What this calculator does
Blockwork quantities are a takeoff problem: how many units, how much mortar, and how many bags to order. This calculator takes the wall dimensions, deducts the openings and returns the block count with a waste allowance plus the bedding mortar volume.
How to use it
- Enter the wall length and height; the calculation assumes standard 400 × 200 × 200 mm blocks at 12.5 per square metre including joints.
- Enter the total area of openings to deduct.
- Set the waste allowance — 5 % is normal for a straightforward wall, more for complex bond patterns.
- Check that the wall is designed for lateral loads; the calculator covers quantities only.
Governing formula
N = A_net × 12.5 ; V_mortar = A_net × 0.028 Where:
-
A_net= Net wall area after openings [m²] -
12.5= Blocks per m² including 10 mm joints [1/m²]
Derived Equations:
bags = ⌈V_mortar / 0.03⌉ Worked example — A 12 × 2.7 m wall with 2 m² of openings
The figures below were produced by running this page's own engine with the example inputs (Wall length (m), Wall height (m), Total opening area (m²), Waste allowance (%)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Blocks to order | 400 no |
|---|---|
| Bedding mortar | 0.85 m³ |
| Mortar bags (approx.) | 29 no |
| Blocks before waste | 380 no |
| Assessment | 30.4 m² of blockwork → 400 standard blocks (incl. 5% waste) and ≈ 0.9 m³ of bedding mortar (≈ 29 mixed bags). |
- The gross area is 32.4 m², and the net area after openings is 30.4 m².
- At 12.5 blocks per m² that is 380 blocks, or about 399 with 5 % waste.
- The bedding mortar is roughly 0.85 m³, which is about 29 bags of dry mortar.
Assumptions & limitations
Engineering calculation. Unit-count estimating from nominal block size and gross wall area, with a bedding-mortar estimate. Does not account for bond patterns, half blocks or grout cores.
- Assumes standard block dimensions; half-blocks, closure blocks and bond patterns change the count.
- Mortar volume is a rule of thumb — the actual quantity depends on joint thickness and workmanship.
- No structural design, reinforcement, grout or lintel quantities.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
Schedule mode — wall runs
A blockwork takeoff is a schedule of wall runs, not one wall. Enter each run once and the totals are the order quantity for the whole floor plate.
The schedule totals are the sum of every item's own engine result, so they match running this calculator once per item. An item the engine rejects is reported in place and contributes nothing to the totals.
- Openings are deducted as a lump area per run; a lintel or a sill is not returned as a separate line.
References & standards
- BS EN 771-1 — clay masonry units.
- BS EN 771-3 — aggregate concrete masonry units.