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Concrete block calculator: how many blocks and mortar bags you need
Estimate how many concrete blocks (CMU) you need for a wall, subtracting doors and windows, and calculate the mortar bags to lay them. Works in metric (standard 40×20 cm block) or US units (16"×8" block), with adjustable coverage per m²/ft² for your actual block size.
Last reviewed: September 2026 · How we calculate
Input data
Add up door and window area to subtract it.
12.5 is the standard for a 40×20 cm block with mortar joint. Adjust if your block differs.
Common reference: 25–30 blocks per 25 kg (55 lb) bag.
Results
Enter your data and press Calculate.
Results are estimates. Always check the manufacturer's or supplier's specifications before buying.
Unit converter
How a block wall is calculated
The calculation has three steps: get the net wall area (subtracting doors and windows), convert that area into a number of blocks using your block's coverage per m², and estimate the mortar bags needed to lay them. Coverage per m² depends on the actual block size: the 40×20 cm (16"×8") standard, joint included, covers 12.5 blocks per m² — adjust the field if your block is a different size.
Formula
Area with waste = Net area × (1 + Waste ÷ 100)
Blocks = Area with waste × Blocks per m² (rounded up)
Mortar bags = Blocks ÷ Blocks per bag (rounded up)
Worked example
A wall 6 m long × 2.4 m high, no openings, standard block (12.5 blocks/m²), 5% waste, and 25 blocks per bag (the calculator's defaults):
| Net area | 6 × 2.4 = 14.4 m² |
|---|---|
| Area with 5% waste | 14.4 × 1.05 = 15.12 m² |
| Blocks needed | 15.12 × 12.5 = 189 blocks |
| Mortar bags | 189 ÷ 25 = 8 bags |
That's exactly what you'll see pressing "Calculate" with the default values: 189 blocks and 8 mortar bags.
Block types: hollow, solid, and lightweight
| Standard hollow block | The most common for partition walls and fences; lets utilities run through the cores |
|---|---|
| Solid block | Higher structural strength; used for load-bearing walls and foundations |
| Lightweight (aerated concrete) block | Less weight and better thermal insulation; same coverage per m² but costs more per unit |
The 12.5 blocks/m² coverage applies to the nominal 40×20 cm (16"×8") size regardless of type (hollow, solid, or lightweight), because the 1 cm mortar joint is already built into that nominal measurement. If you're using a different block dimension, recalculate: blocks/m² = 1 ÷ (actual length × actual height in meters).
Reinforcement: when you need vertical rebar
For low fences and non-structural walls (up to ~2.4 m / 8 ft), mortar-filled joints in hollow block are usually enough. For taller, load-bearing, or seismic-zone walls, vertical rebar is placed inside the block cores every 40–80 cm (16–32 in), anchored to the foundation and filled with flowable grout. This calculator estimates block and joint mortar only — if your project needs reinforcement, add that separately using the concrete calculator for the grout volume.
Common mistakes when calculating block
- Forgetting to subtract openings. A wall with a 2 m² door needs ~25 fewer blocks; skipping this over-buys material.
- Using the generic coverage with a different block. If your block isn't 40×20 cm (16"×8"), 12.5 blocks/m² doesn't apply — adjust the field with the real size.
- Not buying corner or half blocks. In practice you'll need some special pieces for corners and edges; the 5–10% waste margin usually covers them.
- Ignoring the mortar. It's easy to focus only on the block and forget the mortar bags, which are a real cost on their own.
Frequently asked questions
What block size does this calculator use by default?
The standard 40×20×20 cm (16"×8"×8" nominal) block, mortar joint included, which covers 12.5 blocks per m². If your block is a different size, change the "Blocks per m²" field with your own calculation: 1 ÷ (length × height in meters).
Should I subtract doors and windows?
Yes. The "Openings" field subtracts that area from the total wall before calculating blocks, so you don't buy extra.
How many mortar bags do I need per block?
It depends on joint thickness and bag size, but 25–30 blocks per 25 kg (55 lb) bag is a common reference. Adjust the field if your supplier states a different coverage.
Does the calculator include rebar reinforcement?
No. This tool calculates block and joint mortar only. If your wall needs vertical rebar filled with concrete, calculate that volume separately with the concrete calculator.
How much waste should I allow?
5% covers normal breakage and corner pieces on a simple wall. For walls with many angles or edges, go up to 10%.