Mortar Calculator

How the mortar calculator works

Laying 500 modular bricks with ⅜-inch joints takes 10 sixty-pound bags of Type N mortar mix. Those 500 bricks hold about 3.9 cubic feet of mortar in their joints; a 60-lb bag of pre-mixed mortar yields roughly 0.45 cubic feet, and a 10% waste allowance brings the total to 9.5 bags — round up to 10. Put another way: budget 2 bags per 100 bricks, or 3 bags per 100 CMU blocks, at standard ⅜" joints.

Building a floor bed rather than a wall? The third mode, Deck mud (dry pack), sizes the packed sand-and-cement bed used under tile and in shower pans — a 3 × 3 ft shower pre-slope takes 3 sixty-pound bags. It uses a different material and a different yield from the two masonry modes; see deck mud (dry pack) below.

The calculator derives everything from one number: the volume of mortar that actually sits in the joints. For a single-wythe modular brick (7⅝ × 2¼ × 3⅝ inches) each unit carries one bed joint under it and one head joint beside it:

• Bed joint — brick length × brick depth × joint thickness = 7.625 × 3.625 × 0.375 = 10.4 cu in
• Head joint — brick height × brick depth × joint thickness = 2.25 × 3.625 × 0.375 = 3.1 cu in
• Total — 13.5 cu in, or 0.0078 cubic feet per brick

Multiply by your unit count, add the waste factor, and divide by the bag yield. Nothing empirical is smuggled in, which means the "joint volume" and "bags" figures in the results panel always agree with each other — change the joint thickness and both move together, in proportion.

Why this calculator says more bags than the old rule of thumb

You will find "one bag of mortar lays 125 bricks" repeated across the internet, and it is worth explaining because it is half right. That figure is real, but it refers to a bag of masonry cement — the cement-and-lime powder you then mix with about three cubic feet of sand. That combination yields roughly one cubic foot of mortar, and one cubic foot of mortar does indeed lay about 128 modular bricks.

A 60-lb bag of pre-mixed mortar mix — the kind sold at any home center, sand already included — yields only about 0.45 cubic feet. Applying the masonry-cement rule to a premix bag understates what you need by roughly 2.5×. That is the difference between finishing a wall and standing in a parking lot at 4 p.m. with a half-laid course going stiff. If a number you find elsewhere looks generous, check which kind of bag it is counting.

Coverage per bag

Units laid per bag, before any waste allowance. Joint thickness matters more than anything else on this page — going from ⅜" to ½" joints costs you a third of your coverage.

Bag yields used: 0.45 cu ft for 60 lb, 0.60 cu ft for 80 lb. CMU figures assume face-shell bedding — mortar on the two 1¼" face shells only, which is what TMS 602 permits and what masons actually do on standard block work. If you fully bed the webs as well (some DIY builds do), block consumption roughly triples: plan on one bag per 12 blocks instead of 36.

The default waste factor is 10%, which covers normal droppings and what stays stuck in the mixing tub. If this is your first masonry job, raise it to 15–20% in Advanced options. Mortar waste runs higher than tile or mulch waste because a beginner's board life is short — mortar that stiffens past use gets thrown out, and that happens more often when the laying is slow.

Mortar types — pick by exposure, not by strength

ASTM C270 defines five types, named for alternating letters of "MaSoN wOrK." Only four see real use. The proportions column is portland cement : hydrated lime : sand, by volume.

Stronger is not better. This is the single most expensive mistake in masonry repair. Mortar is designed to be the sacrificial element in a wall: it should be weaker than the units it joins, so that seasonal movement cracks the joint — which is cheap to cut out and repoint — rather than the brick. Repoint a soft pre-1920 brick wall with Type S or Type M and the joints will outlast the brick faces, which then spall off in freeze-thaw cycles. That damage is permanent and the brick cannot be replaced without matching a discontinued unit. Soft historic brick wants Type O or a straight lime mortar; if the building has any heritage status, get a mortar analysis before you buy anything.

Worked example 1 — brick veneer wall

A 20-ft × 6-ft single-wythe veneer wall is 120 sq ft. Modular brick runs about 7 per sq ft, so that is 840 bricks. At ⅜" joints: 840 × 0.0078 = 6.5 cu ft of mortar, 7.2 cu ft with 10% waste, divided by 0.45 = 16 sixty-pound bags. At $7.50 a bag that is $120 of mortar against roughly $500–$700 of brick. Mortar is never the expensive part — which is a good argument for buying one bag more than you think you need.

Worked example 2 — CMU garden wall

A 24-ft × 4-ft wall of 8" × 16" block is 96 sq ft, or 108 blocks at 1.125 blocks per sq ft. At ⅜" joints with face-shell bedding: 108 × 0.0126 = 1.36 cu ft, 1.5 cu ft with waste, divided by 0.45 = 4 bags. Use Type S here, not Type N — block walls carry compressive load and the bottom course sits at or below grade. If you are filling any cores with grout for rebar, that is a separate material and a separate volume; do not try to stretch mortar into it.

Mixing your own: masonry cement and sand

Past roughly 300 bricks, mixing your own gets noticeably cheaper. One 70-lb bag of Type N masonry cement (cement and lime pre-blended) takes about 3 cubic feet of mason sand — 18 to 20 shovels — and yields a bit over 3 cubic feet of mortar. Our 500-brick job needs 4.3 cubic feet including waste, so that is 1½ bags of masonry cement plus about 4½ cubic feet of sand.

The economics, using 2026 US planning prices: 10 premix bags at $7.50 is $75. The same job mixed from 1½ bags of masonry cement (~$14 each) plus a quarter-ton of mason sand (~$15 delivered as part of a larger load) is closer to $36. You need a mixer or a strong back and a mortar tub, and you need mason sand specifically — not play sand, not concrete sand. The sand calculator will size the pile.

Mix to a consistency that holds a peak on the trowel and clings to a vertical brick face without sagging. Add water in small amounts; mortar that is too wet loses bond strength permanently, and unlike concrete you cannot fix it by waiting. Mix for a full 3 to 5 minutes after the last water goes in — under-mixed mortar looks fine and bonds badly.

Board life, re-tempering, and weather

Mortar stays workable about 90 minutes, less in heat or wind. You may re-temper once — add a splash of water and remix — within the first 2½ hours, replacing water lost to evaporation. After 2½ hours from initial mixing, throw it out; the cement has begun hydrating and any further water permanently weakens the bond. Never re-temper twice, and never save mortar overnight.

Below 40°F (4°C), standard practice is to heat the mixing water to 70–120°F and protect the finished work for 24 to 48 hours. Never lay on a frozen substrate or with frozen units — the bond will not form and the wall will need to come down. In hot, dry weather the opposite problem appears: high-suction brick pulls water out of the mortar before it can cure. Test by dropping twenty drops of water on the bed face; if it disappears in under 90 seconds, dampen the bricks a few hours ahead so they are damp but surface-dry when laid.

Tooling the joints

Tool when the mortar is thumbprint-hard — firm enough to hold an impression without smearing. Tooling is not cosmetic: compressing the surface is what makes a joint shed water.

• Concave — the default, and the most weather-resistant. Compresses the joint tightly and has no ledge to hold water.
• V-joint — nearly as good as concave, crisper shadow line.
• Weathered — sloped to shed water outward; good performance, harder to strike consistently.
• Struck — slopes the wrong way and leaves a small ledge. Common on older work, mediocre in wet climates.
• Raked — deep shadow, worst water performance. The shelf holds water directly against the joint. Reserve it for dry climates or protected walls.

Repointing math is different

Repointing only fills the outer portion of the joint, so the volume is far smaller than new construction. Rake or grind the old mortar to a depth of about 2 to 2½ times the joint width — ¾" for a standard ⅜" joint, and never less than ½", or the new mortar will not hold. At that depth, 100 square feet of brick wall takes roughly 1.2 cubic feet of mortar, about 3 sixty-pound bags. To use this calculator for repointing, switch to wall-area mode and then take about a quarter of the resulting bag count — or simply enter the number of joints you are actually cutting out, which is usually a fraction of the wall.

Deck mud (dry pack) for shower pans and floor beds

A 3-ft × 3-ft shower pre-slope, 1¼ inches thick at the drain and sloped ¼ inch per foot, takes 3 sixty-pound bags of sand/topping mix — about 1.15 cubic feet of packed deck mud at an average thickness of 1.54 inches. Switch the calculator to Deck mud (dry pack) and it does this arithmetic for any floor size, sloped or flat.

Deck mud — also called dry pack, floor mud, or a mortar bed — is not the same material as the mortar in the rest of this page, and that trips up more shower builds than any other single thing. Wall mortar is a plastic, sticky mix with lime in it, designed to be buttered onto a trowel and to bond two units together. Deck mud is the opposite: a lean, 5 parts mason sand to 1 part portland cement blend, mixed barely damp, that you pack, tamp, and screed to a precise slope. It has almost no bonding job to do. Its job is to hold a shape.

That difference shows up in the yield, which is why deck-mud mode uses a different number from the brick and block modes. A 60-lb bag of pre-mixed mortar yields about 0.45 cubic feet; a 60-lb bag of sand/topping mix — the correct bagged product for deck mud — yields about 0.5 cubic feet, because there is no lime in it and a packed bed compacts differently from a buttered joint. An 80-lb bag yields about 0.67 cubic feet.

Deck mud coverage per bag

Square feet of finished bed per bag, before any waste allowance. Thickness dominates everything here — a bed at 2 inches consumes twice what the same floor does at 1 inch.

How the sloped-bed math works

A shower pre-slope is a shallow cone, not a slab, so its volume is the floor area times its average thickness rather than its thickness at the drain. Plumbing code (IPC 417.5.2) requires a shower floor to fall not less than ¼ inch and not more than ½ inch per foot toward the drain; ¼ inch per foot is what almost everyone builds, and it is what the calculator assumes.

For a drain near the middle of the floor, the average distance from the drain to a point on that floor works out to about 0.38 times the floor's dimension. So the calculator adds ¼" × 0.38 × (length + width) ÷ 2 to your minimum thickness to get the average thickness, then multiplies by the area. On the default 3 × 3 pan that is 1¼" + 0.29" = 1.54" average — which is why 9 square feet of floor needs 1.15 cubic feet of mud rather than the 0.94 you would get by using the drain thickness alone.

Choose Flat bed instead when you are building a level mortar bed — a bathroom floor being brought up to height, a bed over a wire-reinforced subfloor, or the top bed of a traditional shower pan, which sits at a constant thickness because the pre-slope beneath it already carries the fall.

Traditional shower pans need the calculator run twice

A liner-type shower pan is two beds with a waterproof membrane sandwiched between them, and both beds are deck mud:

• Pre-slope — packed on the subfloor, ¾" to 1¼" at the drain, sloping ¼" per foot to the walls. Run the calculator in sloped mode. This layer is what makes the liner itself drain; a liner laid flat holds standing water permanently, which is both a code failure and the usual reason old pans smell.
• Top bed — packed over the liner at a roughly constant 1¼" to 1½", following the pre-slope's fall. Run the calculator again in flat mode at that thickness.

Protect the drain's weep holes with a ring of pea gravel or broken tile before you pack the top bed. Those holes are how water that reaches the liner gets out; mud packed straight over them seals the pan shut and the bed stays saturated.

Pre-slope figures assume 1¼" at the drain, ¼" per foot of fall, and 10% waste. If you are building over a bonded waterproofing membrane instead of a liner — the modern method — there is only one bed, so use the sloped column alone.

Mixing your own deck mud

Deck mud is the easiest masonry mix to make yourself, because the ratio is forgiving and there is no lime to measure. Five parts mason sand to one part portland cement, by volume. A 94-lb bag of portland cement is one cubic foot, so one bag plus five cubic feet of mason sand is a batch — and because the cement mostly fills the voids between sand grains rather than adding bulk, that batch yields roughly five cubic feet of packed mud, enough for four 3 × 3 pre-slopes.

The economics favour bagging for small jobs and mixing for large ones, same as wall mortar. Three bags of sand/topping mix for one pre-slope runs about $25 at 2026 prices. A bag of portland plus five cubic feet of mason sand runs about $30 and does four times the work — but you need a mixing tub, a hoe, and somewhere to put a quarter-ton of sand. Use mason sand, not play sand and not concrete sand; the sand calculator will size the pile.

Water is where deck mud goes wrong. It should be barely damp — on the order of 5–6% by weight, far drier than anything else on this page. Use the snowball test: squeeze a handful hard. It should hold its shape with no water squeezing out, and when you break the ball in half the broken edges should stay crisp rather than crumbling. If it glistens or slumps, it is too wet, and too-wet mud cannot be packed, shrinks as it dries, and cracks. Mud that is too dry crumbles under the tamp and never gains strength; add water a cup at a time.

Packing and screeding

Set your perimeter height first — a band of mud around the walls at the finished height, and a small pad at the drain — then fill between them, tamp with a wood float or a scrap of 2×4, and screed with a straightedge riding those two references. Pack in one lift where you can; deck mud does not bond well to itself once the first layer has set. Finish with a steel or magnesium float. Do not feather the bed below about ½ inch anywhere, and never below 1¼ inch at the drain of a shower — thin deck mud has no strength and will crack under the tile.

Cover the finished bed with plastic and leave it overnight. Most beds are firm enough to tile on the next day; a bed you can kneel on without leaving a mark is ready. If you are working a large flat area that does not need a slope, a self-levelling underlayment is usually faster and flatter than deck mud — deck mud earns its place when you need a slope, a thick build-up, or a bed the pour cannot give you.

One naming note that causes confusion at the counter: floor mud (deck mud) is 5:1 with no lime; wall mud (fat mud) is about 4:1 with lime added so it will cling to a vertical surface. They are not interchangeable. If you are also setting tile over the finished bed, the tile calculator will size the tile, thinset, and grout.

Common questions

Pair this with the brick calculator to size the wall itself, the sand calculator for mason sand if you are mixing your own, and the concrete calculator for footings — a freestanding masonry wall needs one below the frost line. For a deck-mud floor bed, the tile calculator sizes the tile, thinset, and grout that go on top of it.