How Keyway Volume Is Calculated
A keyway is a narrow, continuous groove, so its concrete volume is a small cross-section stretched along a long run. The math is cross-section area times length.
1
Cross-Section Area
A rectangle uses width times depth. A beveled keyway uses the average of top and bottom width times depth. A V-groove uses half of top width times depth.
2
Convert to Square Feet
Divide square inches by 144. A 1.5 in by 1.5 in keyway has 2.25 square inches, which is 0.015625 square feet.
3
Multiply by Length
Multiply the area in square feet by the run in feet. At 100 ft, 0.015625 square feet gives 1.5625 cubic feet.
4
Waste and Yards
Multiply by 1 plus the waste percentage, then divide cubic feet by 27. At 10% waste, 1.5625 becomes 1.719 cubic feet, or 0.064 cubic yards.
Where the Keyway Sits and Why Its Volume Moves
A keyway is a groove formed in the top of a footing so the wall concrete locks into it. The projection that fills the groove is the shear key.
Keyways are often used at wall-to-footing joints, especially when no reinforcement ties the two together. ACI 224.3R-95 notes this practice and describes pressing a slightly beveled 2x4 into the fresh footing.
ACI 332 places the keyway in the middle third of the wall and sets a 1-1/2 in minimum depth and top width. Check the footing size against our footing size chart and the foundation wall thickness chart.
The keyway volume shows up in two pours. It is a void in the footing pour and an addition to the wall pour, so ordering both totals stays the same.
To price the main pour, use the concrete footing calculator and the concrete foundation calculator. Add this keyway volume as a small adjustment.
A keyway is also different from a movement joint. For slab joints, see the concrete joint calculator and our guide to concrete control joints.
Keyway Size Limits by Wall Thickness
The middle-third rule limits how wide a keyway can be. Divide the wall thickness by 3 to get the widest keyway that stays inside the middle third.
| Wall Thickness |
Middle Third Width |
Fits 1.5 in Keyway? |
Fits 3.5 in Keyway? |
| 6 in | 2.00 in | Yes | No |
| 8 in | 2.67 in | Yes | No |
| 10 in | 3.33 in | Yes | No |
| 12 in | 4.00 in | Yes | Yes |
| 14 in | 4.67 in | Yes | Yes |
| 16 in | 5.33 in | Yes | Yes |
The 1-1/2 in minimum depth and top width and the middle-third location come from ACI 332 §8.2.2.1.1 as summarized in an ACI 332-14 handout. Width limits above are wall thickness divided by 3, which is this calculator's reading of that rule.
A square-edge 2x4 laid flat is 3.5 in wide, which only fits walls about 10.5 in or thicker. Thinner walls need a narrower former.
Concrete Volume per 100 Feet of Keyway
Use this lookup to check your result. Values have no waste. Cubic yards equal cubic feet divided by 27.
| Keyway Size |
Area |
Cubic Feet per 100 ft |
Cubic Yards per 100 ft |
| 1.5 × 1.5 in rectangle | 2.25 in² | 1.56 ft³ | 0.058 yd³ |
| V-groove, 3.5 in top, 1.5 in deep | 2.63 in² | 1.82 ft³ | 0.068 yd³ |
| Beveled 2x4, 3.5 / 3 in, 1.5 in deep | 4.88 in² | 3.39 ft³ | 0.125 yd³ |
| Square-edge 2x4 flat, 3.5 × 1.5 in | 5.25 in² | 3.65 ft³ | 0.135 yd³ |
| Square-edge 2x6 flat, 5.5 × 1.5 in | 8.25 in² | 5.73 ft³ | 0.212 yd³ |
Calculated as area in square inches ÷ 144 × 100 ft. The 3 in bottom width on the beveled 2x4 is an assumed bevel, so measure your own former.
Even the largest row needs about 0.21 cubic yards per 100 ft. A full wall footing is usually many times larger, so use the concrete volume conversion chart to convert both pours.
Beginner Walkthroughs
Each walkthrough shows the arithmetic one step at a time. These are sample calculations, not documented projects.
🏠 Basement Wall, Beveled 2x4
Run: 120 ft
Keyway: 3.5 in top, 3 in bottom, 1.5 in deep
Waste: 10%
Step 1: Average width = (3.5 + 3) ÷ 2 = 3.25 in
Step 2: Area = 3.25 × 1.5 = 4.875 in² = 0.0339 ft²
Step 3: Volume = 0.0339 × 120 = 4.06 ft³ = 0.150 yd³
Step 4: With waste = 4.47 ft³ = 0.166 yd³
Bags: 4.47 ÷ 0.60 = 7.45, so 8 bags of 80 lb
This 3.5 in top width needs a wall of about 10.5 in or thicker to stay in the middle third.
📏 Garage Wall, ACI Minimum
Run: 80 ft
Keyway: 1.5 × 1.5 in rectangle
Waste: 5%
Step 1: Area = 1.5 × 1.5 = 2.25 in² = 0.015625 ft²
Step 2: Volume = 0.015625 × 80 = 1.25 ft³ = 0.046 yd³
Step 3: With waste = 1.31 ft³ = 0.049 yd³
Bags: 1.31 ÷ 0.60 = 2.19, so 3 bags of 80 lb
Under a tenth of a yard. Most crews cover this from the leftover in the footing truck.
🔻 Short Wall, V-Groove
Run: 60 ft
Keyway: 3.5 in top, 1.5 in deep, V shape
Waste: 10%
Step 1: Area = 0.5 × 3.5 × 1.5 = 2.625 in² = 0.0182 ft²
Step 2: Volume = 0.0182 × 60 = 1.09 ft³ = 0.041 yd³
Step 3: With waste = 1.20 ft³ = 0.045 yd³
Bags: 1.20 ÷ 0.45 = 2.67, so 3 bags of 60 lb
A V-groove holds less concrete than a flat-bottom keyway of the same top width.
Forming, Dowels, and Joint Prep
🪵
Forming the Keyway
Press a slightly beveled 2x4 into the fresh footing and oil it first. Remove it once the concrete has hardened enough to hold the shape.
🔩
Dowels
An ACI 332-14 handout lists No. 4 dowels at 24 in maximum spacing, extending 36 bar diameters into the wall and 6 in into the footing. Follow your plans.
🧹
Joint Surface
Where shear friction is relied on, ACI 318-19 §26.5.6 calls for a clean surface roughened to about 1/4 in amplitude. Remove laitance and debris before the wall pour.
📐
Footing Minimums
IRC R403.1.1 sets footings at 12 in wide and 6 in thick at minimum. Keyway depth must leave enough footing below it.
For the pour sequence around the footing, see our guide to a concrete footing. Rebar placement and dowel lap details are covered in the does concrete need rebar guide.
Sources and Method
- ACI 332 Code Requirements for Residential Concrete, §8.2.2.1.1 and §8.2.2.1.2 (keyway location, 1-1/2 in minimum size, dowels), as summarized in an ACI 332-14 handout: lwm-info.org. Section numbers may differ in the current edition: ACI 332-20 preview.
- ACI 224.3R-95, Joints in Concrete Construction (keyways in wall-to-footing joints, forming with a beveled 2x4): ACI 224.3R-95 copy
- ACI 318-19 §22.9 (shear friction): Hilti engineering article
- ACI 318-19 §26.5.6 (construction joint surface preparation, roughening to about 1/4 in): TarmacView summary. Read the code text for the exact wording.
- IRC R403.1.1 (minimum footing 12 in wide, 6 in thick): residential footing inspection checklist
- Volume method and waste factor: Length × Width × Depth, then × (1 + waste%) and ÷ 27 for cubic yards: Pearson concrete calculator reference
- Premix bag yields of 0.60, 0.45, and 0.30 cubic feet for 80, 60, and 40 lb bags: confirm on the bag you buy.
The middle-third width limit is this calculator's reading of the ACI 332 rule. It is not quoted from the code.
Last reviewed: October 2026. Reviewed by site author.
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