How the calculation works
A worked example
A wall 10 m long and 3 m high, with no openings, a 40 × 20 cm block face, a 1 cm joint and 5% waste, needs 366 blocks after rounding up.
How many blocks per m²: 12.5 or 11.61?
A 39 × 19 cm actual face with a 1 cm mortar joint forms a 40 × 20 cm module: 12.5 blocks per square metre before waste. KFUPM’s masonry measurement teaching material illustrates this specified-versus-nominal distinction. An actual 40 × 20 cm face with an extra 1 cm joint instead gives approximately 11.61 blocks per square metre.
Enter a nominal 40 × 20 cm module with the joint set to zero, or enter its 39 × 19 cm actual face with a 1 cm joint. Both give the same result in this example. Adding the joint to an already joint-inclusive nominal size counts it twice. The comparison below explains the geometry; it does not establish the dimensions of every block product.
| Face entered | Joint entered | Blocks/m² before waste | 100 m² with 5% waste |
|---|---|---|---|
| 39 × 19 cm actual | 1 cm | 12.5 | 1,313 blocks |
| 40 × 20 cm nominal including joint | 0 cm extra | 12.5 | 1,313 blocks |
| 40 × 20 cm actual | 1 cm | Approx. 11.61 | 1,220 blocks |
Start with the wall area you will actually build
This block calculator estimates pieces for a measured area of masonry. Enter each wall's length and height in metres, then enter the doors and windows that interrupt those walls. A shared wall between two rooms is still one wall for this calculation. Counting it from both rooms would double its quantity.
Keep boundary walls, roof rooms and extensions visible as separate entries if they belong to your order. A villa's floor area cannot tell you its wall area: two homes with the same footprint can have very different partitions, wall heights and openings. A wall schedule gives you a better starting point than a single area taken from the cover of a drawing.
A 30 m² wall, worked through
For a wall measuring 10 × 3 m, the gross area is 30 m². With no openings, the net area is also 30 m². A 40 × 20 cm block face plus a 1 cm joint becomes a calculation module of 0.41 × 0.21 m, covering 0.0861 m².
Divide 30 by 0.0861 to get approximately 348.43 blocks before waste. Multiply by 1.05 for a 5% allowance: 365.85 blocks. The purchase result rounds up to 366. That count includes the selected waste; adding another 5% to the result would count the allowance twice.
Why the mortar joint changes blocks per square metre
Using the same 40 × 20 cm face without a joint gives 12.5 blocks per square metre. Adding a 1 cm joint produces approximately 11.61 before waste because each repeating module occupies more wall area. Neither figure is a universal block rate: both belong to a stated set of dimensions.
Ask the supplier what the printed size describes. If it is already a nominal module including mortar, adding another joint would understate the quantity. The calculator is an area estimate rather than a drawing of individual courses, so corners, cut pieces and special units still need a review of the actual layout.
Keep thickness and quantity separate
A block described as 10, 15 or 20 cm generally refers to its thickness. If all three products share the same face, the calculated count is identical. Thickness, product type and other wall requirements come from your approved specifications; the quantity calculation does not decide whether one block can replace another.
Group the walls by the actual product being ordered. Even when their face dimensions match, different specified products need separate purchase lines. Record all three dimensions and the supplier's product reference alongside the count so the delivery can be checked against the intended wall.
Openings and waste need their own checks
Deduct the opening in the masonry, using its measured width and height. Five windows measuring 1.5 × 1.2 m remove 9 m² in total. Check that the opening belongs to a wall already entered and appears in only one group; duplicating it lowers the net wall area incorrectly.
Waste allows for cuts, breakage and site handling, within the limits of an estimate. Discuss the selected percentage alongside the wall layout and delivery plan. If the contractor's estimate differs, compare the net area, face size, joint and allowance before deciding that either final number is wrong.
Turn the result into a useful order
Print or share the calculation with the wall measurements and assumptions intact. A result that says only ‘366 blocks’ is harder to review than one showing the product size, 30 m² net area, 1 cm joint and 5% waste. Round once for each product group where possible, rather than separately rounding every short wall.
The result covers blocks only. Mortar, lintels and other construction details need separate quantities. If you enter your own price per block, the optional amount uses that price and the rounded purchase count; it is not a supplier quotation and does not add delivery or other charges.
Before you place the order
- Measure each wall once. Deduct doors and windows that belong to those walls before adding waste.
- Confirm whether the supplier's dimensions are actual dimensions or a nominal module that already includes a joint.
- Block thickness is a separate product specification. It does not change the count if the face dimensions stay the same.
- The 5% waste setting is an editable starting point. Agree the allowance with the contractor before ordering.
Sources and product references
These references explain the method or a specific product. Match the product and specification to your project before ordering.
- KFUPM: Measuring Masonry Work
Page 8 explains specified and nominal concrete block dimensions and the 12.5 blocks/m² module. This is a measurement reference, not a product approval.
Prepared by the KSA Home Calc team. Content and defaults await professional review before launch. Updated: .