How this concrete block calculator works
A standard block (nominal 8 × 8 × 16 in) covers 0.89 ft² of wall including its mortar joints, so block count is wall area times 1.125. The calculator subtracts openings, applies the block trade's 5% waste convention, and figures mortar and core fill from published coverage:
mortar = blocks ÷ 13 per 80 lb bag · core fill = 0.84 yd³ per 100 blocks
Worked example: how many blocks for a 20 × 8 wall
The default wall is 20 × 8 ft = 160 ft² of block: 160 × 1.125 = 180 blocks exactly — that's 12 courses of 15 blocks. Add 5% for breakage and cuts and you buy 189 blocks, laid with 15 bags of mortar: about $473 in block and $109 in mortar at the default prices.
How many concrete blocks do I need?
Multiply wall square footage by 1.125 — or read the common wall sizes straight from the engine:
| Wall | Blocks | Mortar bags (80 lb) | Fill all cores |
|---|---|---|---|
| 10 × 4 ft | 48 | 4 | 0.38 yd³ |
| 20 × 4 ft | 95 | 8 | 0.76 yd³ |
| 20 × 6 ft | 142 | 11 | 1.13 yd³ |
| 20 × 8 ft | 189 | 15 | 1.51 yd³ |
| 30 × 8 ft | 284 | 22 | 2.27 yd³ |
| 40 × 8 ft | 378 | 30 | 3.02 yd³ |
Mortar, core fill, and what the wall sits on
- Footing first: block walls bear on a poured footing, typically twice the wall width — size it with the footing calculator.
- Mortar: type S below grade, type N above is the common split; 13 blocks per 80 lb bag is the planning figure.
- Reinforcement: grouted cells with vertical rebar at 24–48 in on center (plus corners and jambs) is typical where required — grout only what the design calls for, not every core.
Concrete block FAQ
How many concrete blocks do I need for 100 square feet?
About 113 standard 8 × 8 × 16 blocks before waste — each block covers 0.89 ft² of wall, so multiply wall area by 1.125. With the usual 5% waste for breakage and cuts, order 119. The calculator subtracts door and window openings before counting.
How many blocks does a bag of mortar lay?
About 13 standard blocks per 80 lb bag of mortar mix, per Quikrete’s published coverage. The default 20 × 8 ft wall (189 blocks with waste) needs 15 bags. Mix in small batches — mortar that sits more than an hour or two is wasted.
How much grout does it take to fill block cores?
Filling every core of standard 8 in block takes about 0.84 yd³ of grout per 100 blocks — the default 20 × 8 wall runs 1.51 yd³. Most walls only grout the reinforced cells (commonly every 24–48 in plus corners), so the real number is a fraction of that; your design drawings govern.
Are cinder blocks and concrete blocks the same for estimating?
Yes. "Cinder block" is the surviving name for the lighter aggregate blocks of decades past; what you buy today is a concrete masonry unit (CMU) either way, in the same nominal 8 × 8 × 16 size — the counts from this cinder block calculator math are identical.
How much does a concrete block wall cost?
Blocks run about $2–4 each in 2026 and mortar $6–9 a bag, so the default 20 × 8 ft wall is roughly $473 in block and $109 in mortar — before footing, core fill, rebar and labor. Laid by a mason, block walls typically price at $15–35 per square foot installed.
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