Excavation planning · Symmetrical geometry only
Calculate excavation top width from an assessed slope ratio
Work out the top width, horizontal run and angle of a symmetrical excavation from an assessed slope ratio.
By HSECalc · Published
Check that this method fits your task
Straight, equal slopes on both sides with a flat bottom. The slope is an input from a competent design or assessment; this tool does not choose a protective system or declare ground safe.
Choose your inputs and work through the steps
- Obtain the slope ratio from a competent assessment or design. The calculator does not select a permissible slope or classify the ground.
- Enter vertical depth, flat bottom width and horizontal run per unit vertical rise. Keep all lengths in metres.
- Use the result for the straight symmetrical cross-section only. Calculate other shapes and unequal sides using the appropriate design method.
Run per side = depth × H:V ratio. Top width = bottom width + 2 × run. Angle above horizontal = arctan(1 ÷ ratio).
A worked example
Illustrative values explain the tool; they are not prescribed inputs for your work.
- With depth 2 m and ratio 1.5H:1V, each side runs 2 × 1.5 = 3 m horizontally.
- A 1 m bottom plus two 3 m runs gives 7 m top width.
- Angle above horizontal = arctan(1 ÷ 1.5) = 33.69°.
Read the result and decide what needs checking
For the fictional example below, 1.5H:1V is only an arithmetic input, not a recommendation for your site.
Ground, water, loading and nearby structures affect protective-system decisions. Do not use a computed width or angle as permission to enter an excavation.
Common questions
Does choosing 1.5H:1V make the trench safe?
No. Geometry alone cannot assess soil, water, loading, nearby structures or protective-system adequacy.
Can I use this for benches or unequal sides?
No. It models one straight symmetrical cross-section only.
Does the tool classify soil?
No. Soil classification and system design require competent assessment under the applicable rules.
References and scope notes
Use the current official guidance and the instructions for your actual equipment. HSECalc is independent; a reference is not an endorsement. Read how we use methods and sources or report a correction.