You have purchased your concrete blocks, laid the rebar, and when it comes time to order the concrete, doubt creeps in: how much do you actually need to fill all those cavities without running short… or paying for too much? This is one of the major questions on home-building projects, especially when you’re working alone or on weekends.
The answer actually depends on the type of block, its dimensions, and whether the wall is load-bearing. Between the classic hollow block and the formwork block, the volumes can differ significantly, but a few numerical benchmarks help you aim accurately. The key lies in a handful of simple figures.
Hollow block or formwork block: correctly identify your wall before calculating
A classic hollow block is mainly used to construct walls where only certain cavities will be filled with concrete, for columns or reinforcements. Blocks sized around 20 x 20 x 50 cm, close to the American format 8 x 8 x 16 inches, hold, when filled completely, about 0.25 cubic feet, which is a little over 7 litres of concrete per block.
Formwork blocks, on the other hand, create a true lost formwork: all the cavities are filled along the planned height. The technical data sheets for many 20 x 20 x 50 cm formwork blocks indicate around 11.5 litres of concrete per block, i.e., nearly 115 litres per square meter, since roughly ten blocks are needed for 1 m² of wall. Some ranges drop to about 100 L/m², while others rise to 160 L/m² depending on thickness.
Concrete quantity per block and per m²: the right benchmarks
American calculations also provide a rough budgetary idea: a 60-pound bag of pre-mixed concrete yields about 0.45 cubic feet of cured concrete, roughly 13 litres. It can thus fill almost two standard 8 x 8 x 16 blocks. These bags cost between $4 and $7 each, about €3.7 to €6.5: for a large wall, the bill adds up quickly.
For a project in France, the most practical references remain those expressed in litres per square meter. For a 20 x 20 x 50 cm formwork block, using around 115 L/m² provides a realistic estimate when the manufacturer’s datasheet isn’t at hand. Many guides give a range of 100 to 160 L/m² depending on models and wall thickness. It is always better to verify the exact value in the technical documentation of the chosen block.
Calculate the volume of concrete and the number of bags for your blocks
The basic method remains the same for all walls filled 100%: concrete volume (in litres) equals wall area (in m²) multiplied by the per-square-meter yield. Then convert to cubic meters by dividing by 1,000, and add a small safety margin of 5 to 10% to avoid running out of concrete during casting.
A concrete example helps put things into perspective. For a formwork-block wall of 12.5 m², with a manufacturer’s yield of 110 L/m², you obtain 12.5 x 110 = 1,375 litres, i.e., 1.375 m³. By adding a 10% margin, you aim for roughly 1,510 litres. If you use ready-mix bags that provide on average 13 litres per 25 kg bag, you would need 1,510 / 13, i.e., a little over 116 bags; rounding up gives 117 bags.
