How the retaining wall calculator works
This free cantilever retaining wall calculator checks stability and concrete strength per foot of wall. Enter the wall height, stem thickness, footing width and thickness, toe length, soil unit weight and friction angle, allowable bearing pressure, surcharge, an optional shear key, stem and footing reinforcement, f'c and fy. Results and the stability diagram update as you type.
What it checks
- Rankine active and passive coefficients: Ka = (1 − sin φ)/(1 + sin φ) and Kp = (1 + sin φ)/(1 − sin φ)
- Active thrust Pa = ½γH²Ka plus the surcharge thrust qKaH
- Overturning safety factor ΣMresisting/ΣMoverturning about the toe (target 1.5)
- Sliding safety factor (μΣV + Pp)/ΣH with μ = 0.5 and passive resistance over the footing depth plus the key depth (target 1.5)
- Bearing pressure at the toe and heel from the resultant eccentricity, trapezoidal or triangular
- Stem flexure and shear with a 1.6 factor on earth pressure, and toe and heel flexure for the bars and spacing you enter
Method
All forces are per linear foot of wall. The resisting weight includes the stem and footing concrete at 150 pcf, the soil over the heel and the surcharge over the heel. The resultant location gives the eccentricity e, and the bearing pressure follows from P/B(1 ± 6e/B) or the triangular distribution when e exceeds B/6.
Assumptions and limitations
- Level backfill with no groundwater (hydrostatic) pressure and no seismic earth pressure.
- The base friction coefficient is fixed at 0.5, and soil above the toe is ignored except for passive resistance at the footing and key.
- The shear key location only changes the drawing. Passive resistance uses the key depth.
- Toe design multiplies the service bearing pressure by 1.4, and heel design ignores upward soil pressure, which is conservative.
- Global (slope) stability and settlement are not checked.
Frequently asked questions
What safety factors does a retaining wall need?
IBC Section 1807.2.3 requires a factor of safety of 1.5 against both sliding and overturning, which is the target this calculator uses. Your local code or geotechnical engineer may require other values.
What is the difference between active and passive earth pressure?
Active pressure acts when the wall moves slightly away from the soil and is the load the wall must resist. Passive pressure develops when the wall pushes into the soil and provides resistance, here in front of the footing and shear key.
When should I add a shear key?
When the sliding safety factor is below 1.5. A key extends the passive resistance deeper below the footing, which raises the sliding resistance.
Does the calculator include surcharge loads?
Yes. A uniform surcharge in psf adds a rectangular pressure qKa on the wall and adds weight over the heel.
Related calculators
For educational and preliminary design purposes only. Always verify results with a licensed PE.