How mortar quantity scales
Mortar quantity comes down to two numbers: how many units (blocks or bricks) your wall needs, and how many of those units a bag of mortar can set. Manufacturers publish yield figures for each — this calculator uses conservative values of 12 blocks or 20 bricks per 80 lb bag of pre-mixed mortar, so you're less likely to come up short mid-job.
Those two yields lead to a counterintuitive result: a single block takes more mortar than a single brick — its bed and head joints are simply bigger — but a block wall still uses less mortar per square foot of finished wall than a brick wall does. A standard 8x16 in block covers about 0.89 sq ft of wall face, so one 80 lb bag (yielding 12 blocks) covers roughly 10.7 sq ft of block wall. A modular brick covers only about 0.15 sq ft, so the same size bag (yielding 20 bricks) covers just under 3 sq ft of brick wall. Put another way, a brick wall needs roughly 3.7 times as many bags per square foot as a block wall of the same area — because brick construction packs far more joints into every square foot, even though each individual joint is smaller.
Worked example: a 100 sq ft block wall
Say you're building a 100 sq ft concrete block wall and buying mortar for it. Here's how the math flows:
100 sq ft × 1.125 blocks/sq ft = 112.5 → 113 blocks (rounded up)
Add the standard 10% waste allowance for dropped and scraped-off mortar, then round up to a whole block again:
113 × 1.10 = 124.3 → 125 blocks
Finally, divide by the conservative yield of 12 blocks per bag and round up to a whole bag:
125 ÷ 12 = 10.4 → 11 bags of 80 lb mortar
This calculator runs the same three steps automatically for both blocks and bricks, whether you enter a unit count directly or let it work the count out from your wall's length and height.
Mortar types: N, S, and M
Mortar isn't one product — it comes in strength grades, and using the wrong one can be a real problem for anything structural. Type N is the general-purpose mortar for most above-grade residential work: garden walls, veneer, chimneys, and interior masonry. Type S has higher bond and compressive strength and is the standard choice for below-grade work, exterior load-bearing walls, and anywhere the wall needs to resist more lateral force. Type M is the strongest of the three and is typically reserved for below-grade structural work — foundation walls, retaining walls, and masonry in direct contact with soil. If your project comes with an engineer's spec or falls under local building code, which is common for anything structural or below grade, that specification overrides any general rule of thumb here.
Pre-mixed bags vs. mixing your own
Traditional mortar is mixed on-site from portland cement, lime, and sand, combined in ratios set by standards like ASTM C270 — this is still how many professional masons work, especially on large jobs where buying raw materials in bulk is cheaper.
For a DIY project, pre-mixed bags are almost always the more practical choice. Each bag already has cement, lime (or a lime substitute), and sand proportioned and blended consistently, so you just add water. There's no need to source, store, or measure three separate materials, and it removes the most common way DIY mortar goes wrong — an inconsistent mix that sets up too weak or too fast.
Don't mix more than you can use
Once water hits the mix, mortar starts to stiffen — most pre-mixed bags stay workable for roughly 90 minutes, though hot, dry, or windy conditions can shorten that noticeably. Mortar that's already begun to set shouldn't be "re-tempered" by splashing in more water to loosen it back up; that weakens the bond even if it looks workable again.
The practical takeaway is to mix in batches you can actually place within that window — a wheelbarrow or two at a time rather than the whole day's supply — rather than trying to save time by mixing everything up front.