Designers make products adjustable because users differ in body size, proportions, strength and mobility. Adjustability allows one product to accommodate a wider user population, improving comfort, safety, usability and inclusion.
In ergonomics, designers apply knowledge about human capabilities and limitations to product design. Anthropometric data provides measurements of the human body, such as stature, shoulder width, seated elbow height and functional reach.
A fixed product designed around an average measurement will not necessarily fit most people. Human dimensions vary independently, so someone of average height may not have average arm length or shoulder width. This makes the idea of an “average user” unreliable.
Designers therefore use percentile data to define an intended user range. A product might be adjustable from approximately the 5th-percentile user to the 95th-percentile user, although the appropriate range depends on the target market and design context.
| Ergonomic requirement | How adjustability helps |
|---|---|
| Reach | Adjustable controls or handles keep components within comfortable reach for smaller and larger users. |
| Posture | Adjustable seat height, backrests and monitor positions support a neutral working posture. |
| Clearance | Movable components provide sufficient space for users with larger body dimensions. |
| Accessibility | Alternative positions can accommodate differences in mobility, dexterity or strength. |
| Shared use | One product can be configured for several users, as with office chairs or bicycle seats. |
Adjustability can also extend a product's useful life, particularly for products used by growing children. However, it may increase cost, weight, mechanical complexity and the risk of incorrect adjustment. Designers must balance these disadvantages against ergonomic benefits.
A common misconception is that designing for the 50th percentile creates a one-size-fits-all solution. It does not: the 50th percentile is only the midpoint for one measured dimension, not a complete “average person.”
For an IB exam response, link adjustability explicitly to anthropometric variation, percentile ranges and ergonomic outcomes. For an evaluate command term, also consider cost, complexity, reliability and whether the adjustment range suits the intended users.
