How many concrete blocks are in 100 square feet?

About 113. One standard block with its mortar joint covers 0.888 sq ft, so 100 ÷ 0.888 = 112.6, rounded up to 113. Add 5% for breakage and cuts and you would order 119.

How many blocks are in a square foot?

1.125. It is the inverse of the 0.888 sq ft a nominal 8 by 16 inch block covers. Multiplying your wall area by 1.125 and multiplying by 0.888 are the same calculation.

Do 4-inch and 12-inch blocks change the block count?

No. Every standard block shares the same 8 by 16 inch face, so the count for a given wall area is identical. Only the depth changes, which affects weight, grout volume per core, footing width and price per unit — not how many you need.

How much mortar do I need for 100 blocks?

About 3 bags for a standard &frac38;-inch joint, so round up to 3. That is a labeled planning typical — a wider joint or a rougher block face uses more, and the coverage printed on your bag is the number to trust.

Should I subtract windows and doors from the block count?

Deduct openings larger than about 10 sq ft; below that, the cut blocks around the opening use up what you would have saved. A 3 by 4 ft window is 12 sq ft, or 13.5 blocks — deduct 13, rounding the deduction down so any error leaves you with blocks rather than short.

Is a cinder block the same as a concrete block?

For counting purposes, yes. "Cinder block" is an older name from mixes made with cinder aggregate; the modern concrete masonry unit is still a nominal 8 by 16 inches on the face, so the same 1.125 blocks per square foot applies.

How many concrete blocks do I need? Block & mortar count

A standard block wall takes 1.125 blocks per square foot. Multiply your wall area by that, round up, add 5% for breakage, and you have your order. This guide shows where the number comes from, how to cross-check it course by course, and what to do about openings, corners, mortar and grout.

By Francesco Zinghinì

The short answer

Measure the wall face in square feet and divide by the area one block covers:

Blocks = wall length (ft) × wall height (ft) ÷ 0.888, rounded up

0.888 sq ft is the face a standard 8×16 block covers once its mortar joint is counted. Inverted, that is 1.125 blocks per square foot — the number masons carry in their heads. A 100 sq ft wall is 113 blocks. Add 5% for breakage, cuts and the odd bad block and order 119. The concrete block calculator does this from a length and a height, and adds the mortar.

Where 0.888 comes from

A concrete masonry unit is not actually 8 inches by 16 inches. It is 7⅝ by 15⅝, which leaves room for a ⅜-inch mortar joint on the top and one end. Block plus joint = a nominal 8 by 16 inches, and that nominal size is what tiles the wall:

8 in ÷ 12 = 0.667 ft × 16 in ÷ 12 = 1.333 ft = 0.888 sq ft per block

This is why the arithmetic never drifts: it is geometry, not a product spec. It also explains why every block dimension in a wall lands on a multiple of 8 inches. A wall designed in 8-inch increments needs almost no cutting; one designed in odd inches needs a cut block in every single course.

A worked example: a 22 ft 6 in wall

Take a garden wall 22 ft 6 in long and 5 ft 4 in high — deliberately not a round number, because real walls rarely are.

Area: 22.5 × 5.333 = 120 sq ft.
Blocks: 120 ÷ 0.888 = 135.1 → 136 blocks.
With 5% waste: 136 × 1.05 = 142.8 → 143 blocks.

Now cross-check it the mason way, in courses. Height 5 ft 4 in is 64 inches, and each course is 8 inches tall: 64 ÷ 8 = 8 courses. Length 22 ft 6 in is 270 inches, and each block eats 16 inches: 270 ÷ 16 = 16.875 → 17 blocks per course (the last one gets cut). 8 × 17 = 136 blocks — the same answer. When the two methods disagree by more than a block or two, you have mismeasured something.

The course check is worth doing because it tells you something the area formula hides: that 0.875 of a block per course means eight cut blocks, one per course, all the same size. If you can shorten the wall to 21 ft 4 in (256 in = exactly 16 blocks) you buy fewer blocks and cut none of them.

Openings, corners and half blocks

Three adjustments turn a clean rectangle into a real wall:

Openings. Deduct anything larger than about 10 sq ft; below that the cut-offs around the opening absorb the saving. A 3 ft × 4 ft window is 12 sq ft = 13.5 blocks, so deduct 13 (round deductions down, so the error stays on the safe side). Our wall drops to 123 blocks, or 130 with waste.
Corners. Running bond means courses alternate, so at every corner one course starts with a full block and the next with a half. Corner-return blocks with a finished end are usually a separate line on the order — budget roughly one per course per corner and buy them as such.
Half blocks. Every wall end and every opening jamb needs a half block on alternate courses. Count them course by course rather than assuming; splitting full blocks works but wastes material and time.

Mortar, and what it does to the order

The planning typical is about 3 bags of mortar per 100 blocks for a standard ⅜-inch joint. Our 143-block wall:

143 × 3 ÷ 100 = 4.3 → 5 bags (round up — you buy whole bags)

Two things move that figure. A wider joint or a rougher block face eats more mortar; and if you are mixing from masonry cement and sand rather than buying premixed bags, plan roughly a ton of sand per 8 to 10 bags of cement. These are labeled planning typicals — confirm the coverage printed on the bag you actually buy, because it varies by product.

Filling the cores

If the cores are being grouted, that is a second material order entirely, and it dwarfs the mortar. A common planning figure for 8-inch block is about 0.3 cubic feet of grout per block with every core filled — roughly 90 blocks to the cubic yard. Our 136-block wall, fully grouted:

136 × 0.3 = 40.8 cu ft ÷ 27 = 1.51 cubic yards

That is a ready-mix delivery, not a bag job — price it with the concrete cost per yard tool, or check the crossover with ready-mix vs bags. If only some cores are filled, scale the figure by the fraction: every other core is half the grout. Core volume varies with the block manufacturer and the web pattern, so confirm it against the block spec sheet before you order.

Vertical bars in grouted cores are a quantity question too, not a count you should guess at. The rebar calculator gives linear feet from a spacing you enter, and rebar spacing and sub-base basics covers the labeled typicals. What bar size goes where in a load-bearing or earth-retaining wall is a structural question for a licensed engineer and your local code — not something a calculator decides.

Does block thickness change the count?

No. This is the single most useful thing to know about block ordering. A 4-inch, 6-inch, 8-inch, 10-inch and 12-inch block all share the same 8 by 16 inch face — only the depth changes. So a 120 sq ft wall is 136 blocks whether it is a 4-inch garden wall or a 12-inch foundation wall.

What does change is everything downstream: weight per block (a 12-inch unit is roughly twice a 6-inch one to lift), grout volume per core, footing width, and price per unit. "Cinder block" and "concrete block" are the same thing for counting purposes — the old name survives from mixes that used cinder aggregate, and the modern unit is still nominally 8 by 16.

Where block counts go wrong

Four recurring mistakes, in rough order of how much they cost:

Using the actual 7⅝ × 15⅝ size instead of the nominal 8 × 16. That gives 1.21 blocks per square foot instead of 1.125 — an 8% over-order, because it ignores the joint that is doing part of the work.
Measuring to the top of the wall but forgetting the footing. The first course sits on the footing, so a stepped footing on sloping ground means the courses per section differ. Count each section separately and add.
Skipping the waste allowance. Blocks crack in transit, get dropped, and get cut wrong. 5% is the floor; go to 10% on a wall with several openings or corners, where cutting is heavy.
Forgetting the pallet. Block is sold and delivered by the pallet — commonly 90 to 108 units, depending on size and supplier. Your 143-block order is two pallets, and the second one is mostly spare. Ask what a partial pallet costs before you assume you can buy exactly 143.

The bottom line

Wall area ÷ 0.888, or 1.125 blocks per square foot, rounded up and plus 5%. Cross-check it as courses × blocks per course. Deduct openings over 10 sq ft, count half and corner blocks separately, allow about 3 mortar bags per 100 blocks, and treat grout as its own cubic-yard order. The count is the same for every block thickness. These are material-quantity figures from labeled industry typicals — confirm coverage and core volume against the block and mortar you actually buy, and leave load-bearing or retaining wall design to a licensed engineer working to your local code.

Where these conventions come from: American Concrete Institute publishes the reference standards for concrete and masonry, ASTM International maintains the dimensional standards that fix a block at a nominal 8 by 16 inches, and Portland Cement Association covers how the work is done in practice.