Cinder Block Calculator
A concrete block plus its mortar joints occupies a fixed 8 by 16 inch rectangle of wall face, so a wall of a given size takes a predictable number of them. This calculator lays the wall out course by course, subtracts your openings, and adds the waste allowance the masonry association publishes. It also prints the area-based rate the masonry association publishes — CMHA, formerly the NCMA, whose TEK notes are cited throughout this page — which is the lower figure and the one that assumes every cut-off half gets laid.
How it works
One standard block covers 8 in × 16 in of wall face, which is 0.8889 ft². Two counts follow from that. Divide the wall area by 0.8889 and a 100 ft² wall comes to 112.5 blocks, or 113 rounded up — the rate NCMA publishes. Lay the same wall out instead, course by course, and you get a different number, because a course that ends mid-block still takes a whole one off the pallet.
The 8 by 16 inch figure is nominal, and the block in your hand is smaller: 7-5/8 in high by 15-5/8 in long, 3/8 in less each way. That missing 3/8 in is exactly the mortar joint. Unit plus joint restores the 8 in module, which is why counting on the nominal face is right and counting on the block itself would leave you short.
Where the wall lands on the module the two agree: a 20 × 8 ft wall is 15 blocks across by 12 courses, which is 180 blocks either way. Where it does not, they part company — a 10 ft length needs 7.5 blocks per course and you set 8, so a 10 × 10 ft wall is 120 whole blocks against the 113 the area division gives. The headline is the laid-out count, because the half you cut off only saves you a block if it starts another course. The area figure sits underneath for the jobs where it does.
Formula: Blocks = ⌈height in ÷ 8 in⌉ courses × ⌈length in ÷ 16 in⌉ per course, less whole blocks displaced by openings
Measuring on site
- Measure the length end to end and the height from the top of the footing to the top course. Do not measure to the ground line if part of the wall is below grade — those courses are blocks too.
- Add up your openings in square feet and enter the total. A 3 × 7 ft door is 21 ft²; a pair of 3 × 4 ft windows is 24 ft². The deduction is in whole blocks and rounds down — that 21 ft² door takes 23 blocks off the order, not the 23.6 the arithmetic gives, because the part block at the jamb still gets cut from a full one.
- Half blocks and corner blocks come out of full ones here. The yard sells them as separate items and you may well buy them that way; the count treats each as a whole block, which is what you pay for and carry.
Waste and what it covers
The default 5% comes from the National Concrete Masonry Association's estimating guide, which is the only waste figure in this calculator with a primary source behind it. It covers breakage, not cutting, and this calculator applies it once: the result states both block counts before the allowance — 120 laid out against 113 by area, on a 10 × 10 ft wall — and adds the 5% only to the figure it asks you to order, which is 126. NCMA's own guide prints 119 for that wall, which is their 112.5 with the same 5% on top. Raise it if the blocks have been sitting on a rough site; the field is yours to set.
| Wall | Wall area | Laid out | Order (+5%) | Courses | By area |
|---|---|---|---|---|---|
| 10 × 10 ft | 100 ft² | 120 | 126 | 15 | 113 |
| 20 × 8 ft | 160 ft² | 180 | 189 | 12 | 180 |
| 30 × 6 ft (garden wall) | 180 ft² | 207 | 218 | 9 | 203 |
| 40 × 8 ft | 320 ft² | 360 | 378 | 12 | 360 |
Questions
- How many blocks do I need for a 10x10 wall?
- 120 whole blocks, or 126 with the 5% breakage allowance. The wall stands 15 courses high and takes 8 blocks per course — 7.5 by the arithmetic, and you set 8, cutting the last one. Dividing the 100 ft² by the 0.8889 ft² a block covers gives the lower figure of 113, which is NCMA's published estimating rate; that is the number to use if the cut-off halves get laid at the other end of the wall, and the one to carry over to the mortar calculator, since a block bought as a spare never gets mortared.
- How many cinder blocks are in 100 square feet?
- 112.5 by area, which NCMA's estimating guide rounds to 113 and prints as 119 once its 5 percent waste and breakage allowance is added. This calculator shows the 113 but headlines the laid-out count instead, because 100 ft² of wall is a different order depending on its shape: a 10 × 10 ft wall takes 120 whole blocks, while a 25 × 4 ft wall of the same area takes 114.
- Is a cinder block really 8x8x16?
- No. A nominal 8×8×16 block is specified at 7-5/8 in each way and 15-5/8 in long, 3/8 in under nominal in every dimension. The shortfall is the mortar joint: laid with a 3/8 in joint the unit occupies a true 8 by 16 inch space, which keeps the 8 in module across the whole wall.
- How do I work out courses and blocks per course?
- Divide the height in inches by 8 for courses and the length in inches by 16 for blocks per course. An 8 ft wall is 12 courses; a 20 ft wall is 15 blocks across. Both round up, so a length that lands mid-block means a cut at one end of every course — and multiplying the two is exactly how this calculator reaches the number it asks you to order.
Sources
- Concrete Masonry & Hardscapes Association (formerly NCMA) — CMU-TEC-001-23 — Concrete Masonry Unit Shapes, Sizes, Properties, and Specifications“Specified 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.”
- Concrete Masonry & Hardscapes Association (formerly NCMA) — TEK 04-02A — Estimating Concrete Masonry Materials“Including a 5 percent allowance for waste and breakage, this translates to 119 units per 100 ft² (9.29 m²) of wall area.”
Figures and sources last checked by The BuilderTally Team in August 2026.