About this calculator
This calculator turns a site mix ratio (cement : sand : gravel) into a shopping list — how many bags of cement, how much sand and gravel by mass and by volume, and how much water — for a given volume of finished concrete. The ratio can be entered by volume or by mass, whichever the recipe on hand uses.
Formula
wet volume = volume needed + waste%
dry volume = wet volume × dry volume factor
Dry, loose cement, sand and gravel take up more space than the finished concrete because compacting and mixing fills the voids between the particles — the dry volume factor (default 154%, i.e. x1.54) accounts for that. It is a practical site estimate, not a normative constant: adjust it for your own materials if you know a better figure.
by volume: cement volume = dry volume × cementRatio / (cementRatio + sandRatio + gravelRatio)
(sand and gravel the same way)
by mass: volumes are chosen so the resulting masses (volume × density) are in the given
ratio, while still summing to the dry volume above
mass = volume × density
cement bags = ceil(cement mass / bag mass)
water mass = cement mass × water/cement ratio
Net mass and the waste allowance are kept separate throughout, so you can see how much of the total is the allowance.
Worked example
Defaults: 1 m³ of finished concrete, ratio 1 : 2 : 3 by volume, water/cement ratio 0.50, 25 kg cement bags, densities 1400 / 1600 / 1650 kg/m³, dry volume factor 154%, no waste.
dry volume = 1 m³ × 1.54 = 1.54 m³
cement volume = 1.54 × 1/6 = 0.2567 m³ -> mass = 0.2567 × 1400 = 359.3 kg -> 15 bags
sand volume = 1.54 × 2/6 = 0.5133 m³ -> mass = 0.5133 × 1600 = 821.3 kg
gravel volume = 1.54 × 3/6 = 0.7700 m³ -> mass = 0.7700 × 1650 = 1270.5 kg
water mass = 359.3 × 0.50 = 179.7 kg -> 179.7 L
FAQ
Does a 1:2:3 mix guarantee a particular strength class? No. EN 206 defines concrete by performance (a strength class, verified by testing a sample), not by a fixed volume or mass ratio of ingredients — the same ratio can give different strengths depending on the cement type, aggregate and water content actually used. For a structural element (foundations, columns, slabs carrying load), specify and order ready-mix concrete of the required class instead of relying on a site mix ratio.
Why does the dry volume exceed the wet volume? Loose cement, sand and gravel have air gaps between the grains. When mixed with water and compacted, the finer material fills those gaps, so the compacted (wet) volume is smaller than the sum of the dry loose volumes. The dry volume factor is the practical multiplier site crews use to buy enough dry material — it is an estimate, not a physical constant.
Volume or mass ratio — which should I use? Use whichever your mix recipe is written in. A "1:2:3 by volume" bucket-count site mix and a "1:2:4 by mass" batching-plant recipe are both common; this calculator supports either without converting one into the other for you (you tell it which one you're entering).
Which standard does this follow? None is cited for the mix ratio or the dry volume factor — see the disclaimers above. The mass-from-volume conversion (mass = volume × density) is plain physics, not a standard.
Assumptions and limits
The cement, sand and gravel bulk densities, the dry volume factor, and the water/cement ratio are all user inputs with example defaults — not data for any specific product or site condition. The 1:2:3 / 1:2:4 / 1:3:5 style ratios sometimes quoted for site mixes are practical rules of thumb, never a substitute for a specified strength class.
Water content here is derived only from the water/cement ratio, not from aggregate moisture, admixtures, or slump requirements — adjust the bought water accordingly on site. No drawing is produced; this calculator's output is a materials list only.