Roboto

Roboto has been refined extensively to work across the wider set of supported platforms. It is slightly wider and rounder, giving it greater clarity and making it more optimistic.

Roboto font weights

Roboto has six weights: Thin, Light, Regular, Medium, Bold, and Black.

Noto

Noto’s vertical metrics are compatible with Roboto.

Noto font weights

Noto Sans CJK (Chinese, Japanese, and Korean) has seven weights: Thin, Light, DemiLight, Regular, Medium, Bold, and Black. The weight of Noto Sans CJK Regular is the same as Roboto Regular.

English and English-like scripts

Latin, Greek, and Cyrillic.

The basic set of styles are based on a typographic scale of 12, 14, 16, 20, and 34.

Dense scripts

Chinese, Japanese, and Korean.

Weight: Since Noto CJK has seven weights that match Roboto, use the same weight settings as English.

Font size: For Title through Caption styles, the font size is 1px larger than that specified for English. For styles larger than Title, the English type size is suitable.

Tall scripts

South and Southeast Asian and Middle Eastern languages, including Arabic, Hindi, and Thai.

Weight: Use Regular weight, as Medium weight is unavailable in Noto. Google recommends avoiding Bold weight, based on feedback from native speakers that Bold is too heavy.

Font size: For Title through Caption styles, font size is 1px larger than that specified for English. For styles larger than Title, the English type size is suitable.

English and English-like scripts

Dense and tall scripts

For all styles, line height is 0.1em larger than the English-like languages. English and English-like languages mostly use a portion of the em box, often the lower portion below the x-height. Chinese, Japanese, and Korean (CJK) ideographic characters use the entire em box. Characters in tall languages often have long descenders and/or ascenders. To achieve the same design intention as English for CJK and to avoid potential text clipping between two lines next to each other for tall languages, the line height needs to be larger than in English for tall and dense languages.

Colors & contrast

A text color that is too similar to the background color is hard to read. Text with too much contrast can also be hard to read. This is especially true of light-colored text against dark backgrounds.

Text should maintain a minimum contrast ratio of at least 4.5:1 (calculated based on luminance values) for legibility. A ratio of 7:1 is preferred.

These color combinations also consider contrast ratios for users who perceive color differently.

Tracking and kerning

Line length

Consider this advice on readability and line length from the Baymard Institute:

“You should have around 60 characters per line if you want a good reading experience. Having the right amount of characters on each line is key to the readability of your text.”

"Too wide – if a line of text is too long, the user’s eye will have a hard time focusing on the text. This is because the length makes it difficult to get an idea of where the line starts and ends. Furthermore it can be difficult to continue from the correct line in large blocks of text.”

"Too narrow – if a line is too short, the eye will have to travel back too often, breaking the reader’s rhythm. Too short lines also tend to stress people, making them begin on the next line before finishing the current one (hence skipping potentially important words).”

Source: “Readability: the Optimal Line Length,”

http://baymard.com/blog/line-length-readability