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Concrete Block Fill Calculator

This concrete block fill calculator will help you determine how much concrete or mortar is needed to fill the void spaces inside a hollow concrete block wall of any size.

Reinforced walls often fill only the cores that hold rebar.
Clear
Grout or concrete needed1.1 cubic yards29.7 cu ft, including 10% waste
Bags of 80 lb mix50
Per block0.27 cu ft8 in block, all cores
Blocks100every core filled
Blocks per cubic yard99filling every core
Weight of fill2.225 US tonsadded to whatever the wall already weighs

The method

fill = blocks × void volume per block

Roughly a yard per hundred blocks

Filling every core of an eight-inch block takes about 0.27 cubic feet, so a hundred blocks come to almost exactly one cubic yard. Twelve-inch block takes twice that.

Most reinforced walls do not need every core filled — only the ones containing rebar, plus the bond beam. Filling selectively can halve the concrete, so check the drawings before ordering for the whole wall.

Grout for filling cores is wetter than concrete for a slab, so it flows into the cavities. Using a stiff slab mix leaves voids you cannot see and cannot fix.