Trench sloping calculator

Pick the soil classification and enter the depth and the bottom width. You get the maximum allowable slope from OSHA's Subpart P Appendix B, the angle, the run on each side and the width of the excavation at ground level. A cross-section shows the shape, and a note tells you when a protective system is required.

Look up the Appendix B maximum slope for the soil type, then work the top width as bottom width plus two runs, with every step shown. A planning aid; the competent person classifies the soil, and 29 CFR 1926 Subpart P governs real work.

The competent person classifies the soil. The slope is only as good as the classification, and 1926.652 requires protection at 5 ft or deeper unless the excavation is in stable rock. Sloping by Appendix B covers excavations 20 ft deep or less.

Trench slope and top width

Soil classificationUnitsDepth (ft)Bottom width (ft)

Soil: Type B
Maximum allowable slope (H:V): 1:1
Angle from the horizontal: 45° (OSHA table: 45°)
Run on each side: 10 ft 0 in (10 ft)
Top width at ground level: 24 ft 0 in (24 ft)

Assumes a simple slope that starts at the bottom edge of the excavation on both sides. A competent person classifies the soil. Confirm every figure against 29 CFR 1926.652 and Appendix B.

How trench sloping works

Sloping is one way to protect workers from a cave-in: you cut the sides of the excavation back at an angle the soil can hold. Appendix B to Subpart P gives the steepest angle allowed for each soil classification in an excavation 20 ft deep or less. Slopes are written as horizontal to vertical, so 1:1 means one foot out for every foot down.

Stable rock: vertical sides, 90 degrees.
Type A: 3/4:1, about 53 degrees. A short-term slope of 1/2:1, about 63 degrees, is allowed in an excavation that is open 24 hours or less and 12 ft deep or less.
Type B: 1:1, 45 degrees.
Type C: 1 1/2:1, about 34 degrees.

Type C is the weakest soil, so it needs the flattest slope and the widest cut. Type A is the strongest of the three soil types. These are the maximum slopes: a flatter slope is always allowed.

The top width follows from the geometry. Each side runs out by the depth times the horizontal part of the ratio, so run = depth × H and top width = bottom width + 2 × run. The angle from the horizontal is the arctangent of 1 ÷ H. OSHA rounds the angles in its table to whole degrees, so the calculator shows both the exact angle and the table figure.

When a protective system is required

Section 1926.652(a)(1) requires protection from cave-ins for employees in an excavation, with two exceptions. Excavations made entirely in stable rock need none. Excavations less than 5 ft deep need none when a competent person examines the ground and finds no indication of a potential cave-in. At 5 ft or deeper in soil, a protective system is required. Sloping is one choice, and benching, support systems and shield systems are the others. Deeper than 20 ft, the system has to be designed by a registered professional engineer.

The soil type comes from the competent person, using the visual and manual tests in Appendix A. Soil that has been disturbed, or soil with water seeping in, usually classifies as the weaker type. This calculator assumes one soil type and a simple slope from the bottom edge. Layered soils, benches and vertical lower sections have their own configurations in Appendix B, and the calculator does not cover them. For the full set of excavation rules, read OSHA excavations (Subpart P) for the CHST.

Worked example 1: 10 ft deep in Type B soil

The trench is 10 ft deep with a 4 ft wide bottom, and the competent person has classified the soil as Type B. These are invented study numbers.

Type B maximum slope is 1:1, so H = 1.
Run on each side = 10 × 1 = 10 ft.
Top width = 4 + 2 × 10 = 24 ft.
Angle = arctangent of 1 ÷ 1 = 45 degrees.
The depth is over 5 ft, so a protective system is required. Sloping at 1:1 or flatter would be one way to meet that.

Worked example 2: 12 ft deep in Type C soil

The trench is 12 ft deep with a 3 ft wide bottom, in Type C soil.

Type C maximum slope is 1 1/2:1, so H = 1.5.
Run on each side = 12 × 1.5 = 18 ft.
Top width = 3 + 2 × 18 = 39 ft.
Angle = arctangent of 1 ÷ 1.5 = 33.7 degrees, which the OSHA table rounds to 34.

A 12 ft trench in Type C soil is 39 ft across at the top, against 24 ft for a 10 ft Type B trench. That gap is why the soil classification matters so much. These are original arithmetic examples, not exam questions.

Trench sloping questions

Does a trench under 5 feet deep need sloping?

Not automatically. Under 5 ft, protection is not required if a competent person examines the ground and finds no sign of a potential cave-in. If the competent person sees one, the excavation needs a protective system whatever its depth. Stable rock is the other exception, at any depth.

Who decides if the soil is Type A, B or C?

A competent person does, using visual and manual tests from Appendix A. The classification is not a choice and it is not a guess from the surface. If the soil is disturbed, wet or in layers, expect the weaker type. A wrong type gives a slope that is too steep.

What if the trench is deeper than 20 feet?

Appendix B stops at 20 ft. For a deeper excavation, the protective system has to be designed by a registered professional engineer. The calculator warns you at that depth and does not treat the slope as a permitted design.

Excavations are in Domain 1 of the CHST, and questions often describe a soil or a change of conditions and ask what applies. The calculator covers the arithmetic, and the guide covers the rest of Subpart P.