Block Calculator — 9 inch and 6 inch sandcrete blocks, mortar and cement

Ten blocks per square metre is the count with no joint in it. A 450 × 225 mm face is 0.10125 m², and 1 ÷ 0.10125 is 9.88, which the trade rounds to 10. That block is never laid touching the next one. Put the 25 mm of mortar most Nigerian sites lay around it and the piece of wall one block occupies is 475 × 250 mm, 0.11875 m², which is 8.42 blocks per square metre. On a 54 m² fence that is 455 blocks against 540. The 10 is not wrong so much as unlabelled: it is the joint-free count, and the 17% it carries over the 25 mm figure is doing the job of a waste allowance without saying so. Use it if you like, but do not add 10% waste on top of it, and do not compare it to a quote built on the joint. This page prints the joint it used in the same row as the count, so the two can be compared.

A three-bedroom bungalow is not an input. The question this page is asked most is how many blocks a 3-bedroom, or a 2-bedroom, or a room, takes. It cannot answer that and neither can any page that does, because the count comes from the length of every wall, external and internal, times its height, less the doors and windows, and none of that is fixed by the number of bedrooms. Two 3-bedroom plans on the same estate differ by a thousand blocks. What the page needs is the total wall length and the wall height off the plan, with the openings added up separately. Take a scale rule to the drawing, add the external perimeter and every internal wall, and put that in the length box. Any figure quoted for a bedroom count without a plan behind it is a guess, and if the person quoting it is also selling the blocks it is not a disinterested one.

One bag of cement to 50 blocks is a moulding yield, not a law, and it moves with the mix. The figure is quoted as if it were printed somewhere. It is what a block yard gets when it moulds 9 inch blocks at a lean mix, and the same bag gives 30 to 35 blocks at 1:6 and fewer still at 1:4. A yard that promises 50 per bag is telling you its mix ratio, not its quality control. The strength of the block falls as the yield rises, which is why the 50-per-bag block is the one that crumbles in the hand at the site gate. No standard was read for this page, so no yield is presented here as a specification. The moulding row above takes whatever figure your yard works to and prints the bags that follow from it, and the default is not to mould at all.

The cement for laying is the softest number on the page, and the trade disagrees with itself about it. At 25 mm joints a square metre of 9 inch hollow wall takes about 0.03 m³ of wet mortar, a figure derived from the joint geometry with the cores left out of the bed. From there the arithmetic runs through a 1:6 mix, a cement bulk density of 1,440 kg/m³ and a dry-volume factor, and every one of those is a convention. Take the factor as 1.3 and the answer is roughly one bag to 50 blocks laid; take 1.54, the concrete figure, and it barely changes, because the joint volume is the part that dominates. Site foremen in Abuja and Lagos report one bag to 35 or 40 blocks, and both are honest: a full bed on a solid block, a fatter joint, sand walked into the ground and mortar dropped off the trowel all come out of the same bag. Three to three and a half bags per hundred is what experienced sites buy. This page prints the derived figure and flags the gap, because a page that prints only one of the two has picked a side without telling you.

A 4 inch block is a partition, and 9 inch is 225 mm, not 228.6. Two things readers meet that are wrong. The 100 mm block is offered above because it is sold, but a wall of it is a screen between two rooms, not a wall that carries a roof or a first floor, and no count of blocks changes that. Which walls carry load is the engineer's decision and this page sizes nothing. The second is smaller: 9 inches is 228.6 mm, but the block is made at 225 mm, and 6 inches is 152.4 mm against a 150 mm block. The inch names are trade names for metric sizes, which is why a wall dimensioned in inches on the drawing comes out a few millimetres short on site over every course. Set out in millimetres.