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Character-related factors

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Font size

Font size refers to the total vertical dimension of a typeface including its ascenders, x-height and descenders. It is measured in points (1 pica point = 0.353 mm). However, proportions vary considerably from typeface to typeface. For the same font size, for example, capital heights may differ by up to 20%, x-heights by up to 40 % and descenders by up to 30%. As a result, different fonts appear to be of different sizes at the same point size. The key factor in assessing readability in mixed notation is x-height1 .

For this reason, the font sizes specified in DIN 1450 and recommended here all relate to x-heights. Where point sizes are given they are provided as approximate values only2 .

When compared, these three fonts – Garamond, Verdana and Futura – show significant differences in the proportions of both their x- and capital heights at the same point size.

Since word-processing and design software does not indicate x-height, it must be measured on a case-by-case basis. An easy-to-install and intuitive plug-in that performs this function is available for Adobe InDesign. Further information and a download are available at typografie.info.

Alongside text genre, a further significant factor in determining font size is viewing distance or the distance from the reader’s eye to the text. Most people read a book held at a forearm’s length in front of them but will generally hold a mobile phone closer to their eyes. A desktop monitor is usually approx. 70 centimetres away from the reader, while it should be possible to read a road sign from at least 10 metres.

Individual eyesight, or visual acuity (VA), and visibility conditions such as lighting and contrast, for example, are the third major factor in determining font size.

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Please note that in its approach to font size calculation leserlich.info confines itself to the three factors set out above. This is a simplified process and there are other methods, in addition to visual acuity testing, for obtaining information about the reading capability of people with sight impairments.

The font size recommendations given in DIN 1450 are based on normal eyesight, i.e., at least 70% (visual acuity ≥ 0.7). For those with poorer eyesight, the given values can be multiplied by a factor based on their particular visual acuity in order to calculate a satisfactory font size. However, the constraints imposed by visual field restrictions, column width, format and technical reading aids, for example, mean that font size cannot be increased infinitely. In practice, factors ranging from 1.4 (VA ≥ 0.5) to a maximum of 1.75 (VA ≥ 0.4) have proven workable and can be used to multiply font size for texts designed specifically for visually impaired readers. The standard font sizes recommended in DIN 1450 are adequate for all other uses.

Where there is any doubt, the best way to ascertain the appropriate font size is to carry out a test in real-life conditions.

As we read our eyes move in small jumps, known as saccades. We recognise ten to twelve letters per saccade – three or four to the left of our centre of vision and seven to nine to the right. It is only in the centre of vision, at the fixation point, that we see characters clearly. People with a restricted visual field, those learning to read and inexperienced readers will recognise fewer letters per fixation.

Font Size Calculator

You can use the Font Size Calculator to work out specific x-heights for common situations in line with DIN 1450. (NB. Where point sizes are indicated they are intended as approximate values only).

As a general rule of thumb, the following visual acuity values can be taken as a starting point:

  • Texts for the general population: VA 0.7
  • Texts for the elderly and the visually impaired: VA 0.4 to 0.5
  • Text in columns less than 10 cm wide: VA 0.6
  • Guidance and wayfinding signs: the German Federation of the Blind and Partially Sighted suggests a VA of 0.1 (according to DIN 32975)

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Signs should be displayed at a height of between 1.0 and 1.6 metres above the ground so that they can be approached easily. Where signs that are designed to provide guidance and assist in wayfinding within a building are hung above head height and cannot therefore be approached, a visual acuity of 0.1 should be used to calculate the required font size.

The height at which signs are displayed is another important factor in determining viewing distance, which can be calculated geometrically as follows. It is the length of a line drawn at 45° from the horizontal centreline of the x-height of the typeface to the point where it intersects with a horizontal line drawn 1.60 metres above floor/ground level, i.e., the height of the average visual axis of a standing person. If signs are displayed higher than this, letter heights may appear compressed (see also DIN 32975).

Minimum size

(Enlarged depiction of visual angle)

You can enter individual values for the ratio of x-height to font-size, light conditions and virtual resolution using the expert version of the Font Size Calculator.

Even where compliance with specific other standards is appropriate, additional checking against all the criteria set out in DIN 1450 and any other relevant standards remains essential.

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Font size

Footnotes

  1. ↑ DIN 1450
    Lettering – Legibility
    April 2013 edition

    Jan Filek
    Read/ability – Typografie und Lesbarkeit
    Niggli 2013

    Indra Kupferschmid
    Buchstaben kommen selten allein
    Niggli 2001

    Gerard Unger
    Wie man’s liest
    Niggli 2009

    Hans Peter Willberg
    Wegweiser Schrift
    Verlag Hermann Schmidt Mainz 2001

    Florian Adler, Sven Neumann
    Leserlichkeit von Schrift im Öffentlichen Raum
    in: J. Eckert, C. Fischer, I. Pfeiffer, P. Schäfer, A. Uebele u.a. (Hg.)
    Schrift und Identität, Niggli 2013
  2. ↑ DIN 1450
    Schriften – Leserlichkeit
    April 2013 edition