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About this tool
Tune optical letter-spacing per font size, weight and case, with a live preview and copyable em, px and 1/1000 em values.
The Letter Spacing Tuner calculates the letter-spacing value a given font size, weight and case should use, expressed in em, pixels and the 1/1000 em unit design apps expect. It applies the optical tracking curve tracking = A + B·e^(C·size) — the exponential fit that makes small text open up and display text tighten — then adds documented corrections for heavy weights, all-caps settings and light-on-dark text. Designers and front-end developers get a defensible starting value plus a live preview instead of guessing at round numbers.
Open Letter Spacing Tuner on AltFTool — it loads instantly in your browser.
Set Font size (px), Font weight from 100 to 900, Text case (Sentence / title case, ALL CAPS or Small caps) and Contrast (Dark text on light or Light text on dark).
Add a Manual nudge (1/1000 em) if you want to override the curve, and type your own Preview text with Preview family set to Sans-serif, Serif or Monospace.
Recommended letter-spacing shows in em with the /1000 em figure, and the rows break out the optical base, case, weight, reversed-text and nudge corrections plus the pixel value; Copy CSS takes the rule.
Tracking follows an exponential curve against font size rather than one fixed value.
Every result is shown in CSS em, rendered pixels and Photoshop's 1/1000 em tracking unit.
The base curve and each correction are listed separately so you can justify the number.
Large headings usually need negative tracking, converging on about -0.022em once the size passes roughly 40px. The optical curve reaches that floor because letterforms at display size already look far apart, so tightening restores the word shape.
Around +0.06em, or 60/1000 em, is the standard editorial allowance for all-caps text. Capitals have no ascenders or descenders to interlock, so without extra tracking the letters read as a solid block; small caps need about half that.
Use em. Letter-spacing in em scales with the font size automatically, so a single value survives responsive size changes, whereas a px value that looks right at 48px will be far too wide at 16px.
Light glyphs on a dark ground bloom optically — an effect called halation — which visually thins the gaps between letters. Opening reversed text by roughly 0.005em restores the rhythm of the same setting printed dark-on-light.