Concrete Mix Calculator
About Concrete Mix Calculation
The Concrete Mix Calculator determines the composition and proportions of a concrete mix from the specified volume, strength class, workability and material properties. It calculates the required quantities of cement, water, sand and coarse aggregate in kilograms and cubic metres, as well as their proportions by mass and volume.
The calculator is intended for preliminary concrete mix design, planning material purchases and preparing concrete on site. All quantities are first calculated for 1 m3 of finished concrete and then scaled to the required volume.
Guidelines and recommendations
Calculating the water requirement
Base water content. The starting value is 190 l per 1 m3 of concrete mix. This quantity is adjusted according to concrete workability, maximum coarse aggregate size and aggregate type.
Workability adjustments. For slump classes S1, S2, S3, S4 and S5, the respective adjustments are -20, -10, 0, +10 and +20 l/m3. Higher required workability leads to a higher calculated water content. The slump classes correspond to cone slump values: S1 - 10-40 mm, S2 - 50-90 mm, S3 - 100-150 mm, S4 - 160-210 mm, S5 - at least 220 mm.
Aggregate size adjustments. For maximum aggregate sizes of 8, 16, 22 and 32 mm, the adjustments are +12, 0, -6 and -12 l/m3, respectively. Larger aggregates generally require less water to achieve comparable workability.
Coarse aggregate type. Crushed stone has an additional adjustment of 0 l/m3, while gravel reduces the water requirement by 5 l/m3. This reflects the differences in particle shape: rounded gravel generally improves the workability of the mix.
Water content = 190 + workability adjustment + aggregate size adjustment + aggregate type adjustment
After applying the adjustments, the calculated water content is limited to 140-240 l/m3. If the result is below 140 l/m3, 140 l/m3 is used. If it exceeds 240 l/m3, 240 l/m3 is used.
Calculating the cement requirement
Water-cement ratio. The cement quantity is determined from the calculated water content and the assumed ratio of water mass to cement mass (W/C). Higher concrete strength classes use a lower W/C, so more cement is required for the same amount of water.
The calculator uses the following indicative W/C values:
- C12/15 - 0.62
- C16/20 - 0.58
- C20/25 - 0.53
- C25/30 - 0.50
- C30/37 - 0.45
- C35/45 - 0.42
- C40/50 - 0.40
Cement strength class. An adjustment factor is applied to the calculated cement quantity to account for differences in cement strength. The factor is 1.08 for class 32.5, 1.00 for 42.5 and 0.95 for 52.5. Thus, for the same concrete strength class, 32.5 cement requires 8% more and 52.5 cement 5% less than the calculation with 42.5 cement.
Cement, kg/m3 = (Water, kg/m3 / W/C) × cement class factor
The calculation assumes that 1 l of water has a mass of 1 kg. For example, for C25/30 concrete with workability S3, 16 mm crushed stone and 42.5 cement, the water content is 190 l/m3 and the cement content is 380 kg/m3.
The assumed W/C values and adjustment factors are intended for preliminary mix estimates. Achieving the specified strength class in practice depends on material properties, mixing quality and concrete curing conditions.
Calculating sand and coarse aggregate quantities
Absolute volume method. Once the water and cement quantities are determined, the volume they occupy in 1 m3 of finished mix is calculated. The air volume is also taken into account. The remaining part of the cubic metre is divided between sand and coarse aggregate.
The following particle densities are used to convert mass into absolute volume:
- Cement - 3150 kg/m3.
- Water - 1000 kg/m3.
- Sand - 2650 kg/m3.
- Crushed stone - 2700 kg/m3.
- Gravel - 2650 kg/m3.
These are particle densities without the voids between grains, not bulk densities. The absolute volume of each component is calculated by dividing its mass by the corresponding density.
Air in the concrete mix. An air content of 2% is assumed for 8 mm aggregate, 1.5% for 16 mm, 1.2% for 22 mm and 1% for 32 mm. The corresponding volume is deducted from the total mix volume.
Aggregate volume = 1 m3 - water volume - cement volume - air volume
Sand fraction. The resulting aggregate volume is divided between sand and crushed stone or gravel. The base sand fraction depends on the maximum aggregate size: 45% for 8 mm, 40% for 16 mm, 37% for 22 mm and 34% for 32 mm.
The sand fraction is then adjusted for workability: S1 - a reduction of 2 percentage points, S2 - 1 point, S3 - no change, S4 - an increase of 1 point, S5 - 2 points. With crushed stone, the sand fraction increases by a further 1 percentage point. With gravel, it decreases by 1 point.
After all adjustments, the sand fraction is limited to 28-52% of the total absolute aggregate volume. If the calculated value falls outside this range, the nearest limit is used.
Sand volume = aggregate volume × sand fraction
Coarse aggregate volume = aggregate volume - sand volume
The masses of sand and coarse aggregate are determined by multiplying their calculated absolute volumes by the corresponding particle densities.
Converting aggregate mass to bulk volume
Bulk volume. For purchasing sand and coarse aggregate, the volume including the voids between particles is used. It differs from the absolute volume used to calculate the concrete mix composition.
The following average bulk densities are assumed to convert mass to bulk volume:
- Sand - 1600 kg/m3.
- Crushed stone - 1500 kg/m3.
- Gravel - 1600 kg/m3.
Bulk volume, m3 = material mass, kg / bulk density, kg/m3
Actual bulk density depends on particle size and shape, moisture content and compaction. Therefore, results in cubic metres are indicative, while the calculated material masses are provided separately.
Calculating mix proportions and cement bags
Proportions by mass. The mass of cement is taken as one part. The masses of water, sand and coarse aggregate are each divided by the cement mass. The result is shown in this order: cement : water : sand : coarse aggregate. For example, 1 : 0.5 : 2 : 3 means that 1 kg of cement is combined with 0.5 kg of water, 2 kg of sand and 3 kg of coarse aggregate.
Proportions by volume. Bulk material volumes are used for the calculation. The assumed densities are 1400 kg/m3 for cement, 1600 kg/m3 for sand, 1500 kg/m3 for crushed stone and 1600 kg/m3 for gravel. Water has a density of 1000 kg/m3. The volume of each component is divided by the bulk volume of cement.
The resulting volumetric proportions can be used for approximate batching with measuring containers of equal capacity, such as buckets. However, the actual amount of material in each container depends on moisture content and compaction.
Number of cement bags. The total cement mass is divided by the specified mass of one bag. The result is always rounded up to the next whole number to ensure a sufficient quantity of cement.
Number of bags = round up (cement mass / mass per bag)
Scaling to the required volume. The component masses and volumes calculated for 1 m3 of concrete are multiplied by the specified mix volume in m3. For example, 3 m3 requires three times as much of every material as 1 m3. The component proportions remain unchanged.
European standards and calculation limits
The following standards are used in European practice when specifying requirements for concrete mixes:
- EN 206-1:2026 - Concrete. Specification, performance, production and conformity. Part 1. Performance, requirements, factory production control and assessment criteria for individual values.
- EN 12350 - Testing fresh concrete. A series of standards, including the determination of workability by the slump test.
- EN 12390 - Testing hardened concrete. A series of standards, including the determination of compressive strength.
The water content coefficients, water-cement ratios and aggregate fractions used by this calculator are design assumptions, not universal standardised concrete mix recipes. Final mix design should take into account the actual properties of the materials and the requirements for the finished concrete.
FAQs
Why does stronger concrete require more cement?
A higher strength class is associated with a lower assumed water-cement ratio. With the same quantity of water, this means more cement is required. For example, the initial W/C for C20/25 is 0.53, while for C30/37 it is 0.45, so the latter mix requires more cement.
Why does cement consumption increase with workability?
For a more workable concrete mix, the calculator assumes a higher water content. To maintain the water-cement ratio assumed for the selected strength class, the cement quantity increases as well. Actual workability also depends on aggregate properties and the conditions under which the mix is prepared.
Why is the bulk volume of sand and crushed stone different from their volume in concrete?
Bulk material contains voids between individual grains, so it occupies more space than its solid particles alone. When concrete is prepared, these voids are partly filled by cement paste and finer particles. Therefore, absolute volumes are used for calculating mix composition and bulk volumes for approximate purchasing quantities.
What is the difference between concrete proportions by mass and by volume?
Mass proportions express the ratios of components in kilograms and are useful when batching by weight. Volume proportions allow cement, sand, crushed stone and water to be measured approximately using containers of equal capacity. These ratios differ because the components have different bulk densities.
How does sand moisture affect the accuracy of concrete calculations?
Wet sand contains additional water, which can change the actual water-cement ratio of the mix. The calculator determines the theoretical water requirement without a separate adjustment for aggregate moisture. When preparing concrete, the amount of added water should be adjusted to account for the actual moisture content of the materials.