Seven factors
The calculation starts from a base value and every unfavourable condition reduces it by a multiplier: the further the load is from the body, the further the hands are from waist height (below as well as above), the further the load travels vertically, the more the trunk twists, the more frequent the lift, the longer the work lasts and the worse the grip — the lower the limit. The harshest factor is usually horizontal distance: bringing the load closer to the body is, on its own, the single biggest improvement in most tasks.
Reading the index
Dividing the actual load by the recommended limit gives an index. Below 1 the task is inside the design target; above 1 strain is raised for part of the workforce; above 3 the task should not be left as it is. The index is a design criterion, not a medical assessment.
Optifora's free tools section runs this calculation on one screen; the ergonomics module keeps it with a task library, measurement history and improvement tracking.