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Work out how much concrete you need to fill the cores of hollow concrete blocks, for retaining walls and reinforced blockwork. Choose the block and how many cores to fill, enter the wall, press Calculate, and see the concrete, the bags or ready-mix to order and the cost.
Concrete To Fill The Cores
0m³
Check the measurements above. The result updates once they look right.
| Method | You Need | Cost |
|---|
The same amount of concrete three ways. Your choice is highlighted.
| Block | Per Block | Per m² (All Cores) | Blocks Per m³ |
|---|
Before any extra for spills. Core space varies between makers.
The result gives the concrete in m³, the ready-mix, cement and ballast, or bags to buy, the number of filled cores and bar length for partly filled walls, the weight of the fill and the cost of each way of getting the concrete.
| Block | Per block | Per m², every core | Blocks per m³ |
|---|---|---|---|
| 140 mm hollow | 4.9 litres | 0.048 m³ | 205 |
| 190 mm hollow | 6.8 litres | 0.067 m³ | 147 |
| 215 mm hollow | 7.7 litres | 0.076 m³ | 130 |
These figures are before any extra for spills. A standard 215 mm hollow block holds about 7.7 litres of concrete, close to the 7.6 litres per block in the well-known American rule of 100 blocks per cubic yard for 8-inch blocks. Core sizes vary between makers, so check the data sheet for your block if you need a precise figure.
| Wall | Blocks | Concrete (incl. 10%) | Mix on site (1:5) |
|---|---|---|---|
| Retaining wall, 8 × 1.2 m, 215 mm, every core | 95 | 0.81 m³ | 10 bags cement + 1.43 t ballast |
| Same wall, every second core | 95 | 0.40 m³ | 5 bags cement + 0.71 t ballast |
| Garden wall, 10 × 1 m, 140 mm, every core | 99 | 0.53 m³ | 7 bags cement + 0.94 t ballast |
| Planter, 5 × 0.9 m, 190 mm, every core | 45 | 0.33 m³ | 5 bags cement + 0.59 t ballast |
| 100 blocks, 215 mm, every core | 100 | 0.85 m³ | 11 bags cement + 1.5 t ballast |
Every m³ of fill weighs about 2.3 tonnes, so a fully filled 215 mm wall gains around 175 kg per m². The foundation must be designed to carry it.
Hollow blocks are lighter to lay than solid blocks, and their cores line up from course to course. Filling the cores with concrete around steel bars turns the wall into reinforced masonry, which is much stronger in bending. This is how most blockwork retaining walls, basement walls and tall freestanding walls are built.
Filled cores also add weight and stability, improve sound insulation and help fire resistance. Some walls are filled only at corners, ends and around openings, where extra strength is needed.
Your engineer's drawings will say which cores to fill. Retaining walls are usually filled solid, because the soil pushes on the whole wall. Freestanding and lightly loaded walls may only need the cores with bars filled, often every second or fourth core, which uses a half or a quarter of the concrete.
With two cores in each 440 mm block, the cores are about 225 mm apart. Filling every second core puts bars about 450 mm apart, and every fourth core about 900 mm apart. Our rebar calculator works out the steel, and the block calculator counts the blocks and mortar.
Block fill needs to flow down narrow cores and wrap round the bars without leaving voids. Order a C25/30 concrete with 10 mm aggregate and a fairly wet (high slump) mix, or a purpose-made core-fill grout. If you mix on site, use 10 mm ballast or pea shingle with sharp sand rather than 20 mm ballast, which can jam in the cores.
For a small wall, ready-mix is usually cheapest: a 0.75–1 m³ load from a mix-on-site lorry costs less than the cement, ballast and time to mix it yourself, and far less than pre-mixed bags. Our cement calculator covers other site mixes.
The core share is worked out from typical block weights: a 215 mm hollow block weighs about 26 kg against about 42 kg if it were solid dense concrete, so around 38% of it is core. You can enter your own figure. Site mix figures match our cement and ballast calculators (300 kg of cement and 1.77 tonnes of ballast per m³), and bags use 2,100 kg per m³, as in our concrete bag calculator.
Wet concrete can burn skin, so wear gloves and eye protection. The HSE guidance on cement explains how to work safely.
About 7.7 litres for a 215 mm hollow block, 6.8 litres for 190 mm and 4.9 litres for 140 mm, before any extra for spills.
About 0.076 m³ per m² when every core is filled, or 0.084 m³ with 10% extra.
About 130 blocks, or roughly 118 once you allow 10% for spills.
Not always. Ordinary walls are often left hollow. Retaining walls and reinforced walls need the cores filled where the engineer specifies, usually around the bars.
A fluid C25/30 mix with 10 mm aggregate, or a purpose-made core-fill grout. Avoid 20 mm stones, which can bridge in the cores and leave voids.
Usually about 1–1.5 m per lift, once the mortar has set enough. Taller lifts need the engineer's approval and clean-out holes at the base.
Not for structural walls. Mortar is too weak and shrinks more. Use concrete or a core-fill grout.
A fully filled 215 mm hollow block wall weighs roughly 450 kg per m², around 175 kg of which is the concrete fill.
Core space is estimated from typical UK hollow dense block weights against solid concrete at 2,050 kg per m³. Block faces are 440 × 215 mm with 10 mm joints. This is a quantity guide only and not a structural design. Reviewed November 2026.