Do all concrete block cores need to be filled?

No — most non-structural, partition, and boundary walls use unfilled hollow block. Filling is for structural, load-bearing, retaining, or high-wind/seismic walls, usually only in the cores containing vertical rebar rather than every core (a "partial" or "lift" fill), unless your structural drawing specifically calls for a full/solid fill.

What is the difference between grout and concrete for core fill?

Grout is a flowable, high-slump mix designed to pour easily into narrow block cores without segregating, while general concrete uses larger aggregate and a stiffer mix better suited to open forms. This calculator's default 1:2:3 cement:sand:gravel mix approximates an all-purpose fill concrete; swap in a grout-specific ratio if your project specifies grout rather than poured concrete.

Does Gibraltar use UK or Spanish block standards?

Gibraltar construction generally follows UK practice, including the harmonised EN 771-3 standard for concrete masonry units, though imported materials and labour often carry a premium over mainland UK prices — factor this into your cost estimate.

Concrete Block Fill Calculator — Grout Fill for Hollow Blocks | Gibraltar

How Much Concrete or Grout to Fill Concrete Block Cores?

Structural and reinforced concrete block (CMU) walls need their hollow cores filled with concrete or grout around the vertical rebar, unlike a standard partition or boundary wall which is usually left unfilled. This calculator works out that fill volume — enter your wall size and block dimensions, and it returns the number of blocks, the core volume per block, and the total fill concrete (plus cement bags) needed to fill them all.

How to Use the Concrete Block Fill Calculator

Enter the wall width and height you need to core-fill.
Pick a block size preset (e.g. 8in x 8in x 16in CMU) or enter your own block, web, and shell dimensions.
Set a waste allowance (5-10% is typical) for spillage and over-pour.
Confirm the concrete mix ratio and bag yield, or use the defaults.
Enter a price per cubic metre of fill concrete for a cost estimate.

The Fill Volume Formula

Core volume per block = inside width x inside thickness x block height, where the inside dimensions subtract the block's outer shell and web thickness from its full face size. Total fill volume = core volume per block x number of blocks, then increased by your waste percentage.

Worked example: a 6m x 2m wall (12 m²) built from standard 8in x 8in x 16in CMU (actual size 406mm x 203mm, 1in webs, 1.25in shells) needs 146 blocks. Each block's hollow core holds about 0.009 m³ (550 in³), so the raw fill volume is 146 x 0.009 ≈ 1.32 m³. Adding 7.5% waste brings the order to about 1.41 m³ of fill concrete — split 1:2:3 (cement:sand:gravel), that is roughly 0.236 m³ of cement, or 14 bags at a 0.017 m³ bag yield.

Full Fill vs. Partial (Lift) Fill

Not every core needs filling. Many structural designs only require grout around the vertical rebar cores — spaced every one, two, or four blocks — rather than a full solid fill, so check your structural drawing before ordering, since full-fill can use 3-4x more concrete than a partial-fill design needs.

On tall walls, pour in lifts of no more than about 1.2-1.5m (4-5ft) at a time, letting each lift take its initial set before the next, to avoid blowing out the block face with hydrostatic pressure from one continuous tall pour.

At a typical Gibraltar price of around £155 per m³ of fill concrete (reflecting the import premium on aggregates), the worked example on this page — a 6m x 2m wall needing 146 blocks and about 1.41 m³ of fill concrete after waste — comes out at roughly £219 in concrete alone, before rebar, mortar, and labour.