Kerning Adjustment Calculator
Optical kerning spacing adjustment estimate.
About this calculator
This calculator estimates the optical kerning adjustment for a character pair based on a five-category shape model rather than reading real kerning-pair tables from a specific font. You classify the pair by its silhouette — straight-straight combinations like H-H need essentially no adjustment (a token -0.5% of the em), straight-round pairs like H-O tighten modestly (-3%), round-round pairs like O-O tighten more (-5%), open-diagonal pairs like T-o or V-A tighten further (-8%), and extreme pairs like T-a or A-V need the most aggressive tightening (-12%). That fraction is converted three ways: into font units based on your specified units-per-em (1000 for PostScript-style fonts, 2048 for many TrueType fonts), into pixels at your given font size, and into a CSS em value. The calculator also adds this kern adjustment to any letter-spacing (tracking) you've already applied, producing a single effective-spacing figure — useful for checking whether your tracking is fighting against or reinforcing the kerning a pair actually needs.
The tightness score is a simple linear scale centered on 50 (neutral) that drops as effective spacing becomes more negative. A warning flag appears above 36px, since kerning errors become far more visually obvious at display sizes than in body text. These are textbook approximations meant for quick sanity-checking, not a substitute for a font's actual GPOS kerning table or your own eye at the size you're designing for.
Inputs
Results
Kern value (UPM units)
-50
How to Use This Calculator
- Enter Font size (px), Shape category (1–5), and Tracking (em).
- Set Units per em (UPM).
- Review the Kern value (UPM units) result.
- Use Kern value (px) and Kern value (em) to inform your decision.
How the result changes with Units per em (UPM)
| Units per em (UPM) | Kern value (UPM units) |
|---|---|
| 500 | -25 |
| 750 | -37 |
| 1,500 | -75 |
| 2,500 | -125 |
What each input means
- Font size (px)
- Font size at which kerning will be applied.
- Shape category (1–5)
- Character pair shape: 1 = straight-straight (HH), 2 = straight-round (HO), 3 = round-round (OO), 4 = open-diagonal (To), 5 = extreme (AV, Ty).
- Tracking (em)
- CSS letter-spacing value in em. Positive = looser, negative = tighter.
- Units per em (UPM)
- Font's units-per-em. Most fonts use 1000 (PostScript) or 2048 (TrueType).
What each result means
- Kern value (UPM units)
- Kerning adjustment in font units. Negative = tighter.
- Kern value (px)
- Kerning adjustment in pixels at your font size.
- Kern value (em)
- Kerning adjustment as a fraction of 1em.
- Effective spacing (px)
- Combined tracking + kerning adjustment in pixels.
- Effective spacing (em)
- Combined tracking + kerning in em — use as CSS letter-spacing.
- Tightness score
- Visual tightness rating: 0 = very loose, 50 = neutral, 100 = very tight.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersFont size (px) = 16, Shape category (1–5) = 3, Tracking (em) = 0, Units per em (UPM) = 1000 = 4 input(s) provided
- Calculate Kern valueKern value-50 = -50
- Calculate Kern valueKern value = kernFraction * fontSizePx-0.8 = -0.8
- Calculate Kern value-0.05 = -0.05
Engine last updated . Checked against 3 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does the same shape category produce a bigger pixel adjustment at larger font sizes?
The kern fraction for each shape category (for example -5% of the em for round-round pairs like O-O) is a fixed percentage, and kern-in-pixels is that fraction multiplied directly by your font size — so a -5% kern is -0.8px at 16px but -3.6px at 72px. This mirrors how real kerning works: optical spacing problems that are imperceptible in body text become visually obvious at display sizes, which is also why the calculator flags a display-size warning at 36px and above.
How does my tracking (letter-spacing) interact with the kerning value?
The calculator simply adds your tracking value (in em) to the shape category's kern fraction to get effective spacing — so if you've already applied positive tracking to loosen a headline, a negative kern adjustment for a tight pair like T-a partially cancels it out rather than stacking on top. Checking the effective-spacing output tells you whether your tracking is reinforcing or fighting the kerning a specific pair actually needs.
Why would I need to know the kern value in UPM units instead of just pixels or em?
Units-per-em (UPM) values are what font-editing tools like Glyphs or FontForge actually use when you're building or adjusting a font's real GPOS kerning table, so converting the estimated adjustment into UPM units (kern fraction times your font's UPM, typically 1000 or 2048) lets a type designer plug a starting value directly into that software. The pixel and em values are more useful for a web or UI designer applying CSS letter-spacing to a specific pair or word.
Why does the tightness score cap at 0 and 100 instead of scaling indefinitely?
The score is calculated as 50 minus effective spacing (in em) times 500, then clamped to the 0–100 range — so it's a bounded visual gauge rather than a raw physical measurement, meant to give a quick 'how tight does this look' read rather than an unbounded number that could otherwise go arbitrarily negative or high for extreme tracking values.
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