How do I calculate how much concrete fill I need for hollow blocks?

Multiply the number of blocks in your wall by the fill volume per block. For a standard NZ 390 x 190 x 190 mm block with two cores, each core is roughly 130 x 130 mm, giving a fill volume of about 6.4 litres per block when filling all cores. Count your blocks (wall area divided by block face area), multiply by the core volume, add a 10 percent waste factor, then divide by your bag yield to get the number of bags. This calculator does all of that automatically.

Do I need to fill all cores in a concrete block wall?

Not necessarily. Fully filling all cores gives maximum strength and is required in structural or earthquake-resistant walls. Partially filling (for example, filling every second core or only cores with rebar) is common in lightly loaded garden walls and retaining walls. The 'fill fraction' setting in this calculator lets you specify what proportion of cores you intend to fill, from 25 percent to 100 percent.

What mix should I use to fill concrete block cores?

The standard fill for concrete block cores is a flowable grout or a wet concrete mix with a maximum aggregate size of 10 mm so it flows around rebar without bridging. NZS 4230:2004 specifies a minimum grout strength of 17.5 MPa for reinforced masonry. A typical site-mixed fill uses a 4:1 sand-to-cement ratio with enough water for a slump of 200 to 230 mm. Pre-bagged concrete mix (such as 25 kg bags yielding 0.012 m3) is practical for small jobs.

Concrete Block Fill Calculator

Work out how much concrete or grout you need to fill the hollow cores of a concrete masonry unit (CMU) wall. Enter your wall dimensions, block type, and what proportion of cores you are filling, and the calculator gives you the fill volume in cubic metres, litres, and number of bags.

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Updated June 2026 Based on NZS 4230:2004 masonry design standard and standard NZ block dimensions.

1. Wall Dimensions

Wall length (metres)

Wall height (metres)

Number of walls (if building multiple equal walls)

walls

2. Block Type and Fill

Block size

Fill fraction (proportion of cores to fill)

Waste factor

Bag size (for bag count estimate)

Fill Volume Required

Fill Volume
-
Cubic metres (incl. waste)

In Litres

Litres (incl. waste)

Bags Required

Based on selected bag size

Blocks in Wall

Total block count

Wall and Block Breakdown

Wall area (total)-

Block face area-

Number of blocks-

Core volume per block-

Cores being filled-

Net fill volume-

With Waste and Bags

Waste factor applied-

Gross fill volume-

Gross fill volume (litres)-

Bag size selected-

Yield per bag-

Bags to buy (rounded up)-

Summary: Enter your wall dimensions to see results.

How to Calculate Concrete Block Fill Volume

Hollow concrete blocks (also called concrete masonry units or CMU) have one or more hollow cores running through them. When you build a wall that needs structural reinforcement or waterproofing, you fill those cores with concrete grout. Calculating how much fill you need involves three steps:

Count the blocks: Divide your total wall area by the face area of one block. For a standard NZ 390 x 190 mm block, the face area is 0.0741 m2 (390 mm x 190 mm).
Calculate fill volume per block: For a standard two-core block, each core is approximately 130 mm x 130 mm. The fill depth equals the block height (190 mm). Volume per core = 0.13 x 0.13 x 0.19 = 0.003211 m3. Two cores = 0.006422 m3 per block (about 6.4 litres).
Apply fill fraction and waste: Multiply by the proportion of cores you are filling, then add your chosen waste allowance (10 percent is typical).

The formula in full:

Fill volume (m3) = (Wall area / Block face area) x Core volume per block x Fill fraction x Waste factor

Standard NZ Block Dimensions and Core Volumes

Block type	Nominal size (L x H x W)	Cores	Core volume per block	Fill per m2 of wall (100% fill)
Standard	390 x 190 x 190 mm	2	~6.4 L	~86 L/m2
Half block	190 x 190 x 190 mm	1	~3.2 L	~86 L/m2
200 series (metric)	400 x 200 x 200 mm	2	~7.2 L	~90 L/m2

Note: Actual core dimensions vary slightly between manufacturers. These figures are based on typical NZ block profiles and are suitable for estimating purposes. For critical structural work, confirm core dimensions with your block supplier.

When to Fill Block Cores

Filling all cores provides maximum compressive strength and is required in NZS 4230:2004-compliant reinforced masonry construction. Partial filling is acceptable in garden walls and lightweight retaining walls where structural loads are low. Common fill patterns include:

All cores (100%): Structural walls, earthquake-resistant construction, basement walls.
Every second core (50%): Medium-duty retaining walls, garden borders with vertical rebar.
Every fourth core (25%): Decorative garden walls with minimal lateral load.

What Mix to Use

For core filling, use a flowable grout or wet concrete with a maximum 10 mm aggregate size. A slump of 200 to 230 mm allows the mix to flow around reinforcing bars without leaving voids. Pre-bagged concrete mix (such as 25 kg bags) is suitable for smaller jobs. For large jobs, ordering ready-mix concrete is more economical and consistent. NZS 4230 requires a minimum grout compressive strength of 17.5 MPa at 28 days for reinforced masonry applications.

Worked Example

A garden retaining wall is 3 m long and 1.8 m high (the calculator's default). Using standard 390 x 190 x 190 mm blocks with two cores, filling all cores (100%), and a 10 percent waste factor:

Wall area = 3 x 1.8 = 5.4 m2
Block face area = 0.390 x 0.190 = 0.0741 m2
Block count = 5.4 / 0.0741 = 72.9, rounded to 73 blocks
Core volume per block = 2 x (0.130 x 0.130 x 0.190) = 0.006422 m3 (6.42 L)
Net fill = 73 x 0.006422 = 0.469 m3
With 10% waste = 0.469 x 1.10 = 0.516 m3 (515.7 L)
25 kg bags (0.012 m3 yield each) = ceil(0.516 / 0.012) = 43 bags

This matches the calculator output with the default inputs.

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Sources and method: Block dimensions and core profiles based on typical NZ concrete masonry block specifications. Fill volume formula: (Wall area / Block face area) x Core volume per block x Fill fraction x Waste factor. NZS 4230:2004 Reinforced Masonry Design (Standards New Zealand). Pre-bagged concrete yield per manufacturer data (approximately 0.012 m3 per 25 kg bag).

This calculator provides indicative fill volume estimates. Core dimensions vary between manufacturers. For structural or code-compliant work, verify dimensions with your block supplier and consult a licensed building practitioner or structural engineer.