Concrete Block Calculator

The count is geometry. Everything after it is a published rule.

One conventional block shows 0.8889 square feet of face. Divide net wall area by that and you have the count. The masonry association whose estimating sheet this page is built on writes it the other way round: For conventional units having nominal heights of 8 in. (203 mm) and nominal lengths of 16 in. (406 mm), the exposed surface area of a single unit in the wall is 8/9 ft2 (0.083 m 2). Including a 5 percent allowance for waste and breakage, this translates to 119 units per 100 ft2 (9.29 m2) of wall area.

That sentence does two things at once. The 0.8889 square feet is arithmetic; the 119 is that arithmetic plus a 5 percent allowance for waste and breakage. A published allowance — the only sourced waste figure on this site — so the field opens at 5, and the default wall here reproduces the source's own answer: 100 square feet, 113 units in the wall, 119 on the order.

Openings come off first, because the same sheet says they must: the area of windows, doors and other wall openings needs to be subtracted from the total wall area to yield the net masonry surface. Lintels, jamb, bond-beam and pilaster units are not in that deduction; they are counted off your plans.

Wall thickness is deliberately absent from it: a 6, 8, 10 or 12 inch unit has the same 8 by 16 inch face and covers the same wall. Thickness moves the grout and the price, never the block count.

Mortar: three ranking pages, three unsourced figures

Of the six results measured for block calculator, one is blocked by a firewall and one is an app listing. Three of the four calculators we could read print a mortar yield: seven bags per hundred blocks, blocks divided by 33.3, one 80 lb bag of Type S per eight or nine blocks — roughly 14, 33 and 8 blocks a bag. None cites a source.

Why they disagree. These are bags of CEMENT batched on site with separate sand, not bags of preblended mortar mix. A 70 lb bag of masonry cement laid with its own sand is not a 60 or 80 lb bag of mix off a pallet, and the yields cannot be swapped.

The sheet we cite gives two rules of thumb, both for hollow units in face-shell bedding. Masonry cement: One 70 lb (31.8 kg) bag of masonry cement will generally lay approximately 30 hollow units if face shell bedding is used. Portland and lime: One 94 lb (42.6 kg) bag of portland cement, mixed in proportion with sand and lime to yield a lean Type S or rich Type N mortar, will lay approximately 62 hollow units if face shell bedding is used. Sand goes with them — a ton per 8 bags, about 240 units to the ton, plus half a ton past three; or 4.25 cubic feet and half a bag of lime per bag of portland.

Put the default wall through both: 4 bags of 70 lb masonry cement with 0.5 ton of sand, or 2 bags of 94 lb portland cement with 1 bag of 50 lb lime and 8.5 cubic feet of sand. Scaled to the round number two of the three search variants keep asking about: 34 bags or 17 per 1,000 blocks — printed here as an output, which none of the pages we could read does.

Both rules of thumb are written for hollow units in face-shell bedding with conventional 8 by 16 in faces, and are independent of wall thickness because only the two face shells are buttered. Change the face and this sheet hides mortar and grout rather than scaling a published figure by a guess.

Switch to 4 by 16 inch half-high units and the count still works — 237 instead of 119 — but mortar and grout go away with the reason attached. In the source’s words, mortar is probably the most commonly misestimated masonry construction material. The same document publishes no yield at all for preblended mortar, so that option ships blank and asks for the figure on your bag's label.

Grout, by wall thickness and by how many cells you are filling

Grout is the one quantity here that depends on wall thickness, and it is published as a table: Table 3 provides guidance for the required volume of grout necessary to fill the vertical cells of walls of varying thickness. Its footnote sets the conditions — Assumes two-core hollow concrete masonry units and 3% waste — so the three percent is already inside the figure and this sheet adds none.

Only that first column is stored here; every other spacing is the published row times eight over the spacing, which is how the table is built — so it is linked, not copied. The default wall fully grouted comes to 36.1 cubic feet, 1.34 cubic yards; at 16 inch centres, 18.05. Four inch walls are not in the table, so this sheet grouts none.

Two ranking pages publish a grout figure per hundred square feet of 8 inch wall: 2.1 cubic yards on one (57 cubic feet) and 1.12 on the other (30), against the 36.1 above. Neither ties its figure to a source.

Nominal, specified, and the joint you must not add twice

A block sold as 8 by 16 inches does not measure that. The industry’s technical note says so plainly: dimensions of concrete masonry units are typically 3/8 in. (9.5 mm) less than nominal dimensions, so that a 4 or 8 in. (102 or 203 mm) module is maintained with 3/8 in. (9.5 mm) mortar joints. By that arithmetic the face is 7.625 by 15.625 inches, and the missing 0.375 of an inch is the joint that puts the module back.

Hence one way to get this wrong: the nominal face already includes the joint, so there is no joint field here. Two of the four readable competitors offer an editable joint thickness, which invites you to add it twice; a third bakes one into a fixed factor without saying where it comes from.

Gross    Ag =  sum( L x H )            every elevation
Openings Ao =  sum( w x h x count )
Net      An =  Ag - Ao                 (sq ft)
Face     Af =  fh x fl / 144           nominal, joint included
Units    U  =  An / Af
Order    Uo =  ceil( U x (1 + waste / 100) )
Mortar   Bm =  ceil( Uo / 30 )         masonry cement, 70 lb
Sand     St =  Bm / 8  ( + 0.5 over 3 tons )
or       Bp =  ceil( Uo / 62 )         portland, 94 lb
Grout    Gv =  An / 100 x G(width) x 8 / spacing   (cu ft)

Worked example: a garage, four walls and four openings

Two 24 foot walls and two 22 foot walls, 9 feet high, with a 16 by 7 foot overhead door, a 3 by 6 foot 9 inch service door and two 4 by 3 foot windows, in 8 inch block grouted every 32 inches.

1. Gross: 828 sq ft over four walls.
2. Openings: 156 sq ft off, 672 sq ft net.
3. Blocks: 756 in the wall, 794 to order.
4. Mortar: 27 bags and 3.88 tons of sand, half-ton waste included.
5. Grout at 32 in centres: 60.6 cu ft, 2.25 cu yd.

Every line is on the takeoff tab with its arithmetic attached and exports to CSV with a link back to this sheet. Lintels, the bond beam and the reinforcing are off your drawings.

Frequently asked

How do I calculate how many concrete blocks I need?

Net wall area divided by the face of one block. The 100 square foot wall above holds 113 units of 0.8889 square feet each; with the published 5 percent for waste, 119 to order.

How many bags of cement do I need for 1000 blocks?

34 bags of 70 lb masonry cement, or 17 of 94 lb portland with about half that many of lime — 1,000 divided by 30 or 62, face-shell bedding, rounded up. Cement batched with sand, not premixed mortar.

How many cement blocks are required for 1000 square feet?

1,182 for 1,000 square feet of wall: 1,125 in the wall plus the published 5 percent. Wall area, not floor area.

How many sq ft does a cinder block cover?

0.8889 square feet of wall face, eight ninths. The nominal 8 by 16 inch face already contains the 0.375 inch joint, so you never add one.

How many 8x8x16 blocks do I need?

119 for this 100 square foot wall. The first 8 is the thickness and changes nothing: 0.8889 square feet of face at 6, 8 or 12 inches thick.

How many blocks do I actually need?

113 go in the wall, 119 on the order: the difference is the 5 percent for waste. Set it to zero and the sheet orders 113. Special shapes are extra.

Limits of this sheet

A-201 turns elevations and openings into a block count, then into mortar and grout through two published rules of thumb and one table — read them at CMHA TEK 04-02A and NCMA TEK 2-1B. It does not design a wall, size reinforcing or estimate labour. No unit weight, pallet count, rebar schedule or price ships here: nothing we opened publishes them for concrete block. Footings go through S-101, mortar sand through C-103. Confirm with your plans, your supplier and your local code.

Sheet A-201 prepared by TakeoffMetric Editorial; no author, credential or field measurement is claimed.