Concrete Block Fill Calculator

Fill = ceil_or_round( area × (1 + wastage ÷ 100) × cores per unit area ) where cores per unit area = blocks per unit area × core volume per block

A

Face area of the wall, in square feet

w

Wastage allowance, as a percentage of the net fill

cores per unit area

The block's core volume multiplied by how many blocks there are to a square foot of wall — 1.125 × 0.2629 ft³ for a US block

core volume

The void inside one block: its actual outside volume times the core ratio, about 50% for a normal-weight unit

bag yield

Volume of concrete one bag makes — 0.6 cubic feet for an 80 lb bag, which is the divisor behind the bag count

Use it when the wall is going to be grouted rather than laid hollow: a reinforced wall, a bond beam course, a wall carrying a heavy point load, or cores that will take anchor bolts. The answer is the fill for every core in the wall, so if only the reinforced cells are being filled, treat it as an upper bound and count the cells you are actually filling. Bear in mind what you are ordering — grout for cores is a wetter, finer mix than a slab mix, and it is placed from a bucket or a pump rather than a chute. For the wall's own concrete, footing and slab, use the concrete calculator; for blocks and mortar alone, use the block calculator.

1. Every core of 100 square feet of 8 inch block, 5% allowed

   1. Blocks: 100 × 1.05 = 105 ft², × 1.125 per ft² = 118.1, rounded up to 119
   2. Core in one block: half of 15.625 × 7.625 × 7.625 in = 0.007443 m³ = 0.2629 ft³
   3. Cores per square foot: 1.125 × 0.2629 = 0.2957 ft³
   4. Fill: 105 × 0.2957 = 31.05 ft³ = 0.879 m³
   5. In 80 lb bags: 31.05 ÷ 0.6 = 51.8, rounded up to 52

   31 cubic feet is 46% of the wall's own volume of 67 cubic feet, which is the fact to take away from this page: a hollow block wall is very close to half hole. 52 bags is over a pallet and a half, and it is why a grouted wall is a decision rather than a default — mixing and placing that much grout from a bucket is a day's work for the wall alone.

2. Every core of 100 square feet of 390 × 190 mm block, 5% allowed

   1. Blocks: 100 ft² = 9.290 m², × 1.05 = 9.755 m², × 12.5 per m² = 121.9, rounded up to 122
   2. Core in one block: half of 0.390 × 0.190 × 0.190 = 0.014079 m³ → 0.007040 m³ = 7.04 L
   3. Cores per square metre: 12.5 × 0.007040 = 0.08800 m³ = 88.0 L
   4. Fill: 9.755 × 0.08800 = 0.858 m³ = 30.31 ft³
   5. In 80 lb bags: 30.31 ÷ 0.6 = 50.5, rounded up to 51

   The metric block is slightly smaller than the US one but has the same 50% core, so the two come out within 2% of each other — 30.3 cubic feet against 31.1. What does not stay the same is the block count: 122 metric blocks for 119 American ones, because the metric unit is a little smaller on the face and the joint is 10 mm rather than ⅜ inch.

3. Every core of 40 m² of 90 mm block, 5% allowed

   1. Blocks: 40 × 1.05 = 42 m², × 12.5 per m² = 525
   2. Core in one block: half of 0.390 × 0.090 × 0.190 = 0.006669 m³ → 0.003335 m³ = 3.33 L
   3. Cores per square metre: 12.5 × 0.003335 = 0.04168 m³ = 41.7 L
   4. Fill: 42 × 0.04168 = 1.751 m³ = 61.82 ft³
   5. In 80 lb bags: 61.82 ÷ 0.6 = 103.0, rounded up to 104

   Halving the block's thickness halves the fill: 41.7 litres per square metre against 88.0 for the 190 mm unit, while the block count stays at 12.5 either way. A partition wall is therefore much cheaper to grout — but it is also the wall least likely to need grouting, since it carries nothing. Where a thin wall is grouted, it is usually every core for sound or for a fixing, not alternate ones for reinforcement.

The core ratio is an order of magnitude, not a specification: manufacturers run between about 40% and 60% void depending on the unit, and every volume on this page is that ratio applied to the block's outside dimensions. The answer covers every core in the wall, which is the upper bound and not usually the order: a reinforced wall is often filled only in the cells with vertical steel, and a partially grouted wall may be filled every second or third cell depending on the engineer's design, so count the cells you are actually filling and scale the result. Grout is not the same material as slab concrete — it has to flow down a narrow core and around reinforcement, so it is mixed wetter and with a smaller aggregate — and this page returns a volume rather than a mix, so the strength and the proportions come from the supplier or the specification. A cross-section that loses volume to a bond beam, a lintel or a solid-filled jamb is not modelled: the page multiplies a uniform wall by a rate. And nothing here deals with the wall's own concrete, the footing, the rebar, the horizontal joint reinforcement or the labour of placing half a wall's worth of grout from a bucket.

How much concrete does it take to fill block cores?

About 46% of the wall's own volume, which surprises most people. A normal-weight 8 inch block is roughly half void by volume, and the block occupies about 93% of the module it lays in, so 100 square feet of 8 inch wall — 67 cubic feet of wall — holds 31 cubic feet of cores. That is 52 bags of 80 lb mix. The thin 90 mm block is the exception: it has the same void ratio but half the depth, so its cores are 41.7 litres per square metre rather than 88.

Is core fill the same as grout?

In blockwork, yes — the trade calls the fill grout, and it is placed into the cores of the wall. It is not the same mix as a slab, though: it has to flow down a narrow core and around vertical steel, so it is mixed wetter and with a smaller aggregate than a slab mix would be. This page returns the volume to order; the mix and its strength are the supplier's or the engineer's call, not this page's.

Do I have to fill every core?

No, and most walls do not. A reinforced wall is commonly filled only in the cells that contain vertical steel, and how often those come depends on the design — every cell, every other cell, or every third. A bond beam or a lintel course is filled solid along its length. This page multiplies the whole wall by a rate, so its answer is the figure for a wall that is filled everywhere: an upper bound. For a partially filled wall, work out what fraction of the cells are being grouted and take that share.

How many bags of concrete is a cubic foot?

The other way round: an 80 lb bag of ready mix makes about 0.6 cubic feet, so a cubic foot takes 1.67 bags. The examples here are all read off that one number — 31.05 cubic feet divided by 0.6 is the 52 bags in the first example. A 60 lb bag yields about 0.45 cubic feet, so it takes 2.2 bags to the cubic foot and the same wall would need 69 of them.

Why does this page ask for the wall area and not the number of blocks?

Because the block count is not something you can supply without doing this same arithmetic first. The page derives the blocks from the area, the same way the block calculator does — 1.125 to the square foot for a US unit, 12.5 to the square metre for a 390 mm one — and then multiplies by the core volume per block. It reports the block count as a fourth output so the two can be checked against each other and against the block calculator.

How much wastage should I allow for core fill?

Five percent where the grout is placed carefully from a bucket into clean cores, and 10% or more where it is pumped, where the pour is tall enough that some is lost down the face, or where the cores have to be flushed out first. The allowance is applied to the fill rather than to the wall area, so it moves the volume and the bags and leaves the block count alone. The field takes anything from 0 to 30.

• Approximate Conversions from U.S. Customary Measures to Metric — the chart behind the cubic foot, pound and inch figures used here — National Institute of Standards and Technology
• The International System of Units (SI) brochure — the metre, the cubic metre and the litre, the units the core volumes are reported in — Bureau International des Poids et Mesures (BIPM)