Hollow Block Calculator — CHB per square metre, mortar and cell filling

12.5 blocks per square metre is true of a 400 × 200 mm module, and a module is the block plus its joint. A block moulded at 390 × 190 mm and laid on a 10 mm joint occupies 400 × 200 mm of wall, 0.08 m², and 1 ÷ 0.08 is 12.5. That is where the trade figure comes from and it is exact once those three numbers are stated. A block moulded at the full 400 × 200 mm, which is what many yard-made CHB are, occupies 410 × 210 mm on the same joint, and the count drops to 11.6. The fields above take whatever face and joint you measure and print the module, so the figure on this page is arithmetic on your block and not a number copied from a table. If the count and the delivery do not agree, measure a block before blaming the mason.

10 blocks per square metre is a different block. That figure belongs to the 450 × 225 mm block of Nigeria, Ghana and much of India, which with its joint makes a 450 × 225 module of 0.101 m². A page that prints 10 per m² for CHB is describing a block a Philippine yard does not sell, and it under-orders by a fifth. The US CMU pages on this site work from an 8 × 16 inch nominal face, 1.125 blocks per square foot, which is 12.1 per m²; close enough to 12.5 to be confused with it, and different enough to matter over a house.

The mortar the joints hold and the mortar the trade orders are not the same quantity, and this page prints both. The joint row is geometry: the module area less the block face, times the full thickness of the block, as if the bed and head joints were solid across the whole block. For a 6 inch wall at 12.5 blocks per m² that is about 0.011 m³ per square metre. Turned into dry material at the 1.3 factor mortar is quoted at, and split 1 : 3, it is about an eighth of a 40 kg bag per square metre. The trade rule of one bag of cement to 50 blocks is a quarter bag per square metre, about double that, because it carries what falls off the trowel, what fills the cores by accident, the mortar mixed at four in the afternoon and thrown away at five, and the thicker joints a wall actually gets. Neither figure comes from a standard read for this page. Order by the trade rule; use the joint row to see how much of the order is allowance.

1 : 3 is what the trade quotes for laying, and it is quoted here as convention. Philippine estimating tables list mortar by class letter rather than by ratio, and public works specifications for masonry are widely reported to call for one part cement to three parts sand by volume. This page did not read that specification, so it does not cite an item number for it, and it does not put class letters on the options. The 1 : 3 default is the mix a Philippine mason will quote you; if the project specification says otherwise, the specification governs. Cement is counted in 40 kg bags, the bag the Philippine market sells, at a loose density of 1,440 kg/m³, which is a convention; the mass on the bag is the fixed thing.

The bar spacing is a field you fill in, not a recommendation, and this page does not size reinforcement. 10 mm bars at 400 or 600 mm vertically and every third course horizontally is what you will hear quoted for a house wall, and that is a habit of the trade, not a design. A wall's bars come from its plan: the engineer sets them from the wall's height, its exposure to wind and earthquake, whether it carries load and how it is tied to columns and beams, and none of those is a field here. What this page does is count. Enter the spacing on your drawing and it prints how many bars that spacing puts in the wall, their length, their mass at 7,850 kg/m³ of steel, and how many 6 m lengths that is before laps, which it does not add. The cell filling likewise takes the cores as a share of the block volume that you can edit, because the void of a yard-made three-hole CHB is not published anywhere this page could read; 40% is a starting figure, not a fact.