20 ft × 8 ft
160 ft² ÷ (128/144 ft²) = 180 block equivalents. At 10% waste: 180 × 1.10 = 198 blocks.
The numbers behind the wall.
A concrete block takeoff starts with one wall face. This calculator turns its net area into full-block equivalents, applies your waste allowance, and rounds up once.
The concrete block formula
- Gross area = wall length × wall height.
- Opening area = the sum of opening width × opening height × quantity.
- Net area = gross area − opening area.
- Block face = nominal block length × nominal block height, in matching area units.
- Order estimate = round up [net area ÷ block face × (1 + waste ÷ 100)].
Waste is applied to the unrounded block equivalent. The displayed base count is rounded for readability; it is not used as an intermediate rounded value in the final calculation. Enter each rough masonry opening once. The tool rejects total openings equal to or larger than the wall, but cannot check their physical positions or overlap.
Worked examples
A 4 ft × 3 ft opening
From the same wall, subtract 12 ft²: 148 ÷ (128/144) × 1.10 = 183.15. Round up to 184 blocks.
10 m × 2 m
A 400 × 200 mm nominal face covers 0.08 m². 20 ÷ 0.08 × 1.10 = 275 blocks. This module is different from the US 16 × 8 in module.
Choose the installed block module
| Block face | Area per block | Blocks per area |
|---|---|---|
| 16 × 8 in | 8/9 ft² | 1.125 per ft² |
| 400 × 200 mm | 0.08 m² | 12.5 per m² |
For conventional mortared modular masonry, nominal dimensions include the joint module. The actual physical block is smaller. Entering the physical block dimensions without the joint module can overestimate quantities. For dry-stack or another system, choose Custom and use the effective installed face dimensions supplied for that system.
A 6 × 8 × 16 in block and an 8 × 8 × 16 in block share the same face and give the same area count. Their thickness, price, weight and supplier pallet quantity may differ.
Block material cost and pallets
Block-count cost = estimated blocks × your price per block. Full-pallet quantity = round up (estimated blocks ÷ supplier blocks per pallet). Full-pallet block cost = pallets × blocks per pallet × the same unit price.
For 198 blocks at $1.85, block-count cost is $366.30. With 90 blocks per pallet, three full pallets contain 270 blocks and cost $499.50. These figures exclude labor, tax, delivery, supplier pallet fees, mortar, grout and reinforcement. This is not an installed concrete block wall cost estimate.
What can change the real order?
This is an area model for one rectangular, single-layer wall. It does not lay out individual blocks or optimize offcuts. Bond patterns, opening positions, cuts, corners, half units, lintels and bond-beam units can change the purchase list. Calculate separate walls separately and round each estimate before combining them. Do not count both faces of the same wall.
The starting 10% waste allowance is an editable example, not a universal recommendation. Choose an allowance with the mason or supplier for the actual layout and handling conditions. Structural design, retaining-wall stability, foundations, core-fill volumes, mortar and block weight are outside this calculator.
Precision and input limits
Calculation uses exact decimal arithmetic with rational unit conversions, rather than a floating-point tolerance near whole-block boundaries. Switching display units preserves the original measurements until you edit them. Choosing another block preset intentionally changes the block module.
Dimensions must be between 0.000001 and 100,000 metres or their equivalent. The tool accepts up to 30 non-overlapping opening rows, opening quantities and pallet sizes from 1 to one billion, waste from 0% to 50%, and estimates up to one billion blocks. Unit prices are entered in USD with at most two decimal places, and totals must stay within the supported exact-cent range. Invalid inputs produce an error instead of an order estimate.
The arithmetic has independent known-answer, boundary and randomized test coverage. Passing those cases does not establish a real-world error probability of 0% or 0.1%; material ordering still depends on the assumptions above.
Sources and corrections
CMHA explains modular block dimensions and area-based masonry estimating. This site's examples and software are original; the sources do not endorse this calculator.
- CMHA: concrete masonry unit dimensions
- CMHA: estimating concrete masonry materials
- CMHA: modular layout of concrete masonry
Reviewed 18 October 2026. Found a mismatch? Send the inputs and expected result for review.