The big idea: A work envelope is the three-dimensional space a user can reach without moving from their position.
Everything the product asks them to touch has to be inside it — and how often they touch it decides which part of it.
Normal reach, maximum reach and knee clearance on one seated user. Step through to see which percentile sets each.
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Normal reach
- Upper arm at the side, forearm extended
- Sustainable for hours
- For controls used constantly — a mouse, a till keypad, a gearstick
- And for anything safety-critical, however rarely used
Maximum reach
- Whole arm extended, shoulder rotated forward
- Causes shoulder fatigue if repeated
- For controls used occasionally — a service switch, a rarely changed setting
- Never for something touched hundreds of times a shift
Frequency decides: The same distance can be right or wrong depending on how often the user goes there. A reach that is fine twice a shift is an injury risk 400 times a shift.
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The rule again: Reach is sized from the 5th percentile — the shortest-armed user. Clearance is sized from the 95th percentile — the largest user.
They pull in opposite directions, which is why so many workstations adjust.
Where the two collide
Why a footrest exists at all
Standing envelopes differ
How this is tested — placing controls inside a work envelope and sizing clearances correctly. It comes up two ways:
Paper 1 — multiple choice
- Identify which zone a named control belongs in.
- Pick the percentile that sets a stated clearance.
Paper 2 — analysing a product
- Explain the layout of a workstation in terms of reach zones.
- Explain why a fixed-height desk cannot suit both the 5th and 95th percentile.
The trap: Treating the envelope as one zone. "Inside reach" is not enough — say normal or maximum, and justify it by how often the control is used.
A supermarket checkout has a scanner used on every item, a receipt printer refilled twice a day, and a till the operator sits at. Explain how reach and clearance should be applied to its layout.
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