Block Fill Grout Calculator
Works out grout for filling the cores of a block wall. Pick how many cores are being filled — solid, or every second, third or fourth — and it gives the volume.
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The formula
blocks = wall area × blocks per m² cores filled = blocks × 2 × fill fraction grout = cores × void per core × (1 + waste)
A standard block has two cores, so filling every second core works out at one core per block. The void figures are the hollow volume of the block, not its outside dimensions — roughly 45% of a standard CMU is air.
What to watch out for
Core fill is structural, and how it is poured matters as much as how much:
- Pour in lifts. No more than about 1.5 m at a time, consolidated with a vibrator or a rod. A single full-height pour bridges on the way down and leaves hollow sections.
- Grout is not concrete. It is a high-slump mix with small aggregate, designed to flow into a narrow cavity. Stiff concrete will not fill the core.
- Reinforced cores must be filled. Wherever there is a rebar, that core is grouted — the bar does nothing in an empty cell.
Which cores get filled is a structural decision from the drawings, not a choice made on site.
Frequently Asked Questions
How much grout to fill a block wall?
About 0.0034 m³ per core for 8 in CMU. Filling every second core in a 20 m² wall is roughly 250 cores, which comes to about 0.9 m³ once waste is counted.
How many blocks does a cubic yard of grout fill?
Around 110 solid-filled 8 in CMU, since each takes roughly 0.24 ft³ of void. Filling every second core doubles that to about 220 blocks.
What is the difference between grout and mortar?
Mortar beds the blocks and is stiff enough to hold them. Grout fills the cores and is deliberately wet and fluid so it flows into the cavity. They are not interchangeable.