The big idea: A percentile is the value that a given percentage of the population falls below.
The 5th percentile of arm reach: 5% of people reach less far than this. The 95th percentile of hip breadth: 95% of people are narrower than this.
Percentiles exist so a designer can answer one question: how many people does this dimension exclude? A measurement with no percentile attached cannot answer it.
Step through each percentile to see the slice of the population it covers, and the design decision it serves.
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The rule worth memorising: Reach comes from the smallest user — the 5th percentile. Clearance comes from the largest user — the 95th percentile.
If the shortest person can reach it, everyone can. If the largest person fits through it, everyone fits.
5th percentile
- Height of a shelf or a control
- Distance to a foot pedal
- Force needed to open a lid
- Anything the user must REACH or OVERCOME
95th percentile
- Doorway and headroom height
- Legroom and knee clearance
- Seat and gangway width
- Anything the user must FIT INTO or THROUGH
50th percentile
- Height of a door handle
- Position of a light switch
- Used only where being wrong either way is harmless
- Almost never the answer for reach or clearance
The classic error: "Design for the average user" fails half the population by construction. Nobody is average in every dimension at once — a person at the 50th percentile for height is rarely at the 50th for arm length.
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Most workstations, seats and vehicle cabs are designed to suit the 5th to 95th percentile range — one product that adjusts across 90% of the population.
Why 5th to 95th, and not everyone?
When is excluding 10% not acceptable?
Which percentile for a bus grab rail?
How this is tested — choosing the right percentile for a stated design decision, and justifying it. It comes up two ways:
Paper 1 — multiple choice
- Identify the percentile appropriate to a given dimension.
- Spot the dimension where the 50th percentile is the wrong choice.
Paper 2 — analysing a product
- Justify the percentile chosen for a named part of a product.
- Explain who a chosen percentile excludes, and whether that matters.
The trap: Deciding by the size of the number rather than the kind of decision. "It should suit big people, so 95th" is wrong for a control that has to be reached — bigger users can always reach further.
A manufacturer is designing a domestic washing machine. Justify the percentile used for (a) the height of the detergent drawer and (b) the width of the door opening.
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