Governing Formula
Masonry walls are laid on a nominal grid: every unit occupies a cell of unit length + joint, and unit height + joint. The block count is the number of full cells that fit the elevation, with openings deducted from the wall area first.
N = ceil(H / course_h) × ceil(L / course_w) Where:
-
L= Length of the masonry wall [m] -
H= Height of the masonry wall [m] -
A_open= Total area of window/door openings deducted from the wall (optional) [m²] -
b= Nominal length of the masonry unit [mm] -
b_h= Nominal height of the masonry unit [mm] -
j= Bed and perp joint thickness [mm]
Derived Equations:
A = L × H A_net = A – A_open Blocks/m² = 1 / ((b+j)/1000 × (b_h+j)/1000) N = ceil(H/courseH) × ceil(L/courseW) × (1 – A_open/A) V_mortar = V_bed + V_perp How the Calculation Works
Starting from the gross wall area, the calculator subtracts any opening area you enter to arrive at the net elevation to be filled by masonry. A course grid is then built from the unit dimensions plus the joint: 400 mm blocks on 10 mm joints occupy 410 × 210 mm cells.
The number of courses and of units per course are both rounded up so the wall is always covered without cutting gaps. The resulting block count is then inflated by the wastage allowance you specify to cover cut units, breakage, and site losses. Mortar volume is computed separately from the bed joints and the perp (head) joints over the full wall thickness.
Worked Engineering Example
Design Scenario: 10 m × 3 m Wall, 400×200×200 Block, 10 mm Joints, 5% Wastage
- Gross wall area:
A = 10 × 3 = 30.00 m² - Course grid:
course_w = (400 + 10) / 1000 = 0.41 m·course_h = (200 + 10) / 1000 = 0.21 m - Blocks per square meter:
1 / (0.41 × 0.21) = 1 / 0.0861 = 11.61 blocks/m² - Course counts and block total:
nCourses = ceil(3 / 0.21) = 15·perCourse = ceil(10 / 0.41) = 25·N = 15 × 25 = 375 blocks - Include 5% wastage:
N_order = ceil(375 × 1.05) = 394 blocks - Mortar volume (bed + perp joints):
V = 10 × 0.01 × 0.2 × 16 + 24 × 0.01 × 0.2 × 0.2 × 15 = 0.32 + 0.144 = 0.464 m³
Engineering Notes & Design Benchmarks
| Wall Type | Modular Unit (nominal) | Typical Coverage |
|---|---|---|
| CMU 400×200 (200 thick) | 400 × 200 mm | ≈ 11.6 blocks/m² |
| Standard brick 200×100×65 | 200 × 100 mm (10 mm joints) | ≈ 44.4 bricks/m² |
| Thin brick 240×70 | 240 × 70 mm (10 mm joints) | ≈ 51.8 bricks/m² |
Allow a wastage factor of 3–5% for blockwork and 5–10% for brickwork, where cutting and breakage are more common. Where openings are concentrated, deduct their area before counting rather than afterwards.
Assumptions & Limitations
- The count is based on a nominal modular grid; from an actual wall layout, cut units at ends and around openings may shift the exact number by a few units.
- Opening deduction is proportional to area only — individual lintel, sill, and jamb details are not modelled.
- Mortar volume assumes full rectangular jointing over the whole wall thickness; tooled or raked joints use marginally less.
- Block and mortar quantities are preliminary; confirm against the architectural drawings and the structural masonry schedule.
Frequently Asked Questions
How many 400×200 concrete blocks are in a square meter?
With 10 mm joints, one block occupies a 410 mm × 210 mm cell, so 1 / (0.41 × 0.21) ≈ 11.6 blocks per m². The exact count depends on the joint thickness you use.
Should I deduct window and door openings from the block count?
Yes. Enter their combined area in the optional opening field and the calculator scales the block count down proportionally. Lintels, jambs, and sills are still added separately by the detailer, which is why a small over-run allowance is recommended.
How much mortar do I need for a typical block wall?
For a 200 mm thick, 400×200 block wall with 10 mm joints, expect roughly 0.464 m³ of mortar per 30 m² of wall (about 0.015 m³ per m² of elevation). Actual consumption varies with joint finish and workmanship.
Engineering Disclaimer
Engineering Note: This calculator provides preliminary masonry quantity estimates for planning. Final quantities must be reconciled with the approved drawings, the structural masonry design, mortar specification, and any shop-detailed opening schedules.
Technical References
- ASTM C90/C90M: Standard Specification for Loadbearing Concrete Masonry Units, ASTM International.
- ASTM C270: Standard Specification for Mortar for Unit Masonry, ASTM International.
- TMS 402/602: Building Code Requirements and Specification for Masonry Structures, The Masonry Society.