Excavation Slope Calculator uses OSHA soil ratios to find setback per side. Formula: S = depth × slope ratio. Enter depth, bottom width, and soil type to calculate top width, area, and volume. outputs
What the Excavation Slope Calculator Finds
The Excavation Slope Calculator computes the geometry of a sloped trench wall based on excavation depth, bottom trench width, and OSHA soil classification. For each calculation, it returns the horizontal setback required per side, the slope ratio applied, the top opening width, the slope face length, the slope angle in degrees, the trapezoidal cross-section area, and the excavation volume per linear foot or meter. All outputs follow directly from the OSHA simple-slope ratios defined in 29 CFR 1926 Subpart P Appendix B.
Excavation Slope Formulas
The calculator applies the following formulas to every soil type. Inputs are excavation depth, bottom trench width, and the slope ratio assigned to the selected OSHA soil classification.
$$\text{Setback per side} = \text{Depth} \times \text{Slope ratio}$$
$$\text{Top opening width} = \text{Bottom width} + 2 \times \text{Setback per side}$$
$$\text{Section area} = \frac{(\text{Top width} + \text{Bottom width})}{2} \times \text{Depth}$$
$$\text{Volume per linear foot} = \frac{\text{Section area}}{27}$$
The slope face length is the hypotenuse of the right triangle formed by the depth and the horizontal setback:
$$\text{Face length} = \sqrt{\text{Depth}^2 + \text{Setback}^2}$$.
The slope angle is the arctangent of depth divided by setback, expressed in degrees.
OSHA Soil Slope Ratios Used by the Calculator
The calculator assigns a slope ratio to each OSHA soil classification as specified in 29 CFR 1926 Subpart P Appendix B for simple slopes. These ratios express horizontal run per unit of vertical rise (H:V).
| OSHA Soil Type | Slope Ratio (H:V) | Approximate Angle |
|---|---|---|
| Stable Rock | Vertical (0:1) | 90° |
| Type A | ¾:1 | ~53° |
| Type B | 1:1 | 45° |
| Type C | 1½:1 | ~34° |
These are simple-slope ratios only. The calculator validates simple sloping geometry and does not produce benching configurations, shoring designs, or any other protective system layout.
Example: Type B Excavation Slope Calculation
The following example uses an excavation depth of 10 ft, a bottom trench width of 4 ft, and Type B soil. Type B cohesive soil has unconfined compressive strength greater than 0.5 tsf and less than 1.5 tsf. The assigned simple-slope ratio is 1H:1V.
- Setback per side: 10 ft × 1.0 = 10 ft
- Top opening width: 4 ft + (2 × 10 ft) = 24 ft
- Slope face length: √(10² + 10²) = √200 ≈ 14.14 ft
- Slope angle: arctan(10 ÷ 10) = 45°
- Section area: ((24 ft + 4 ft) ÷ 2) × 10 ft = 140 sq ft
- Volume per linear foot: 140 ÷ 27 = 5.19 yd³/ft
With a 10 ft deep Type B trench, each sloped wall moves 10 ft horizontally from the trench floor edge. The total surface opening of 24 ft is more than six times the 4 ft bottom width, which is important for staging, spoil placement, and right-of-way planning.
How to Read the Calculator Results
Required Horizontal Setback
Setback per side is the horizontal distance the slope must extend from the bottom edge of the trench to the top cut line on each side. It is the minimum clear distance required at grade level beyond each wall of the trench floor. A larger setback means more surface area disturbed on both sides of the trench.
Top Opening Width
Top opening width is the total distance measured at the original ground surface from one cut edge to the other. It determines the footprint of the excavation at grade and is used for right-of-way assessment, spoil pile placement distance, and surface utility identification. The top width grows rapidly with depth in Type C soil due to the 1.5:1 slope ratio.
Section Area
Section area is the trapezoidal cross-sectional area of the excavation in square feet or square meters. It represents the area of one vertical slice perpendicular to the trench centerline. The calculator uses the average-width formula for a trapezoid: the mean of the top and bottom widths multiplied by depth.
Volume per Linear Foot
Volume per linear foot is the section area divided by 27 to convert cubic feet to cubic yards for one foot of trench length. Multiply this figure by the total trench run in feet to estimate total excavation volume. This output is a unit rate, not a project total.
PE Design Status
The calculator flags when an excavation depth exceeds 20 ft. Per OSHA 29 CFR 1926 Subpart P Appendix B, sloping and benching for excavations deeper than 20 ft must be designed by a registered professional engineer. The simple-slope ratios in this calculator do not apply to those excavations without PE-stamped design documents.
What This Calculator Does Not Design
This calculator validates simple sloping geometry only. It does not produce or substitute for any of the following:
- Benching geometry — multiple-step bench configurations are not modeled.
- Shoring design — timber shoring, hydraulic shoring, and similar systems are outside the scope of this tool.
- Shielding and trench box selection — the calculator does not size or specify trench boxes or shields.
- Layered or mixed soil profiles — the tool assumes a single uniform soil classification for the full depth.
- Surcharge, groundwater, vibration, or load analysis — no external load conditions are evaluated.
- Competent-person soil classification — the OSHA soil type entered must be determined in the field by a qualified competent person, not selected arbitrarily.
Common Mistakes When Estimating Excavation Slopes
- Using bottom width as top width. The bottom width is the trench floor dimension. The top opening width is always wider once setback is applied. Entering top width as the bottom width understates the actual surface footprint.
- Forgetting setback applies to both sides. The slope ratio generates a setback on each side independently. Total surface width increase is twice the single-side setback.
- Treating volume per linear foot as total project volume. The volume output is a unit rate per foot of trench. Total volume requires multiplying by the full trench length.
- Selecting soil type without field classification. OSHA soil type must be determined by a competent person using visual and manual tests on-site. Using an assumed or default soil type can produce non-conservative slope geometry.
- Applying simple-slope tables to excavations over 20 ft. OSHA requires a registered professional engineer to design slopes and benching for excavations exceeding 20 ft in depth. The ratios in this calculator do not extend to those depths without PE involvement.
Frequently Asked Questions
How do you calculate excavation slope setback?
Multiply the excavation depth by the slope ratio for the applicable OSHA soil type. For a 10 ft deep Type B excavation with a 1:1 ratio, the setback per side is 10 ft. The same formula applies on both sides of the trench, so total top width increase is 2 × setback.
What is the OSHA slope for Type B soil?
OSHA 29 CFR 1926 Subpart P Appendix B specifies a maximum allowable slope of 1H:1V (45°) for Type B soil under simple-slope conditions. Type B cohesive soil has an unconfined compressive strength greater than 0.5 tsf and less than 1.5 tsf. This ratio applies to excavations 20 ft deep or less.
How wide is the top of a sloped trench?
Top opening width equals the bottom trench width plus twice the horizontal setback per side. The setback depends on depth and the OSHA slope ratio for the soil classification. For a 10 ft deep Type B trench with a 4 ft bottom width, the top opening is 24 ft wide.
Does a 5 ft trench need a protective system?
OSHA 29 CFR 1926.652 generally requires a protective system for excavations 5 ft or deeper unless the excavation is made entirely in stable rock or a competent person determines that a protective system is not required due to site-specific conditions. Sloping, benching, shoring, and shielding are all recognized protective system options under Subpart P.
Can this calculator be used for benching?
No. This calculator computes simple-slope geometry only. Benching involves a series of horizontal steps cut into the trench wall and follows different geometric requirements defined in OSHA Appendix B. Benching dimensions are not produced by this tool.
What happens if the excavation is deeper than 20 ft?
OSHA requires that sloping and benching for excavations deeper than 20 ft be designed by a registered professional engineer. The simple-slope ratios in OSHA Appendix B apply only to excavations at or under 20 ft. This calculator will flag that condition, but the resulting geometry should not be used for construction without a PE-prepared design.
Who determines the OSHA soil type?
OSHA requires that soil classification be performed by a competent person at the excavation site using the visual and manual tests described in 29 CFR 1926 Subpart P Appendix A. The soil type input in this calculator must reflect an on-site field classification, not an assumed or default value.
Does excavation volume per linear foot equal total excavation volume?
No. Volume per linear foot is the cubic yards of material in a one-foot-long cross section of the trench. To estimate total excavation volume, multiply the volume-per-linear-foot result by the total trench length in feet. The calculator returns only the unit rate for a single linear foot.
References
- OSHA 29 CFR 1926 Subpart P Appendix B — Sloping and Benching. Defines maximum allowable slopes and configurations for Stable Rock, Type A, Type B, and Type C soils.
- OSHA 29 CFR 1926 Subpart P Appendix A — Soil Classification. Establishes the classification system and field testing methods used to determine OSHA soil type on excavation sites.
- OSHA 29 CFR 1926.652 — Requirements for Protective Systems. Sets the trigger depths and conditions under which a protective system is required for excavations.