Retaining Wall Design Calculator
Soil Pushes Harder on a Wall Than Its Weight Alone Suggests
Rankine's active earth pressure theory describes the lateral thrust that retained soil exerts on a wall as it's allowed to yield slightly outward — a fraction of the vertical soil pressure, set by the active earth pressure coefficient Ka, which depends only on the soil's friction angle. A looser, lower-friction soil pushes noticeably harder than a well-compacted, higher-friction one, even at identical unit weight and wall height.
The Formula
Pa = 0.5 × γ × H² × Ka
Pa is the total active thrust per unit length of wall, γ is the soil unit weight, H is the wall height, and φ is the soil's friction angle. The resultant of this triangular pressure distribution acts at H/3 above the base of the wall.
How Friction Angle Alone Changes the Thrust
| Friction angle | Ka | Total active thrust Pa |
|---|---|---|
| 20° | 0.4903 | 39.71 kN/m |
| 25° | 0.4059 | 32.87 kN/m |
| 30° | 0.3333 | 27.00 kN/m |
| 35° | 0.2710 | 21.95 kN/m |
| 40° | 0.2174 | 17.61 kN/m |
Going from a 20° friction angle to a 40° friction angle cuts the total thrust by more than half on this example — a direct incentive to specify well-compacted, higher-friction backfill behind a retaining wall wherever practical.
Where This Gets Used
- Sizing a retaining wall's structural resistance — the active thrust is the primary lateral force a wall's sliding and overturning checks need to resist.
- Comparing backfill material choices — a granular, well-draining, higher-friction backfill reduces lateral thrust compared to a poorly draining, lower-friction soil.
- Early-stage wall sizing — a quick estimate of the governing lateral force before a full geotechnical and structural design.
How to Use This Calculator
- Choose Unit system — Metric (kN/m³, m) or Imperial (pcf, ft).
- Enter Soil Unit Weight, Wall Height, and Soil Friction Angle.
- Select Calculate to see the active pressure coefficient and total active thrust.
Related Calculations
Check the bearing capacity under the wall's footing with the Soil Bearing Capacity Calculator, or evaluate the slope this wall is retaining with the Slope Stability Calculator.