The size of a font influences both the legibility and the effect of the text. But what exactly is font size? Where is it measured, and why are the letters of a Times font larger than those of an English script font, even though you enter exactly the same size value?

There are still general rules on font sizes circulating on the Internet, especially for students, but also for other target groups. For example, some universities still require dissertations to be in 12 point, possibly in Times New Roman, and you wonder who would actually want to read that. The uncertainty is no coincidence, as there are hardly any compact and at the same time comprehensible guidelines regarding font size. It’s time to bring light into the darkness.

Table of contents

• The typographic measurement system
• Definition: What is font size?
• Different measurement results with the same font size
• Different size effect
• Measure font size
• Which font size is suitable for what?
• Conclusion

The typographic measurement system

When working with type, we do not work with the metric system of measurement, but with the typographic system of measurement. We therefore do not measure and define in metres and millimetres, but historically in the smallest typographic unit, the point. Over the years and centuries, there have also been different sizes. For many years, the Didot point, a further development of the Fournier point, was the standard in the typographic world. At 0.375 mm, it was slightly larger than the current PostScript or DTP point, which is 0.353 mm. So if you enter a font size of 10 points in Microsoft Word or Adobe InDesign, you specify that your font has a size of 3.53 mm. Actually.

Definition: What is the font size?

Let’s first define the term font size or font size. Strictly speaking, it is misleading, because neither in InDesign nor in Word or other software colleagues do we define the size of the font by entering points in the “Font size” input field. Instead, we define the size of the cone. The type cone is a cuboid metal cone from lead typesetting. The printing character, the actual typeface, sits mirror-inverted and raised on the upper part; around and next to it is more or less a lot of “air”, i.e. a non-printing area, also known as flesh or punch.

When Gutenberg set a 12-point typeface, the cone – and not the measurable printed letter – had a size of 12 points. This means that the cone size is generally always larger than the actual typeface.

Now, we are not called Johannes Gutenberg, nor do we fortunately still have to carry typesetting boxes around. In the age of digital fonts, there is therefore no longer a physical cone. Nevertheless, the term and, above all, the meaning of the type cone in font design remain. Because if you define 12 points as the font size in InDesign, you define precisely this cone size.

The PT Sans, defined in InDesign with 12 points; the grey area behind the font has a height of 12 points, i.e. 4.233 mm, and symbolises the cone size.

Different measurement results with the same font size

You could now assume that you would simply have to remove, say, 15 per cent of the flesh, i.e. non-printing material, around the typeface for each font. By entering 12 points, each font would then have an actual measurable size of 10.2 points at 15 per cent. But this is not the case.

You are probably familiar with this: two fonts placed next to each other in the same size can either appear the same, but can also have significant differences in size. Apart from the size effect, which also plays a significant role, the utilisation of the cone has a considerable influence on the actual measurable final size.

Cone utilisation – a decision for the font designer

This cone utilisation, i.e. the design of the proportions of the printed character and the air around it, is up to each font designer. Typographers who like their typefaces to be more airy cut their typefaces with a lot of flesh; other typefaces have a large cone utilisation and – mentally transferred to the lead cone – protrude to the edge of the cone and even beyond.

The smaller the cone utilisation, the smaller the actual measurable font size; the larger the cone utilisation, the larger the font size.

Different size effects

When deciding on a size and also assessing it, we must not forget the effect. There are fonts that not only have an identical cone size (we draw the fonts in 12 point), but also an identical measurable letter size (we measure an identical size of the capital letters with the typometer). And yet they appear different sizes and can be read differently, especially in small sizes – in other words, they make a different impression. Why is that?

A letter has many different forms; a “p”, for example, protrudes downwards into the basement, an “a” remains in the centre, a “b” reaches upwards, an “E” stays in the middle and at the top, an “O” protrudes a little further upwards. Capital letters with accents are the winners in the size competition. The following overview illustrates this.

Decisive: the centre length

In addition to the shapes, weights and thicknesses of the letters, it is above all the centre length that influences the size effect. The centre length, also known as x-height, is the height of lowercase letters such as “m”, “a” or “z”. This size is different in relation to the capitalisation height, i.e. the “M”, “A” or “Z”. The letters often have a size ratio according to the golden ratio – the factor is 1.6, meaning that the “M” is 1.6 times larger than the “m”.

However, these proportions are, as always and fortunately, only an average; many fonts use other proportions that also work. And even if you have to be careful with general statements on the subject of font size, it can be said that fonts with large centre lengths appear larger than fonts where the centre lengths are relatively small.

Measure the font size

What do you do if you have a template and need to determine the font sizes used there, for example because a flyer is to be laid out according to an old template?

There are various approaches here, but unfortunately no binding standards. Printed fonts can be measured using a typometer, i.e. a ruler for the typographic measuring system. But here, too, there are different measurement variants, different typometers and ultimately different point sizes.

Measuring font size with the hp height

It is common to measure the hp height, i.e. the total height from the upper edge of an “h” or “b” or “d” or “l” to the lower edge of a “p” or “q”. Most typometers have printed rectangles for this purpose; the motivated designer selects the rectangle in which the characters “h” and “p” fit exactly and has thus read out the font size. If it weren’t for the different cone utilisation described above …

The cap height

Alternatively, some designers work by entering the capitalisation height. This simply measures the size of a capital letter without rounding, such as an “H” or “M” – but these are also difficult to measure, especially in handwritten and script fonts. This size is of course not the font size that programmes such as InDesign expect under font size, as they want the cone size. This means that either experience is required or you know where to look in order to get from the cap height to the font or cone size.

Which font size is suitable for what?

Due to the situation just explained, it is hardly possible to give serious recommendations on font size selection. A good reading size for longer texts such as magazines and books is usually between 8 and 12 points; if you want to narrow it down further, you need to know the font used in order to make a reliable statement.

The same applies to business cards or letterheads, where sizes between 8 and 12 points are also possible. Captions, copyright information, footnotes and similar additional notes can also be legible in 7 point. However, this is highly dependent on the font used and other details such as line length, the amount of text or the background. Fonts in 6 point usually require a good pair of glasses on the nose and are often used as a deterrent for the “small print” in the contract.

Conclusion: We know that we know nothing

There is no universal basis for cone utilisation or the font size that ultimately results. Font sizes are relative, and the trained eye and experience are the two helpers that, in addition to a few basic values, are most effective in choosing the right size.