Percentiles in Design
Use 5th, 50th and 5th–95th percentile thinking to make reach, clearance, fit and adjustability decisions.
By the end of class, you should be able to…
Interpret percentiles
Explain what 5th, 50th and 95th percentile values represent.
Choose appropriately
Identify when 5th, 50th or 5th–95th percentile approaches fit a design scenario.
Use design strategies
Explain adjustability and ranges of sizes.
Apply key dimensions
Use work envelopes, reach and clearance in design reasoning.
A percentile is a position within a population distribution.
The 50th percentile is the middle value for a specific measured dimension in a defined population—it is not “the perfect average human.”
Smaller end
5% of the population is below this measured value.
Median
Half the population is below and half above for that dimension.
Larger end
95% of the population is below this measured value.
Different design problems need different percentile strategies.
Choose the percentile based on what failure you are trying to avoid.
Design for smaller reach
Controls that most users must reach may use low-percentile reach data.
Design for larger bodies
Openings and spaces may need high-percentile body dimensions.
50th is sometimes useful
A median value may be appropriate in limited contexts, but often excludes many users if fixed.
Pick the percentile strategy
A wall-mounted emergency stop button must be reachable by nearly all intended adult users.
Which starting strategy is strongest?
Many products aim to accommodate most of the population, not one point.
One product, variable geometry
Example: chair height, monitor position, bicycle seat.
Several fixed variants
Example: gloves, helmets, footwear, protective equipment.
Adjustability and size ranges solve different problems.
Both strategies can extend inclusion, but they introduce trade-offs.
Flexible + shared use
Can accommodate different users without changing products.
Complexity + cost
Adjustment mechanisms can add parts, weight, setup time or failure points.
Optimized fixed fit
Multiple sizes can provide simpler, lighter products for defined body ranges.
Inventory + selection
Users must choose the correct size; more variants may increase production and stock complexity.
Three ergonomic ideas shape the space around a user.
Can the user access it?
Controls, handles and objects should fall within usable reach for the intended population.
Is there enough space?
The product or environment must allow bodies, hands, clothing or mobility aids to pass safely.
What space can the user operate within?
The reachable and usable area around a person during a task.
A percentile decision workflow
Percentiles support decisions; they do not replace user testing.
Define the measurement
Reach? clearance? seat height? grip diameter?
Identify the population
Who needs to use the product, and under what conditions?
Choose the exclusion risk
Would failure occur if the user is smaller, larger, or outside a fixed value?
Select a strategy
5th, 50th, high percentile, 5th–95th, adjustability or size range.
Test with users
Validate the design rather than assuming the dataset guarantees usability.
Audit one classroom chair or desk.
Three mistakes to avoid.
“95th percentile is better.”
Higher is not better; suitability depends on the design problem.
“50th fits most people.”
A fixed median dimension may fit relatively few users well.
“Percentile = person.”
A person can be at different percentiles for height, hand size, reach and other dimensions.
Percentile design vocabulary
Retrieve first. Then check.
Can you retrieve it without looking back?
Evaluate whether a fixed-height public kiosk should be designed around the 50th percentile user.
Consider reach + viewing + accessibility
Explain who may be excluded, what dimensions matter, and whether adjustability or a range-based strategy is feasible.
End with a reasoned conclusion
“Evaluate” requires weighing strengths and limitations before deciding whether the approach is appropriate.
Before you leave…
IA Day 3 — User & Product Research
Keep the evidence chain moving: user → research → decision → test.
A1.1.3 percentiles · A1.1.4 adjustability / range of sizes · A1.1.5 work envelopes, reach and clearance.