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Trench Slope & Excavation Calculator

Enter the trench depth, bottom width, length and the soil type your competent person classified. The calculator returns the maximum allowable slope from OSHA Table B-1, how far back to cut each side, the resulting top width and the excavation volume, and flags the Subpart P thresholds that apply at that depth.

ft
Measured from the lowest point of the excavation to the ground surface.
ft
Width needed at the bottom for the pipe, the bedding and the crew.
ft
Appendix A classification. When in doubt, Type C is the conservative choice.

Results are estimates for planning purposes and do not replace a competent person's evaluation or an engineer's design. Follow the OSHA standards that apply to your work and your employer's written program.

OSHA Table B-1 — maximum allowable slopes

Maximum allowable slopes (H:V) for excavations less than 20 feet deep — 29 CFR 1926 Subpart P, Appendix B
Soil or rock typeMaximum allowable slope (H:V)Angle from horizontalCut back per foot of depth
Stable rockVertical90°0 ft
Type A3/4:153°0.75 ft
Type A — short term (24 h or less, 12 ft or less deep)1/2:163°0.5 ft
Type B1:145°1 ft
Type C1 1/2:134°1.5 ft

Slopes are measured from the horizontal. The table applies to excavations less than 20 feet deep; sloping or benching for deeper excavations must be designed by a registered professional engineer. If the soil shows signs of distress, Appendix B requires the slope to be cut back to at least ½ horizontal to 1 vertical less steep than the maximum allowable slope.

The depths that change what you must do

Excavation thresholds in 29 CFR 1926 Subpart P
DepthWhat it triggersCitation
Any depthSpoil, material and equipment kept at least 2 ft from the edge; daily inspection by a competent person before the start of work, as needed through the shift, and after every rainstorm or other hazard-increasing occurrence1926.651(j)(2); 1926.651(k)(1)
4 ftStairway, ladder or ramp in trench excavations, no more than 25 ft of lateral travel away; atmospheric testing before entry where a hazardous atmosphere exists or could reasonably be expected1926.651(c)(2); 1926.651(g)(1)(i)
5 ftProtective system required — sloping, benching, shoring or shielding — unless the excavation is entirely in stable rock, or a competent person examines the ground and finds no indication of a potential cave-in1926.652(a)(1)
20 ftTable B-1 no longer applies: sloping and benching must be designed by a registered professional engineer1926 Subpart P App. B

Who decides the soil type

A competent person classifies the soil under Appendix A, using at least one visual and one manual test, and re-classifies it whenever conditions change — after rain, when the excavation is deepened, when water shows up, when vibration from traffic or equipment starts. Nothing on this page replaces that classification, and the calculator cannot see your ground: it converts the type you pick into geometry.

Two common traps: soil that has been previously disturbed is Type B at best and usually Type C; and a trench next to a road, a building or a spoil pile carries a surcharge load that pushes the classification down. When in doubt, Type C is the choice that keeps people alive.

How it's calculated

Maximum allowable slope (Table B-1)
H:V = 0 (stable rock) | 3/4:1 Type A | 1/2:1 Type A short term | 1:1 Type B | 1 1/2:1 Type C
Table B-1 applies to excavations less than 20 ft (6.09 m) deep. Sloping or benching deeper than 20 ft must be designed by a registered professional engineer.
Cut-back distance per side
offset (ft) = depth (ft) × H
H is the horizontal component of the maximum allowable slope: 0.75 for Type A, 0.5 for Type A short term, 1 for Type B, 1.5 for Type C, 0 for stable rock.
Top width
top width (ft) = bottom width (ft) + 2 × offset (ft)
Excavation volume (simple slope, both sides)
V (ft³) = (bottom width + top width) / 2 × depth (ft) × length (ft)
Trapezoidal prism. It ignores the slope at the two ends of the trench and any benching, and it is bank measure — material swells when excavated.

Frequently asked questions

1½ horizontal to 1 vertical, an angle of 34 degrees from the horizontal, for excavations less than 20 feet deep (29 CFR 1926 Subpart P, Appendix B, Table B-1). In practice that means cutting back 1.5 ft on each side for every foot of depth: an 8 ft deep trench in Type C is 24 ft wider at the top than at the bottom.

From Table B-1, for excavations less than 20 feet deep: stable rock vertical (90°); Type A 3/4:1 (53°); Type B 1:1 (45°); Type C 1½:1 (34°). A short-term maximum slope of 1/2:1 (63°) is allowed in Type A soil in excavations 12 feet or less in depth, where short term means the excavation is open 24 hours or less.

Employees in an excavation must be protected from cave-ins by an adequate protective system, except when the excavation is made entirely in stable rock, or when it is less than 5 feet deep and examination of the ground by a competent person provides no indication of a potential cave-in (29 CFR 1926.652(a)(1)). Under 5 feet is not automatically safe — it depends on that examination.

Table B-1 only applies to excavations less than 20 feet deep. Sloping or benching for excavations greater than 20 feet deep must be designed by a registered professional engineer (Appendix B). The same applies to support and shield systems used outside the tabulated data options of 1926.652(c).

A stairway, ladder, ramp or other safe means of egress must be located in trench excavations 4 feet or more in depth so that no employee has more than 25 feet of lateral travel to reach it (29 CFR 1926.651(c)(2)). That is a separate threshold from the 5 ft protective system rule — a 4 ft trench needs egress even if a competent person concludes no protective system is required.

Employees must be protected from excavated material and equipment by keeping it at least 2 feet from the edge of the excavation, or by retaining devices, or both (29 CFR 1926.651(j)(2)). Two feet is the minimum, not a target: spoil surcharges the wall, so further back is better, and the calculated top width is measured before the spoil setback.

No. It computes the geometry of a simple slope from the soil type you select, so you can plan the footprint, the excavation volume and the space you need. Classifying the soil is the competent person's job under Appendix A — visual and manual tests, updated whenever conditions change — and shoring, shielding, benching and any excavation over 20 feet deep require the tabulated data or an engineer's design.

Yes. A competent person must inspect the excavation, the adjacent areas and the protective systems daily, before the start of work and as needed throughout the shift, and after every rainstorm or other hazard-increasing occurrence (29 CFR 1926.651(k)(1)). Atmospheres must also be tested before entry into excavations more than 4 feet deep where a hazardous atmosphere exists or could reasonably be expected (1926.651(g)(1)(i)).

Sources & references

Content checked against these sources — last reviewed August 27, 2026.

By — Editorial team of SteelToeTools.com (published by LSEA SAS) Updated v1