The stem is modeled as a trapezoidal prism. The average thickness is the mean of top and base thicknesses.
- Stem volume: Vstem = L × H × (Ttop + Tbase) ÷ 2
- Footing volume (optional): Vfoot = L × Wfoot × tfoot
- Shear key volume (optional): Vkey = L × Wkey × Dkey
- Waste allowance: Vgross = Vnet × (1 + waste% ÷ 100)
- Select your unit system and keep all dimensions consistent.
- Enter wall length, stem height, and stem thicknesses (top and base).
- Enable the footing and shear key if they exist, then add their sizes.
- Set a waste factor for placement losses and site conditions.
- Add density to estimate weight and a unit rate to estimate cost.
- Press Calculate, then export your report as CSV or PDF.
| Scenario | Length (m) | Height (m) | Top (m) | Base (m) | Footing (m × m) | Waste | Gross (m³) |
|---|---|---|---|---|---|---|---|
| Garden boundary | 10 | 1.2 | 0.15 | 0.25 | 1.0 × 0.25 | 5% | 3.02 |
| Driveway cut | 18 | 1.8 | 0.18 | 0.32 | 1.2 × 0.30 | 7% | 7.94 |
| Basement edge | 24 | 2.4 | 0.20 | 0.35 | 1.4 × 0.35 | 8% | 13.94 |
Quantity takeoff inputs that matter
Accurate retaining wall quantity starts with consistent dimensions along the wall run. Measure length on plan, then confirm the built line in the field for curves and offsets. Use the stem height from top of footing to finished wall top, and record the top and base thickness at the same section. If wall thickness varies, split the wall into segments and calculate each segment separately. Small changes in thickness can shift total concrete significantly on long runs, especially for walls above 2.0 units of height.
Trapezoid stem modeling and checks
This calculator models the stem as a trapezoidal prism, using the average of the top and base thicknesses. The approach matches common estimating practice for tapered stems and gives a fast, reliable volume. Confirm that the top thickness does not exceed the base thickness, and double-check units before exporting. For stepped walls, treat each step as a separate stem with its own height and thickness pair, then add the totals.
Footing and shear key allowances
Many walls include a rectangular footing, and some include a shear key beneath the footing. When enabled, footing volume is calculated as length multiplied by footing width and footing thickness. The shear key is computed as length multiplied by key width and key depth. These options help align the estimate with typical construction details without requiring manual side calculations. If only part of the wall includes a key, segment that portion and combine results.
Waste factor, weight, and cost planning
Waste covers placement losses, pump priming, spillage, and minor overbreak. A practical range is 3–10%, depending on access, form tightness, and pour sequencing. Weight is estimated from gross volume and density, supporting lift planning and delivery staging. Cost uses the gross volume in your chosen basis, so the estimate reflects what you will likely purchase rather than the net geometric volume alone.
Example data for a quick validation
Example: length 18, height 1.8, top 0.18, base 0.32, footing width 1.2, footing thickness 0.30, waste 7%. Net stem volume is 6.48, footing volume is 6.48, net total is 12.96, and gross total is 13.87.
| Field | Value | Notes |
|---|---|---|
| Length | 18 | Wall run measured on plan |
| Height | 1.8 | Stem height above footing |
| Top / Base thickness | 0.18 / 0.32 | Tapered stem section |
| Footing | 1.2 × 0.30 | Width × thickness |
| Waste | 7% | Moderate placement allowance |
1) What does “gross volume” mean?
Gross volume is the net geometric volume plus the waste factor. It is a practical ordering quantity that helps avoid short pours and last-minute top-ups.
2) Why use a trapezoid for the wall stem?
Many retaining wall stems taper from base to top. Averaging top and base thickness provides a fast, standard estimate for a tapered section without detailed sectional modeling.
3) When should I add a footing in the calculation?
Enable the footing when your wall detail includes a base slab under the stem. Use the full wall length, footing width, and footing thickness from the drawings or field measurements.
4) What is a shear key and how is it counted?
A shear key is a projection beneath the footing that improves sliding resistance. The calculator treats it as a rectangular prism: length × key width × key depth.
5) How do I handle stepped or varying wall heights?
Split the wall into sections where height or thickness changes. Run the calculator for each section and add the gross totals. This approach improves accuracy on irregular geometry.
6) What waste factor is reasonable for concrete?
A typical range is 3–10%. Tight access, pumping, and complex forms tend to require higher waste. Well-controlled placement with short runs may need less.
7) Does this calculator design the wall structurally?
No. It estimates concrete quantity, weight, and optional cost. Structural checks such as reinforcement, bearing, sliding, and overturning must be verified by a qualified engineer.