Core Fill Concrete Calculator
Estimate the volume of concrete or grout needed to core-fill a hollow concrete-block (besser / masonry) wall. Enter the wall size and block dimensions and the calculator counts the blocks, works out the fill per core and totals the volume in cubic metres and litres, with an allowance for filling only every Nth core and for waste.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the wall length L and height H in metres (gross wall size).
- Set the block face length and height in millimetres — the default 400 × 200 mm suits a standard 200-series block.
- Enter the fill per block in litres (default 1.2 L for a standard 200-series core filled solid) from your block's data sheet if you have it.
- Set 'fill every Nth core' to 1 for a fully filled wall, or 2, 3 … if only reinforced/starter cores are filled, adjust the waste allowance, then read the total core-fill volume.
How it works
The wall is divided into whole blocks: blocks per row = ceil(L ÷ block face length) and rows = ceil(H ÷ block face height), so the block count is rows × columns (rounded up). The fill volume is blocks × (litres per block ÷ spacing), converted from litres to cubic metres by dividing by 1000, then increased by the waste allowance. For example a 10 m × 2.4 m wall of 400 × 200 mm blocks is 25 columns × 12 rows = 300 blocks; at 1.2 L each that is 360 L = 0.36 m³, or 0.378 m³ with 5% waste.
Worked example
Worked example. A 10 m long, 2.4 m high wall built from 400 × 200 mm blocks needs 25 blocks per row (10 ÷ 0.4) and 12 rows (2.4 ÷ 0.2) = 300 blocks. Filling every core at 1.2 L per block gives 300 × 1.2 = 360 L = 0.36 m³. Adding a 5% waste allowance gives 0.36 × 1.05 = 0.378 m³ of core-fill concrete.
Common mistakes
- Using the nominal 1.2 L per core for every block type — void volume varies with the block series, manufacturer and web layout, so check the data sheet, especially for 150- or 300-series blocks.
- Forgetting the 'fill every Nth core' setting when only reinforced cores (e.g. every second core) are grouted, which doubles the estimate.
- Not subtracting large door and window openings, which overstates the block count and fill volume.
- Ordering the exact calculated volume with no waste — core-fill is easily lost to spillage, over-fill and pump lines, so keep the 5% (or more) allowance.
Frequently asked questions
How much concrete fills a 200mm block core?
A standard 200-series (200 mm) hollow block holds roughly 1.2 litres of fill per block when every core is filled solid, but the exact void volume depends on the block's shape and webs — confirm it from the manufacturer's data sheet. The default here is 1.2 L.
What does 'fill every Nth core' mean?
Many block walls are only grouted at reinforced or starter-bar cores rather than solid. Setting the spacing to 2 fills every second core, 3 every third, and so on, which reduces the fill volume proportionally. Use 1 for a fully filled wall.
Should I use concrete or grout to core-fill?
Core-fill is usually a high-slump, small-aggregate concrete or a proprietary masonry grout so it flows into the cores. This calculator estimates the volume either way; choose the mix to suit the engineer's specification and the core size.
Do I need to allow for waste?
Yes. Spillage, over-fill and material left in pump lines mean the delivered volume should exceed the theoretical fill. A 5% allowance is a reasonable default; increase it for pumped work or difficult access.
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