Figure My Build

Concrete Block (CMU) Calculator

Enter the wall length and height and get the block count and mortar bags for a standard 8×8×16 block wall, waste included, in whole purchasable units.

Diagram of a wall showing its length and height, with a door and a window
One wall at a time: length × height is the area every wall material covers.

Your buy-list

Enter your measurements

How block and mortar quantities are calculated

Every standard concrete block, whether it's 4, 6, 8, 10 or 12 in thick, has the same 8 × 16 in face. Add the standard 3/8 in mortar joint and each block covers 8/9 of a square foot, so a wall needs 1.125 blocks per sq ft (the Concrete Masonry & Hardscapes Association's estimating guide, TEK 04-02A, lands on the same rate). Blocks = wall area × 1.125, plus your waste allowance, rounded up.

For mortar, QUIKRETE's Mortar Mix data sheet states one 80 lb bag lays 13 standard blocks; Sakrete's Type S sheet says about 15. This tool uses 13, the conservative figure, so you don't run short mid-wall. Mortar bags = blocks to buy ÷ 13, rounded up.

Openings aren't deducted: the waste allowance and cutting cover small ones. For a wall with a big door or window opening, measure and run each solid section separately and add the results. For a brick veneer or garden wall, the Brick Calculator uses the same mortar maths with brick sizes.

Sources

The figures behind this calculator come from published manufacturer data and industry standards:

Worked examples

20 × 8 ft foundation wall

160 sq ft × 1.125 = 180 blocks, +10% waste; mortar at 13 blocks per bag.

Buy 198 blocks + 16 mortar bags

10 × 4 ft garden wall

40 sq ft × 1.125 = 45 blocks, +5% waste.

Buy 48 blocks + 4 mortar bags

40 × 10 ft workshop wall

400 sq ft × 1.125 = 450 blocks, +10% waste.

Buy 495 blocks + 39 mortar bags

Frequently asked questions

How many concrete blocks do I need for a 20×8 ft wall?
180 blocks exactly (160 sq ft × 1.125), or 198 with the usual 10% waste allowance, plus 16 bags of 80 lb mortar mix at 13 blocks per bag.
How many concrete blocks are in a square foot?
1.125. A block's face is 8 × 16 in; with a standard 3/8 in mortar joint it covers 8/9 sq ft of wall, and 1 ÷ (8/9) = 1.125.
Does block thickness change how many blocks I need?
No. 4, 6, 8, 10 and 12 in blocks all share the same 8 × 16 in face, so the count per sq ft is identical. Thickness changes the wall's strength and the price per block, not the quantity.
How many bags of mortar do I need per 100 blocks?
About 8 bags of 80 lb mix. QUIKRETE rates a bag at 13 laid blocks and Sakrete at roughly 15; this calculator uses 13, so 100 blocks ÷ 13 ≈ 7.7 → 8 bags.
Should I subtract door and window openings?
Not for small openings. The waste allowance covers the cuts. For large openings, split the wall into solid rectangles, run each through the calculator, and add the buy-lists together.

Related calculators

Concrete Bag Calculator

How many 40, 60 or 80 lb bags of concrete you need, and what they'll cost.

Concrete Slab Calculator

Cubic yards of ready-mix for a slab, plus the bag count if you mix it yourself.

Concrete Volume Calculator

Cubic yards and cubic feet for any pour shape.

Rebar Calculator

Rebar pieces and grid layout for a slab.

Estimate disclaimer: results are planning estimates based on standard manufacturer yields and coverage figures. Site conditions, product choice and installation method change actual quantities. Verify before you buy, and follow local building code for structural work. This tool does not replace professional engineering advice.

Use this calculator on your own site

Free to embed. Paste this code where you want the calculator to appear. It carries a small "by Figure My Build" credit link, which is all we ask.