Three bags per hundred block is the figure every supply house prints. It is also three times short if you walk into a hardware store and buy premix. Both are here.
Masonry supply houses publish the same estimating figures because they all sell the same thing: a 70 lb bag of masonry cement that you mix on site with roughly two and a half parts sand, about sixteen shovels. Three bags lay a hundred block. Seven bags lay a thousand modular brick. One ton of sand covers seven bags.
Those figures are internally consistent — three bags per hundred block works out to four tons of sand per thousand block, which is exactly the separate rule of one ton per 250 block. That is a good sign the numbers are real and not copied around.
Premixed mortar is a different product. The sand is already in the bag, so an 80 lb bag lays about a dozen block rather than a third of a hundred. Same wall, same joints, roughly two and three quarter times the bag count and no sand order at all.
You can work out the volume of a mortar joint exactly, and it is worth knowing that the answer comes out five to six times smaller than what masons actually buy. Mortar falls off the board, gets buttered on and squeezed out, drops down cells, stiffens in the tub and gets thrown away at the end of a board life.
The trade figures have all of that in them and the geometry does not. So this page uses the published unit-count figures rather than a volume formula dressed up to look precise. Every yield is editable — if your crew has a number that works, use it.
After unit type this is the biggest lever. A ½″ joint runs about half again what a ⅜″ joint does — the published figures move from three bags per hundred block to about four and a half. The scaling here is calibrated to hit both of those points.
| Type | Strength | Cement : lime : sand |
|---|---|---|
| M | 2500 psi | 1 : ¼ : 3 |
| S | 1800 psi | 1 : ½ : 4½ |
| N | 750 psi | 1 : 1 : 6 |
| O | 350 psi | 1 : 2 : 9 |
Proportions are by volume, from the ASTM C270 proportion specification. Stronger is not better. Mortar is meant to be the weak, sacrificial part of a wall — it should crack and be repointed rather than let the units crack.
That matters most on old soft brick. Repointing nineteenth-century brick with a modern Type N or S traps moisture and spalls the faces off, which is why conservation work uses Type O or a lime mortar softer still.
Three of masonry cement plus sand, or about eight of premix. Check which bag is in front of you.
A ton per seven bags of masonry cement. None with premix.
About ninety minutes, and you can retemper with a little water inside that window. Once it has started to set, water will not bring it back — it just makes weak mortar that looks fine going in.