Work at height · 4 up : 1 out · geometry
Estimate leaning-ladder length using the UK 1-in-4 rule
Estimate base offset and ladder length using four units vertically up for every one unit horizontally out.
By HSECalc · Published
Check that this method fits your task
Geometry for a straight leaning ladder on level ground. This UK-style rule is an approximation, not a complete ladder-selection or safe-access assessment.
Choose your inputs and work through the steps
- Measure vertical support height on level ground. This is not the sloping length of the ladder.
- Specify any required vertical projection above the support for the assessed access task. The tool converts that projection to extra length along the ladder.
- Review the geometric result against the manufacturer’s usable length, section overlap and the suitability of the access method.
Offset = support height ÷ 4. Length to support = √(height² + offset²). Add projection × length-to-support ÷ height for extra ladder length.
A worked example
Illustrative values explain the tool; they are not prescribed inputs for your work.
- Support height 4 m gives a base offset of 4 ÷ 4 = 1 m.
- Length to support = √(4² + 1²) = 4.123 m.
- A 1 m vertical projection adds 1.031 m along the ladder: total geometric length 5.154 m.
Read the result and decide what needs checking
The UK four-up/one-out geometry gives about 75.96°. Do not mix it with a rule that measures one quarter of the ladder’s working length.
Geometry does not establish safe access. Pre-use checks, securing, ground conditions, handhold arrangements and the actual task still matter.
Common questions
Why does the calculation show about 76° rather than exactly 75°?
Four units vertically up and one out gives arctan(4), about 75.96°. HSE describes this practical rule as about 75°.
Can I buy a ladder of exactly the calculated length?
Check usable extended length, required section overlap, labels, task and access requirements with the supplier.
Is this the US OSHA working-length rule?
No. This page explicitly uses the UK vertical-up rule. Do not mix the two reference lengths.
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.