The Retaining Wall Design Calculator runs a preliminary stability check on a simple gravity retaining wall — a wall that resists soil pressure with its own weight.
It calculates the active earth pressure with Rankine’s theory (Ka = tan²(45° − φ/2)), then checks the wall against overturning, sliding, base eccentricity (middle-third rule) and soil bearing pressure. It is a sizing aid for low walls, not a substitute for an engineered design.
Retaining Wall Design Calculator
Preliminary stability check for a gravity retaining wall: Rankine earth pressure, overturning, sliding, base eccentricity and bearing pressure.
Formula
- Active pressure coefficient: Ka = tan²(45° − φ ÷ 2)
- Lateral force per unit length: P = ½ × Ka × γs × H² + Ka × q × H
- Overturning moment: Mo = Ka × γs × H³ ÷ 6 + Ka × q × H² ÷ 2
- Wall weight: W = γw × B × H; resisting moment Mr = W × B ÷ 2
- Overturning safety factor: FSo = Mr ÷ Mo
- Sliding safety factor: FSs = μ × W ÷ P
- Eccentricity: e = B ÷ 2 − (Mr − Mo) ÷ W (OK if e ≤ B ÷ 6)
- Toe bearing pressure: q_max = W ÷ B × (1 + 6e ÷ B) when e ≤ B ÷ 6
Variables Explained
| Symbol | Meaning | Unit |
|---|---|---|
| H | Retained height | ft, m |
| B | Wall base width | ft, m |
| γw, γs | Unit weight of wall and soil | pcf, kN/m³ |
| φ | Soil friction angle | degrees |
| q | Uniform surcharge on top of the soil | psf, kPa |
| μ | Base friction coefficient | — |
| Allowable bearing | Soil capacity from a soil report or code table | psf, kPa |
How the Calculation Works
Step 1: The soil pushes on the wall with a triangular pressure (plus a rectangle for any surcharge). Its resultant acts at H ÷ 3.
Step 2: The wall’s weight resists overturning about the toe and sliding along its base.
Step 3: The resultant force must fall in the middle third of the base so the whole base stays in compression, and the peak soil pressure must not exceed the allowable bearing.
Example Calculation
Given: H = 4 ft, B = 2.5 ft, γw = 130 pcf, γs = 120 pcf, φ = 30°, no surcharge, μ = 0.5, allowable bearing 1,500 psf
Ka = tan²(30°) = 0.333; P = ½ × 0.333 × 120 × 16 = 320 lb/ft
W = 130 × 2.5 × 4 = 1,300 lb/ft; Mr = 1,625; Mo ≈ 426.7
FSo ≈ 3.81 (OK); FSs = 0.5 × 1,300 ÷ 320 ≈ 2.03 (OK)
e ≈ 0.33 ft ≤ B/6 = 0.42 ft (OK); q_max ≈ 930 psf ≤ 1,500 psf (OK)
Engineering Assumptions
- Rectangular gravity wall with a vertical back and level backfill.
- No water pressure (the backfill is assumed to drain), no passive soil resistance at the toe, no seismic load.
- Wall height equals retained height; embedment below grade is not modeled.
- The 1.5 targets shown are common preliminary criteria; your code or engineer may require different factors.
Walls over about 4 ft, or those supporting slopes, driveways or buildings, generally require design by a licensed engineer and may need a permit.
Formula Variations
Cantilever (L- or T-shaped) concrete walls also count the soil on the heel as resisting weight, and segmental block walls often use geogrid reinforcement. Those systems need their own design methods or the manufacturer’s design tables.
Units and Dimensions
You can work in feet and pcf or in meters and kN/m³; forces are per unit length of wall (lb/ft or kN/m).
Frequently Asked Questions
What is a good factor of safety for a retaining wall?
1.5 is a commonly used minimum for overturning and sliding in preliminary checks; your governing code sets the actual requirement.
What is Ka in retaining wall design?
The active earth pressure coefficient. For level backfill with Rankine theory, Ka = tan²(45° − φ/2); φ = 30° gives 1/3.
What does the middle-third rule mean?
If the resultant falls within the middle third of the base, the entire base presses on the soil and none of it lifts.
How wide should a gravity retaining wall base be?
Wide enough to pass all the checks; try different base widths in the calculator.
Does water behind the wall matter?
Yes, a lot — this check assumes good drainage. Water pressure can greatly increase the load.