Concrete & masonry · free tool
Concrete Block Calculator
How many concrete blocks do you need? Enter your wall and block size, and get the blocks and bags of mortar to buy — with waste built in.
Concrete Block Calculator
How many blocks & bags of mortar you need.
1 Your wall
2 Your estimate
Order 80-lb pre-mixed mortar — about 1 bag per 13 blocks.
One 80-lb bag of Mason Mix lays about 13 blocks (~8 bags per 100). Mixing your own: ~1 cement bag per 33 blocks plus 1 yd³ of sand per 7 bags.
Estimates only — block sizing, joints and waste vary; confirm quantities before ordering. See how we calculate · Updated monthly
Method · how it works
How to calculate how many blocks you need
Wall area times the blocks each square foot holds, plus waste. A standard 8x8x16 block covers about 0.89 sq ft, so a wall needs roughly 1.13 blocks per sq ft.
Measure the wall
Multiply length × height in feet for the wall area in square feet.
Multiply by coverage
Times the blocks per square foot — ~1.125 for a standard 8x8x16 (2.25 for a half block).
Add waste
Add 5-10% for cuts and breakage. Subtract large openings only if the wall is mostly glass or doors.
Add mortar
Plan about 1 bag of 80-lb mortar per 13 blocks — roughly 8 bags per 100 blocks.
A 10 × 8 ft wall, 8x8x16 block
80 × 1.125 = 90 blocks
90 × 1.10 = 99 blocks
99 ÷ 13 = 8 mortar bags
10% waste in
Sources: NCMA block coverage · QUIKRETE Mason Mix yields · standard CMU sizes
Verified against published block + mortar data · Updated monthly
Estimates only. Block sizes, joint width and mortar coverage vary — order the waste shown, and confirm structural walls with an engineer.
Coverage · by block
How many blocks per square foot?
Nearly all CMUs share the same 8 x 16 in face, so most sizes need the same count — pick yours in the calculator above.
The standard block — 113 per 100 sq ft.
Most commonSame face, thinner wall — still 113 per 100.
PartitionsSame count, but heavier and more mortar.
FoundationsThin veneer or backing block; same 113 per 100.
VeneerHalf the length, so double the count — 226 per 100.
Ends & cutsThe thickness (4-12 in) doesn't change the count — only the face height × length does. Add 5-10% waste on top.
Know your block
Concrete block sizes
CMUs are named by nominal width x height x length. The height and length stay 8 x 16 — only the width changes, which sets the wall's thickness and strength.
Thin partition, veneer or backing walls. Light and non-structural on its own.
Interior walls and light retaining. A common step up from 4 in.
The workhorse — foundations, garages and load-bearing walls.
Taller or higher-load walls and basements needing more strength.
Heavy foundations and tall walls — the strongest common size.
Nominal vs actual: a "16 in" block is really 15 5/8 in, and "8 in" is 7 5/8 in — the missing 3/8 in is the mortar joint. Estimating always uses the nominal size, which is why 8x8x16 works out to ~1.125 per sq ft.
Mortar · how much
How much mortar for a block wall?
The easy path is bagged, pre-mixed Type S mortar: one 80-lb bag lays about 13 standard blocks. Here is what that works out to.
Pre-mixed 80-lb bags
Bags you need, by block count
| Blocks | 80-lb bags |
|---|---|
| 100 | 8 |
| 250 | 20 |
| 500 | 39 |
| 1,000 | 77 |
Rule of thumb: ~1 bag per 13 blocks (about 8 bags per 100) with standard 3/8 in joints. Thicker blocks and joints use more.
Mixing your own?
Per 100 blocks, plan roughly:
Need just the mortar figure for any block or brick job? The dedicated mortar calculator breaks down bags and sand on its own.
Structural walls
Filling cores & reinforcement
CMUs are hollow. For structural and retaining walls, some or all of the cores get filled with grout around steel — here's how to plan it.
When to fill
structural / retainingFill cores where the wall carries load or resists soil — typically the cells with vertical rebar, plus a bond beam course. A garden wall may need none; a basement may need many.
How much grout
~0.67 yd³ / 100 blocksA standard 8 in block holds roughly 0.18 cu ft of grout per fully filled block — about 0.67 yd³ per 100 blocks. Toggle "Fill cores" in the calculator to estimate it.
Rebar & bond beams
vertical + horizontalVertical bars run in filled cells; horizontal bars sit in bond-beam blocks at the top and mid-height. Spacing and bar size are set by code — check your plans or an engineer.
Estimating grout by the yard? Use the concrete calculator for the fill volume — and always follow the structural drawings for reinforcement.
Answers
Concrete block FAQs
How many blocks are in a square foot?
A standard 8x8x16 block covers about 0.89 sq ft, so a wall needs roughly 1.125 blocks per sq ft — about 113 per 100 sq ft.
How many blocks do I need for a wall?
Multiply wall area by 1.125, then add 5-10% waste. A 10 x 8 ft wall is 80 sq ft, so about 99 blocks with 10% waste.
How big is a standard concrete block?
The common size is 8 x 8 x 16 in nominal — actually 7 5/8 x 7 5/8 x 15 5/8, with the missing 3/8 in being the mortar joint.
How much mortar do I need per 100 blocks?
About 8 bags of 80-lb pre-mixed mortar (~1 bag per 13 blocks). Mixing your own: roughly 3 cement bags plus sand per 100 blocks.
Do I need to fill the block cores?
Only for structural or retaining walls — fill the cells with vertical rebar plus a bond beam. A standard 8 in block takes about 0.18 cu ft of grout when fully filled.
Do I subtract doors and windows?
For accuracy on walls with large openings, yes — deduct their area. For a mostly solid wall, leaving them in gives a safe cushion of cut blocks.
Are cinder blocks and concrete blocks the same?
Roughly — cinder block is an older term for CMUs made with coal ash. Today most are concrete masonry units (CMUs); the count and mortar are the same.
How much waste should I add?
5-10% for cuts and breakage — more for walls with lots of corners, openings or curves.