Mortar Calculator
Work out mortar mix bags for laying bricks or concrete blocks, or for repointing old joints — in 80-lb and 60-lb bags, with a site-mix equivalent.
Mortar, three ways
Mortar is bought by the bag but consumed by the joint. This calculator works the way the bag manufacturers publish their rates: an 80-lb bag of pre-mixed mortar lays about 36 standard bricks or about 12 standard concrete blocks at 3/8-inch joints, and a 60-lb bag holds three quarters as much. Those field rates already carry the realities of the trowel — furrowed beds, struck joints, dropped mortar — that a pure joint-volume calculation would miss.
Bags = units to lay ÷ units per bag · 80-lb: 36 bricks / 12 blocks · 60-lb: 27 bricks / 9 blocks
Example: laying 240 concrete blocks takes 240 ÷ 12 = 20 bags of 80-lb mortar mix exactly — 22 bags with the 10% waste allowance in, or 30 bags of 60-lb, or the site-mix equivalent of about 3 bags of portland cement, 3 of hydrated lime and 17 cu ft of sand.
Repointing is different work
Repointing fills only the raked-out joint, so it is figured from the wall itself: the fraction of the face that is joint (about 18% for brick, 7% for block) times the depth you rake out — ¾ inch is the standard repointing depth, roughly twice the joint width. Because struck, wasted and over-raked mortar is unavoidable, the bags are figured at a placed-joint yield of about 0.35 cu ft per 80-lb bag, which lands on the trade's planning figure of roughly 25 sq ft of brick wall per bag at ¾-inch depth.
Site mix
For bigger work, pros mix on site. The classic Type N recipe is 1 part portland cement : 1 part hydrated lime : 6 parts sand by volume; a batch built on one 94-lb bag of cement yields about 6 cu ft of mortar. Type S (stronger, for below-grade and load-bearing work) doubles the cement: 2 : 1 : 9. The site-mix rows in the result use the Type N proportions against the same mortar volume the bags represent.
Frequently asked questions
What type of mortar should I buy?
Type N is the general-purpose choice for above-grade brick and block walls. Type S is stronger and is specified below grade, in foundations and for retaining work. Type O is soft and belongs in repointing old, soft historic brick — hard modern mortar can damage it.
How long does mixed mortar stay workable?
About 2½ hours in mild weather, less on hot windy days. Retemper with a little water as it stiffens, but discard mortar that has started to set — and mix half-bags on small jobs so less goes to waste.
Why do the laying rates differ from the joint volume?
Because a trowel is not a syringe. Bed joints are furrowed, head joints are buttered generously, joints are struck and the excess falls. Manufacturer bag rates are measured on real walls, which is why this calculator uses them for laying and reserves pure geometry for repointing, where the joint really is all you fill.
Can I use mortar mix as concrete for a small slab?
No. Mortar has no coarse aggregate, so it shrinks and cracks as a slab and is far weaker in compression across a span. It is a bonding material — the gravel in concrete mix is doing structural work.
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