Retaining Wall Calculator
A Retaining Wall Is a Volume Problem With a Lower Waste Margin
Poured concrete retaining walls are formed on both sides, which means the finished volume is much closer to the design volume than an open slab pour is — there's no broadcasting extra concrete across a wide open surface to compensate for a shortfall. That's part of why this calculator defaults to a tighter 5% waste allowance rather than the 10% used for slab work.
The Formula
Wall length and height are entered in feet (or meters), and thickness in inches (or centimeters), since a wall's thickness is a much smaller number than its length or height. The calculator converts thickness to feet, multiplies through to get cubic feet, divides by 27 for cubic yards, and applies the waste allowance — 5% by default — to account for formwork seepage and minor over-pour.
Where This Calculation Matters
- Grade change retention — a poured wall holding back a sloped yard or driveway cut is sized directly by this length-height-thickness formula, independent of the soil pressure calculations an engineer would separately run to confirm the wall's structural adequacy.
- Basement and foundation walls — the same volume math applies to a poured foundation wall, which is functionally a retaining wall holding back backfill around a basement.
- Tiered garden walls — shorter poured walls stepping down a slope use the same formula per tier, with each section's own length, height, and thickness.
Worked Example
A 40 ft long, 4 ft tall wall, 8 in thick, with the default 5% waste allowance:
| Step | Calculation | Result |
|---|---|---|
| Volume | 40 × 4 × (8 ÷ 12) | 106.67 cu ft |
| In cubic yards | 106.67 ÷ 27 | 3.95 cu yd |
| With 5% waste | 3.95 × 1.05 | 4.15 cu yd |
How to Use This Calculator
- Select Imperial (ft / in) or Metric (m / cm) units.
- Enter the Wall Length and Wall Height.
- Enter the Wall Thickness.
- Optionally adjust the Waste % — defaults to 5%.
- Select Calculate to see the total concrete volume needed.
Related Calculations
Reinforcing the wall before the pour? The Rebar Calculator sizes the steel grid, and the Gravel Calculator covers the drainage backfill typically placed behind a retaining wall.