. So in a 1:2:4 mix ratio, the proportion of s is 2, that of g is 4, and there are seven parts altogether. The weight of powder and number of bags depends on its density.

The bulk density of normal Portland cement is about 1,440 kg per cubic meter, which is approximately 90 lb per cubic foot. Therefore a 50 kg bag of cement contains about 0.0347 cubic meters or 34.7 liters of powder.

Why dry volume is larger than wet volume:

This is a step that's often overlooked but affects about one-third of orders.

When dry cement, sand and aggregates are stacked up, a large part of it is air. When water is added, fine particles penetrate the voids between coarse particles. The mixture compacts, resulting in volume less than the original volume.

Engineers take this into account and use a dry to wet conversion factor of between 1.52 and 1.57 for concrete. This calculator starts at 1.54 and can be changed. If the dry volume is already given, it will be set to 1; if pure cement mortar with no aggregates, this should be reduced to a value close to 1.

Nominal mixing ratio, strength class and compressive strength:

The nominal mix ratio is not a laboratory developed ratio but one that can be mixed with quantities taken by shovels. The designation of the grade indicates the compressive strength which the mixture should achieve after 28 days. M20 means 20 megapascals.

The number of bags is calculated using a dry to wet conversion factor of 1.54, a cement density of 1440 kg per cubic meter and a bag weight of 50kg without wastage. Changing any one of these values will result in different results. A comparison table for calculation tools performs these calculations based on your own settings.

For construction applications, the mixing ratios specified by the supplier are usually better than nominal mixing ratios. Nominal mixing ratios are suitable for foundations, roads, underlays, small slabs and general on-site construction work.

Example calculation:

Suppose you pour concrete cubes with a side length of 200mm into a steel mold using the lowest strength standard mix ratio of 1:5:10.

The mixing ratio of 1:5:10 consists of 1+5+10, which is a total of 16 parts. One part of this is cement.

The sand makes up five parts or 0.00385 cubic meters. The aggregates make up ten parts or 0.0077 cubic meters. With a water-cement ratio of 0.5, this mix will require 0.55 liters of water.

To convert this to a full cubic meter of concrete in the ratio 1:2:4 you would need 0.22 cubic meters of cement, which is 316.8 kg or 6.34 bags of 50kg, plus 0.44 cubic meters of sand and 0.88 cubic meters of aggregate.

The water-cement ratio determines strength.

Water is measured by weight relative to the cement, not volume. A water-cement ratio of 0.5 means that half the weight of the cement is used as water. So 25 liters of water would be equivalent to one bag (50kg) of cement.

The usual water-cement ratio for construction concrete is normally between 0.45 and 0.55. For concrete mixed on site by hand this figure can go up to 0.6, as a wetter mix is easier to handle.

If this value is exceeded the strength rapidly decreases and after excess water evaporates voids are left. Frost damage or corrosion of reinforcement often begins in these voids. If the mix is too stiff to work it is better to use a plasticizer rather than simply adding a bucket of water.

Standards for cement bags worldwide.

Cement is sold in bags and the quantity indicated on purchase is important but standards vary by region.

When calculating the result, the quantities are rounded up because you can't buy 6.34 bags of cement for example. As cement has a shorter shelf life after opening the package, it is more economical to buy larger amounts even if this results in small residual amounts.

Mortar, plaster and pure cement.

If you select a particular ratio for mortar, the proportion of rock is automatically hidden since mortar does not contain aggregates.

The 1:4 ratio is typically used for exterior walls or load-bearing masonry while 1:5 or 1:6 is suitable for interior walls and plastering work, and 1:3 for structural construction or underground works. A higher cement content does not always mean better quality. The strength of the mortar must be lower than that of the stones or bricks used. Otherwise, in case of movement, it will be the mortar which cracks first before the stones break.

If you mix cement and water only (no sand), you get a slurry or paste that is used for grouting, adhesives, and small construction items. It is not suitable to be applied in thick layers because it shrinks and cracks badly.

Common mistakes to avoid:

A common and serious problem is to order cement based on the wet volume. If the conversion factor from dry to wet is omitted, this will eventually result in a shortage of about one third.

Another common problem is the incorrect order of mixing ratios. The order is cement, sand, aggregate. So 1:2:4 means one bag of cement, two bags of sand and four bags of aggregate. Reading it in reverse gives a mix with four times as much cement which will cost accordingly more.

Measuring by scooping with a shovel rather than using a bucket can also lead to inaccuracies. A shovelful of sand and a shovelful of cement powder are not the same volume, and wet sand can expand in volume by up to one-third. Using measuring containers or buckets will allow precise adherence to mixing ratios.

Do not try to increase the strength of a bag of cement by reducing water without increasing the amount of cement itself. The most important ratio is that of water to cement. If you add only water without increasing the amount of cement, the material will be significantly weaker than M15 unnoticed with a mix ratio of 1:2:4.

This calculator is for planning and educational purposes only. The actual amount required will vary depending on the actual moisture content of sand, gradation of aggregate, local conditions and standards used. Therefore please consider the results as an estimate that is approximate to reality. For construction work where bearing capacity is needed follow construction drawings and seek advice from supplier.