The FontUnit Bridge is designed to 1) provides an easy conversion between font size units, and 2) offers ballpark estimates for common usage scenarios when device information and viewing distance are unknown or vague. Ultimately, this tool aims to enable researchers, practitioners, clinicians, and the public to speak the same language regarding font size, fostering communication and collaboration.
A researcher is conducting a literature review to summarize how font size affects reading speed. This tool helps them quickly convert font sizes reported in different studies into a common unit and estimate values when viewing distance or device information is not reported.
A designer is helping a coffee shop create menus for the counter, tables, and mobile devices. They can use the tool to calculate the font size for each viewing scenario (in pt, px, or mm), ensuring a consistent visual angle. In addition, seeing font sizes in all these units simultaneously helps the designer contextualize the measurements and address other practical constraints, such as device size and resolution, and the dimensions and placement of the printed menus.
A clinician is seeing a patient who has recently experienced a reduction in visual acuity. Based on the patient’s updated acuity level, the clinician can use the tool to simulate various reading scenarios and determine appropriate accommodations for each, such as moving closer or increasing font size to ensure text legibility, or identifying the minimum magnification level when using assistive devices.
Questions or feature requests? Please contact Yingchen He, yingchen_he@ncsu.edu.
He, Y., & Sutherland, N. (2025). FontUnit Bridge: Unifying the language of font size for practical application. Proceedings of the Human Factors and Ergonomics Society Annual Meeting (OnlineFirst). https://doi.org/10.1177/10711813251371647
Depending on the conversion, information about the device or viewing distance may be required. If these values are not not available, or if users want to explore a range of scenarios, they can select from a list of representative device categories and viewing situations for a ballpark estimate. Below is the full list of the estimates:
These units describe how large something appears to the eye, considering viewing distance.
These conversions require the Pixel Per Inch ($PPI$) information. For example:
$ \text{Size (px)} = \text{Size (in)} \times \text{PPI} $
These conversions require the Viewing Distance ($D$) information, assuming the fixation is at the center of the object. The calculation uses basic trigonometry. For example, the conversion between a physical size ($PS$) and the angle ($\theta$) subtended at the eye is given by the tangent function:
$$PS = 2 \times D \times \tan\left(\frac{\theta}{2}\right)\text{, and}$$ $$\theta = 2 \times \tan^{-1}\!\left(\frac{PS}{2 \times D}\right) .$$Here, $\theta$ must be in radians, which can be converted from degrees: $\theta_{\text{rad}} = \theta_{\text{deg}} \times \pi/180$.
A device pixel (or physical pixel) is the smallest unit of a display, i.e. the actual dots that make up the screen image. The size of a device pixel varies depending on the model. In contrast, a CSS pixel (also called a reference pixel) is a logical unit used by browsers in CSS for consistent sizing. It is defined as the size of a pixel on a "standard" device with a pixel density of 96 PPI. The CSS pixel is used if the layout uses HTML/CSS rules.
In addition, text rendering can be influenced by antialiasing strategies, as well as the specific software and operating system. These factors need to be taken into account if pixel-level precision is required.
While the conversion formulas are straightforward, additional considerations arise when applying the results to real-world scenarios. For example, does letter size refer to cap height (the distance from the baseline to the top of capital letters), or x-height (the height of lowercase letter x)? Many acuity charts use cap height, whereas x-height is more commonly used in research studies, which requires an additional conversion.
Also, visual acuity describes only the smallest discernible letter size, whereas fluent reading at maximum speed requires larger text. Other factors such as contrast, layout, or font type also affect reading experience, but they are not considered in this tool.