Use this concrete block fill calculator for the grout or concrete specified inside hollow CMU cells. It also serves cinder block, cement block and masonry core fill estimates when you supply the correct unit data. Joint mortar is a separate material. Start with the blocks actually being filled, rather than the larger quantity purchased for cuts and breakage.
Maintained by BlockCalculatorPro. Check our reference cases.
How this calculator works
Fill volume = installed block equivalents × documented void volume per block × fill fraction. Apply the entered grout allowance, then divide by mixed bag yield and round bags up.
Confirm the actual block void volume and specified fill pattern. The named reference is an estimating assumption, not a universal cavity size.
Calculate the hollow space you will actually fill
Base fill = installed blocks × void volume per block × fill percentage ÷ 100. The void input must describe all the cells represented by one block. If your supplier gives a value for one cell, first convert it to the matching volume per unit. Do not multiply by two again when the published figure already covers both cells.
Order fill = base fill × (1 + allowance percentage ÷ 100). Keep the base volume and allowance separate so another person can check your order. If a supplier table already includes waste, avoid adding that same allowance twice.
For an area based estimate, divide net masonry area by the nominal face module to get equivalent units. Deduct windows and doors first. Separate bond beams, pilasters and unusual units where their void volume differs. Purchased spare blocks do not need grout unless they become part of the wall.
Why there is no universal fill value for an 8 inch block
Two blocks sold as 8 × 8 × 16 can have different face shells, webs and cell shapes. Nominal width alone is insufficient for a precise grout takeoff. Request the manufacturer, product identifier, cell configuration and published core volume for your actual block.
Fairbanks Materials publishes an estimating reference for standard two cell units in an 8 inch wall with every cell filled: 1.00 cubic yard per 100 blocks. This is equivalent to 0.27 cubic foot per block. It is a named reference assumption, rather than a guaranteed volume for every 8 inch CMU. Its table also distinguishes different grouting patterns.
CMHA explains that unit configuration and grout delivery method affect quantities. Horizontal grouting can require additional material. Published estimates apply to the units and conditions described, so the supplier's data for your actual product takes precedence before ordering.
Worked example: 400 blocks with a documented reference
Suppose a takeoff contains 400 installed blocks, every cell is specified to be filled, and the applicable supplier estimate is 0.27 cubic foot per block. The base fill is 400 × 0.27 = 108 cubic feet, or 4 cubic yards. A chosen 5% ordering allowance adds 5.4 cubic feet, producing 113.4 cubic feet or 4.2 cubic yards.
Angelus publishes an approximate yield of 0.70 cubic foot for one 90 lb bag of PMP Spec Concrete 3000, a coarse core fill masonry grout. At that product yield, 113.4 ÷ 0.70 = 162 bags. This example demonstrates quantity arithmetic. Confirm that the selected material suits the project specification and cell geometry.
| Quantity | Cubic feet | Cubic yards |
|---|---|---|
| Base fill | 108 | 4.00 |
| 5% allowance | 5.4 | 0.20 |
| Order estimate | 113.4 | 4.20 |
Partial filling, reinforced cells and bond beams
The fill percentage is a quantity input taken from the project schedule. It does not choose which cells need reinforcement or grout. A 50% entry is appropriate only when half of the relevant void volume is actually being filled. It does not necessarily mean every second block.
- For regularly spaced cells, count the specified filled cells and convert them using the documented cell volume.
- For different block types, calculate each group separately and add their volumes.
- For bond beams, use the channel volume for that product. Deduct overlapping vertical cell volume if another calculation already includes it.
- Use any documented adjustment for reinforcement displacement consistently. A general allowance cannot establish the required reinforcement layout.
The result estimates a material quantity. It does not select a structural fill pattern or approve a wall design.
Convert fill volume into yards, litres and bags
Divide cubic feet by 27 for cubic yards. One cubic foot is 28.316846592 litres, and one cubic yard is 0.764554857984 cubic metre. Convert the volume and bag yield into the same unit before dividing.
Bags to buy = order volume ÷ mixed yield per bag, rounded up. Bag weight is not a substitute for mixed yield. Two 80 lb products can have different yields and intended uses. A grout product, ordinary concrete mix and laying mortar are not interchangeable solely because they come in similar bags.
To reverse the calculation, divide 27 by the documented cubic feet required per fully filled block. The 0.27 reference gives 100 blocks per cubic yard before an allowance. With a 5% allowance, the same yard supports about 95 complete blocks under that assumption. Actual product data and fill scope can change this substantially.
Check these details before placing the fill order
- Match the void volume to the block manufacturer and shape, including open end or bond beam units.
- Confirm whether the source figure is per block, per cell, per 100 blocks or per wall area.
- Keep joint mortar, footing concrete and core grout on separate material lines.
- Ask the supplier about delivery minimums, pump residue and the allowance appropriate to the placement method.
- Check whether the quote uses cubic yards, cubic metres, bulk bags or individual bags.
For a small repair, bags may be convenient. For a larger pour, compare the complete bagged and delivered grout quotes, including handling and any delivery minimum. There is no universal volume at which one purchase method becomes cheaper.
Common questions
How much concrete fills an 8 inch cinder block?
It depends on the unit's actual cells and the specified fill pattern. The Fairbanks Materials standard two cell estimating table implies 0.27 cubic foot per fully filled 8 inch unit. Treat this as a named reference, then confirm the void volume for your own block. Do not use its outside envelope as the fill volume.
How many 8 inch blocks will one yard of grout fill?
At 0.27 cubic foot per block, one cubic yard fills 100 blocks before waste. Using a different supplier void volume changes that count. An ordering allowance also reduces the number of complete blocks covered by a fixed yard.
How many bags do I need to fill cinder blocks?
Multiply the installed block count by the applicable void volume and fill fraction, add your allowance, then divide by the grout product's mixed yield. Round the final bag count up. Check the actual product sheet rather than assuming every bag of a particular weight has the same yield.
Can I calculate fill for 12 inch or custom concrete blocks?
Yes, using the documented void volume for that exact unit. A 12 inch block is wider than an 8 inch unit, but its fill volume should not be scaled solely by the ratio of nominal widths. Internal webs and shells also affect the cavity.
Can I use this for Besser blocks in Australia or New Zealand?
Use metric block modules and the local manufacturer's core volume and specified fill material. The same volume method applies to Besser and other hollow masonry blocks, but a US nominal size reference does not establish the cavity size or local material specification.
Does the core fill result include mortar joints?
No. Laying mortar belongs between the units, while core grout fills the specified cavities. Calculate those materials separately and combine their costs only after both quantities have been established.
Sources and product references
Named product references support the examples. Confirm the currently supplied dimensions, yields and purchasing conditions before ordering.
- Fairbanks Materials: grout calculations for concrete blocksThe named estimating table gives 1.00 yd³ per 100 fully filled standard two cell blocks in an 8 inch wall. This equals 0.27 ft³ per block and is an estimating reference, not a universal cavity volume for every 8 inch unit.
- Angelus PMP Spec Concrete 3000 core fill grout data sheetThe manufacturer identifies coarse core fill masonry grout and an approximate mixed yield of 0.70 ft³ from one 90 lb bag. Product suitability follows the project specification.
- CMHA: Estimating Concrete Masonry Materials, TEK 04 02AExplains the effect of unit configuration, separate horizontal grouting and waste on grout estimating.