Calculate Excavation Slope
Results
How to Use the Excavation Slope Calculator
Get your safe trench slope estimate in under two minutes:
- Enter excavation depth — type the total planned depth of your trench in feet. Use the Feet / Meters toggle if you prefer metric.
- Select soil type — choose the OSHA classification your competent person identified on site: Stable Rock, Type A, Type B, or Type C. If you're unsure, always default to Type C — it's the most conservative.
- Set the trench bottom width — enter the width at the bottom of the cut. Most utility trenches default to 2 feet; adjust for wider excavations like manholes or footings.
- Add trench length (optional) — enter a length to get the estimated excavation footprint area, useful for spoil pile planning and permit applications.
- Review your results — the calculator shows the OSHA slope ratio, horizontal setback per side, total top opening width, and a plain-English safety note. Use Print or Copy to save them.
Always review results with your on-site competent person. Field conditions — groundwater, nearby structures, equipment vibration, and weather — can require a more conservative protective system than simple sloping alone.
OSHA Soil Classifications & Slope Ratios Explained
OSHA 29 CFR 1926 Subpart P classifies excavation soils into four categories. The classification determines the maximum allowable slope angle. A competent person must classify the soil at your specific site using visual observation and manual tests before work begins.
Stable Rock
Vertical — 0:1
Natural solid mineral matter that can be excavated with vertical sides and remains intact while exposed. No horizontal setback required. Rare in most field conditions.
Type A Soil
3/4:1 — 0.75 H : 1 V
Most stable classified soil. Cohesive soils with unconfined compressive strength ≥1.5 tsf. Examples: clay, silty clay, sandy clay. Cannot be Type A if fissured, subject to vibration, or previously disturbed.
Type B Soil
1:1 — 1 H : 1 V
Moderately stable soil. Cohesive soils with unconfined compressive strength between 0.5 and 1.5 tsf. Examples: angular gravel, silty loam, previously disturbed soils, and soils subject to vibration.
Type C Soil
1½:1 — 1.5 H : 1 V
Least stable classified soil. Cohesive soils with unconfined compressive strength <0.5 tsf. Examples: gravel, sandy loam, submerged soil or rock, or soil where water is freely seeping.
Quick Reference: OSHA Excavation Slope Ratios
| Soil Type | Slope Ratio (H:V) | 10 ft Depth — Setback/Side | 10 ft Depth — Top Width* |
|---|---|---|---|
| Stable Rock | 0 : 1 (Vertical) | 0 ft | 2 ft |
| Type A Soil | ¾ : 1 (0.75 H:V) | 7.5 ft | 17 ft |
| Type B Soil | 1 : 1 (1.0 H:V) | 10 ft | 22 ft |
| Type C Soil | 1½ : 1 (1.5 H:V) | 15 ft | 32 ft |
* Top width based on 2 ft trench bottom width. Source: OSHA 29 CFR 1926.652 Appendix B.
Frequently Asked Questions
Per OSHA 29 CFR 1926.652, Type C soil requires a slope ratio of 1.5:1 (horizontal to vertical). That means for every 1 foot of excavation depth, the wall must be cut back 1.5 feet on each side. A 10-foot-deep trench in Type C soil would need 15 feet of horizontal setback per side, giving a top opening 32 feet wide for a 2-foot-wide bottom. Use the Type C soil slope calculator above to estimate your specific dimensions.
The formula is simple: Horizontal Setback = Depth × Slope Ratio. Then: Top Width = Bottom Width + (2 × Horizontal Setback). For example, a 6-foot deep trench in Type A soil (0.75 ratio) needs 4.5 ft of setback per side, so a 2-foot-wide trench bottom becomes 11 feet wide at the surface. This trench slope calculator does that math automatically.
Sloping is just one of three acceptable protective systems under OSHA 1926.652. When there isn't enough room to slope (in a tight right-of-way, next to a structure, etc.), contractors use shoring (timber, hydraulic, or pneumatic bracing) or shielding (trench boxes). The competent person at your site must determine which system is appropriate based on soil conditions, depth, water, surcharge loads, and available space.
Under OSHA, a competent person is someone capable of identifying existing and predictable hazards and who has the authority to take prompt corrective action. For excavations, this person must classify the soil type using visual and manual tests, inspect the excavation at least daily and after rain events, identify surface encumbrances, and ensure proper protective systems are in place before workers enter.
OSHA requires a protective system for any excavation 5 feet or deeper — unless the excavation is entirely in stable rock. Even for trenches less than 5 feet, OSHA may still require protection if the competent person determines that hazardous ground movement could occur. Never enter an unprotected excavation. Falls and cave-ins are among the leading causes of construction fatalities.
OSHA's classification ranks soil from most to least stable: Type A soils (clay, hardpan) are the most stable and allow a 3/4:1 slope. Type B soils (silty loam, angular gravel) are moderately stable and require a 1:1 slope. Type C soils (sand, gravel, saturated soil) are the least stable and require a 1.5:1 slope. Soil can be downgraded but never upgraded. Disturbed or fissured soil is always classified as Type C at minimum.
OSHA 29 CFR 1926.652 requires that all excavations 5 feet or deeper have an adequate protective system — unless the excavation is entirely in stable rock. Appendix B of the standard establishes maximum allowable slopes: 0.75:1 for Type A, 1:1 for Type B, and 1.5:1 for Type C soil. Simple sloping (what this calculator estimates) is one of three accepted protective methods alongside shoring and shielding. The employer must ensure a competent person inspects the site daily.
No — this trench slope calculator only estimates open-cut (sloped) excavation dimensions. Shoring and trench box (shielding) design require separate analysis based on lateral soil pressure, surcharge loads, and manufacturer specifications. If sloping is not practical due to space or soil conditions, consult your competent person. For excavations deeper than 20 feet, a licensed professional engineer must design the protective system.
Significantly. Any soil that is submerged, subject to water seepage, or has been exposed to rainfall must be classified as Type C — regardless of what it may have been before. A competent person must re-inspect the excavation after every rain event, at the start of each shift, and whenever water conditions change. Never assume a prior soil classification still applies. When in doubt, always assume Type C and use a 1.5:1 slope or switch to shoring.
Not always. Sloping works well when space permits and soil is stable, but it's not always practical. In tight urban rights-of-way, near existing foundations, or in deep Type C excavations, shoring or shielding may be safer and more cost-effective. All three protective methods are equally valid under OSHA 1926.652 when properly designed and implemented. The on-site competent person — not this calculator — determines which system is appropriate for your specific project conditions.