Masonry: Mortar Bags for a Block Wall

All versions

Mortar comes in 80-pound bags, and a bag lays a predictable number of block. Count the block, divide by the yield, and round up — running short mid-course means a stiff joint and a trip to the yard.

Quick formula: Blocks over the per-bag yield, rounded up:

=ROUNDUP(B2/C2,0)

480 standard block at about 33 per bag is 14.5 bags — so you buy 15.

Functions used (tap for the full reference guide):

ROUNDUP

The example

Three walls with their block counts, all using a mortar yield of 33 block per bag.

	A	B	C	D
1	Wall	Blocks	Per bag	Bags
2	Garden wall	480	33	15
3	Planter	320	33	10
4	Retaining	900	33	28

The formula

ROUNDUP converts the fractional bag count into whole bags to buy:

=ROUNDUP(B2/C2,0) // block count over blocks-per-bag, rounded up

How it works

One division, always rounded up:

B2/C2 divides the block count by the per-bag yield — 480 over 33 is 14.5.
ROUNDUP(...,0) makes that 15, because you cannot buy half a bag of mortar.
Keep the yield in its own cell — type S, type N, and different block sizes change how far a bag goes.

Around 33 block per bag is a common figure for standard 8 by 8 by 16 CMU with a 3/8-inch joint, but it swings with joint size and block type. Pin it down on your own first course.

Try it: interactive demo

Interactive

Enter the block count and how many blocks one bag of mortar sets.

Blocks

Blocks per bag

Variations

Add sand by the shovel

Type S mortar mixes roughly a bag of mix to a set volume of sand; scale sand with the bags.

=ROUNDUP(B2/C2,0)*18

Bags with a waste factor

Pad for droppings and a fat last joint.

=ROUNDUP(B2/C2*1.1,0)

Pitfalls & errors

Do not round the bag count down. A wall that needs 14.5 bags and gets 14 stops with two courses to go and no mortar to finish them.

Yield drops fast with wider joints. A 1/2-inch joint uses noticeably more mortar than 3/8, so measure your actual joint before trusting a book number.

Practice workbook

📊

Download the free Masonry: Mortar Bags for a Block Wall practice workbook

Edit the yellow block and per-bag cells; the bag count recalculates.

Frequently asked questions

How many block does a bag of mortar really lay?

For standard CMU with a 3/8-inch joint, roughly 30 to 35 block per 80-pound bag of type S is a common range. Your joint size, block type, and mixing habits move it, so verify on the job.

What about brick instead of block?

Same formula, different yield. A bag of mortar lays far more brick than block — often over a hundred — so just change the per-bag number for a brick job.

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Function references: ROUNDUP