How to use the concrete block calculator
- Enter the wall length and height, in whichever unit you measured.
- Choose a block size, or enter your own.
- Add any doors and windows.
- Set the waste allowance and, if you like, a price per block.
How the block quantity is calculated
- Multiply wall length by height to get the gross wall area.
- Subtract the area of every door and window.
- Divide the net area by the wall area one block covers.
- Add the waste allowance and round up to a whole block.
Example: a 40 × 8 ft wall is 320 sq ft. With 8 × 16 in blocks that is 360 blocks, or 378 with 5% waste.
Common concrete block sizes
Standard 6, 8 and 12 in concrete masonry units (CMU) share a nominal 8 × 16 in face, so they cover the same wall area and give the same count. A 400 × 200 mm nominal face is included as a metric example.
Sizes vary between manufacturers. Confirm them with your supplier.
Nominal vs actual block sizes
A standard U.S. 8 × 8 × 16 in block actually measures about 7 5/8 × 7 5/8 × 15 5/8 in. The missing 3/8 in is the mortar joint, so the nominal size already describes how much wall each block covers. If you enter a block's physical size, choose “Actual” and the calculator adds the joint once.
How doors and windows affect the estimate
The area of each door and window, times its quantity, is subtracted from the wall before waste is added, so you don't buy extra blocks for holes in the wall.
How waste allowance works
Waste covers cut blocks at ends and openings, breakage and handling. The percentage is added to the net block count, then rounded up to a whole block. 5% is a common start.
How block cost is estimated
Cost is the final block count, including waste, times the price per block you enter. The currency is a label only; no exchange rates are applied.
Frequently asked questions
How many concrete blocks are in a square foot?
A standard nominal 8 × 16 in block covers 128 sq in, so about 1.125 blocks per square foot of wall.
Does block thickness change the count?
No. Standard 6, 8 and 12 in CMU share the same 8 × 16 in face, so they cover the same wall area. Thickness can affect weight, wall design, and material requirements, but not the block count.
Should I add the mortar joint to the block size?
Only for actual (physical) dimensions. U.S. nominal sizes like 8 × 8 × 16 already include a 3/8 in joint. Adding it again undercounts blocks.
How much waste should I allow?
5% is a common starting point. Use more for walls with many cuts, corners or openings.
Are cinder blocks the same as concrete blocks?
People often use the names interchangeably. Most blocks sold today are concrete masonry units (CMU). For counting, what matters is the face size, so the calculator works the same either way.
Is this estimate exact?
It is a planning estimate. Corners, bond patterns, lintels and site conditions can change the real quantity. Confirm with your supplier.
This is a planning estimator. It does not provide structural design, engineering certification or building-code compliance.