Concrete Block Calculator

How this calculator works

You don’t have to work any of this out by hand - the calculator does every step. This is just what is happening behind the scenes, in case you like to know.

A block wall is measured like any wall: first work out the gross wall area (each wall’s length times its height), then take off the doors and windows to leave the net area - the part you actually fill with block. The rest is turning that net area into a number of blocks, plus the mortar and the sand:

blocks per area = 1 ÷ ((blockL + joint) × (blockH + joint))
gross area      = sum of (wall length × wall height)
net area        = gross − (door & window areas)
blocks          = ceil( net area × blocks/area × wastage )
mortar volume   = net blocks × block depth × (cell − block face) × wastage   (full-bed)
sand (mixing)   ≈ mortar volume      mortar mix bags = ceil( mortar volume ÷ bag yield )

Each block takes up its own size plus one mortar joint on two sides (one on top, one to the side), so the space one block fills is (block length + joint) × (block height + joint). A standard 8 × 8 × 16 in block (actually 7-5/8 × 7-5/8 × 15-5/8 in) on a 3/8 in joint fills an 8 × 16 in space, which works out to exactly 112.5 blocks per 100 sq ft. A metric 390 × 190 × 190 mm block on a 10 mm joint is 12.5 blocks per m². The mortar fills the gaps around every block; this is a generous (full-bed) estimate, so hollow blocks - which are only buttered with mortar around the edges - use a little less. Buy your mortar ready-mixed by the bag, or mix your own from roughly that volume of sand plus masonry cement.

Worked example

This example follows the unit system you pick in the calculator above.

An 8 m × 2.4 m single-skin (one block thick) wall in 390 × 190 × 190 mm block, minus a 0.9 m × 2.1 m doorway, 5% waste:

1. Gross wall area: 8 × 2.4 = 19.2 m².
2. Deduct the door: 0.9 × 2.1 = 1.89 m², so net = 17.31 m².
3. Blocks per m²: 1 ÷ (0.40 × 0.20) = 12.5 /m² (face plus a 10 mm joint each way).
4. Add 5% waste: ceil(17.31 × 12.5 × 1.05) = 228 blocks.
5. Mortar: about 0.25 m³ with face-shell bedding (mortar on the block edges only) - roughly 26 bags of 20 kg mortar mix, or about 0.25 m³ of sand if you mix your own.

Concrete block sizes and how many per area

Block is sold by the nominal size (the actual unit is 3/8 in or 10 mm smaller to leave room for the joint). These are the common sizes and their per-area counts at a standard joint:

The face size (length × height) sets the count; the depth (140, 190 or 290 mm) changes the wall thickness and the mortar, not the number of blocks per area. So a 140 mm and a 290 mm block both come out at 12.5 per m² for a standard 390 × 190 mm face - the thicker block just uses more mortar per block.

Tips for ordering concrete block

• Measure each wall length times height and take off the doors and windows - the calculator works out each wall on its own, so a big opening on one wall never eats into another.
• Pick the block preset that matches your unit - the 390 × 190 × 190 mm block and the US 6, 8 and 12 in CMU (concrete block) sizes are all there - or type in your block's real size (its face, plus how deep it is) and the joint. The number of blocks comes from the face size; the mortar comes from the depth and the joint. See concrete block sizes for the full dimensions chart.
• Check the weight before you book delivery - a standard block is about 16 kg (35 lb) and a pallet is over a tonne. See how much does a concrete block weigh.
• For a budget, open the optional cost section: enter your price per block and per bag of mortar to price the materials, or a single rate per block laid to price a bricklayer. The footing, rebar, core fill (the concrete poured into the block holes) and labour are extra.
• For the half blocks you cut in at corners and openings, the waste percentage covers the offcuts - bump it to 10% on a wall with lots of openings.
• Buy mortar whichever way the result shows it: ready-mixed bags by the count, or that volume of sand plus masonry cement to mix your own. See how much mortar do I need.
• For the footing the wall sits on, work out the concrete with the concrete calculator; if you are laying brick instead, the brick calculator uses the same method. For the full method and block counts by wall size, see how many concrete blocks do I need.

Standard brick and block sizes where you are

In the United Kingdom, the brick most jobs use is the standard metric brick, 215 × 102.5 × 65 mm (a coordinating 225 × 112.5 × 75 mm once a 10 mm joint is added) - about 60 bricks per m², and the concrete equivalent is a concrete block (sometimes a "breeze block"), commonly 440 × 215 × 100 mm. Both are laid to a 10 mm joint, which is built into the coordinating size, so the per-m² count comes straight off it. Enter your unit's actual face and depth, plus the joint, in the calculator above - or pick the matching preset - so the per-area count and the mortar come out right for the unit you are actually buying.

For the recognised specification and laying detail, see BS EN 771 for the masonry units and BS EN 1996 (Eurocode 6) for masonry design.