Typography Legibility Grader
Grade font size, line height and line length against legibility best practice.
Comfortable to read, size, rhythm and measure all sit in the sweet spot. Long-form copy will hold attention, which is exactly what you need on pages that sell with words.
Guidance for body copy, not display headings. On mobile, favour 16px+ and shorter measures; the line-length ideal is about the reading column, not the viewport.
About this calculator
Long-form copy that sells, product pages, blog posts, sales letters, lives or dies on whether people can actually read it without fatigue. This grader scores three typography fundamentals against the ranges typography research consistently points to as ideal: font size, line height (leading), and line length (measure), and tells you which one is dragging your legibility down.
How to use it
- Enter your body font size in pixels.
- Enter your line height as a multiplier of font size (1.5 means 1.5× the font size).
- Enter your line length in characters per line, roughly how wide your text column reads.
- Read the overall legibility score plus each factor's individual match against its ideal range, and the single weakest factor to fix first.
Methodology
Each input is scored against an ideal band using a band-scoring function: full marks (100%) if the value falls inside the band, decaying linearly toward zero as it moves outside, at a rate set by a tolerance value. Font size targets 16-20px (tolerance 8), line height targets 1.4-1.7 (tolerance 0.5), and line length targets 50-75 characters (tolerance 30).
The three band scores are weighted 35% font size, 30% line height, 35% line length, and summed into a 0-100 legibility score. The lowest-scoring factor is surfaced as the specific fix to prioritise, with directional guidance (e.g., "cap line length near 65-75 characters" if you're over, "widen the measure" if under).
These bands describe desktop body copy conventions specifically. Mobile reading generally favours the same or larger font sizes with a shorter measure, since the viewport itself constrains line length; the tool's guidance is a starting point, not a strict cross-device rule.
FAQ
Below 16px, body text starts requiring visible effort to read on typical screens, particularly on mobile, and it's also the size below which browsers may auto-zoom on input focus. 16-20px is the range most legibility research and style guides converge on for comfortable long-form reading.
Lines that run too long (over ~75 characters) force the eye to work harder to track back to the start of the next line, increasing reading fatigue even if the font size and spacing are otherwise fine. Lines that are too short cause the eye to jump lines too frequently, which has the opposite but related effect.
No, the bands here are calibrated for body copy meant to be read continuously. Headings and short display text follow different legibility rules since they're typically scanned rather than read line by line.