How Font Similarity Is Calculated
FontAlternatives uses a deterministic scoring system to identify free fonts that can serve as practical alternatives to premium typefaces. This page explains how similarity scores work and what they represent.
What "Similarity" Means
A similarity score measures how closely two font families align in structural and functional terms. Scores range from 0 to 100, where higher values indicate stronger structural compatibility.
Similarity scores do not claim visual identity or legal equivalence. They indicate whether a free font can serve as a functional substitute for a premium font in common use cases.
Classification Gate
Fonts are only compared when they share the same broad classification:
- Sans-serif fonts are compared only to other sans-serif fonts
- Serif fonts are compared only to other serif fonts
- Display fonts are compared only to other display fonts
- Monospace fonts are compared only to other monospace fonts
If classifications do not match, the similarity score is automatically zero. This prevents misleading comparisons between fundamentally different typographic systems.
Scoring Dimensions
The similarity algorithm evaluates five dimensions, each weighted according to its importance for practical substitution:
| Dimension | Weight | Values |
|---|---|---|
| X-height | 25% | small, medium, large |
| Width | 20% | condensed, normal, extended |
| Stroke contrast | 20% | low, medium, high |
| Intended use | 15% | ui, editorial, display, code |
| Language coverage | 20% | Overlap percentage of supported scripts |
X-height affects perceived size and readability at the same point size.
Width determines how much horizontal space text occupies.
Stroke contrast describes the variation between thick and thin strokes.
Intended use captures whether a font is optimized for user interfaces, long-form reading, display headlines, or code.
Language coverage measures how well script support overlaps between fonts.
How Scores Are Calculated
Each dimension contributes to the final score based on how closely the fonts match:
- Exact match (e.g., both "medium" x-height): 100 points for that dimension
- Adjacent match (e.g., "medium" vs "large"): 50 points
- Distant match (e.g., "small" vs "large"): 0 points
For intended use and language coverage, the score reflects the percentage of overlap between the two fonts. The final score is the weighted sum of all dimensions, rounded to the nearest integer.
Example Calculation
Comparing a premium sans-serif font (medium x-height, normal width, low contrast, ui+editorial use, Latin+Cyrillic support) to a free alternative (medium x-height, normal width, low contrast, ui+editorial use, Latin-only support):
X-height: 100 × 0.25 = 25
Width: 100 × 0.20 = 20
Contrast: 100 × 0.20 = 20
Intended use: 100 × 0.15 = 15
Language coverage: 50 × 0.20 = 10 (1 of 2 scripts)
Total: 90%
Confidence Levels
Scores are grouped into confidence levels:
| Score Range | Confidence | Interpretation |
|---|---|---|
| 75–100 | High | Strong structural match |
| 60–74 | Medium | Acceptable alternative with differences |
| Below 60 | Low | Limited compatibility |
| Below 50 | Not recommended | Insufficient structural similarity |
Alternatives below 50% are not displayed as recommendations.
What Is Not Considered
The scoring system intentionally excludes:
- Brand popularity or market adoption
- Visual trends or aesthetic preferences
- Foundry reputation
- Licensing cost or commercial value
- Personal opinion or editorial judgment
This ensures scores remain consistent, reproducible, and independent of subjective factors.
Content Tiers
FontAlternatives uses a tiered content system:
Tier 1
Pages cover high-priority fonts and include human-written analysis explaining why specific alternatives work well in practice. The underlying similarity scores remain system-generated.
Tier 2
Pages are programmatically generated and rely entirely on algorithmic scoring without additional human commentary.
In both cases, the numeric similarity score comes from the same deterministic algorithm.
Limitations
Similarity scores indicate structural compatibility, not guaranteed visual equivalence. Factors that affect real-world suitability include:
- Specific letterform details (terminals, apertures, curves)
- OpenType features and stylistic alternates
- Hinting quality at small sizes
- Licensing terms for specific use cases
Designers should always test fonts in real layouts and at target sizes before final selection.
Data Sources and Updates
Font metrics are maintained in a structured database. Premium font measurements are derived from foundry specifications and type analysis. Free font measurements are based on published font files.
Metrics and similarity calculations are periodically reviewed. Corrections can be submitted for consideration.
This methodology applies to all similarity scores displayed on FontAlternatives.