Table of Contents

Font Metrics

FontDescription tells you what a face is called. FontMetrics tells you how that face behaves.

Once you know what a font is, the next question is usually how it behaves. FontMetrics is where you inspect the measurements and coverage data that explain line spacing, decoration placement, variation support, and glyph availability.

How to get FontMetrics

The most direct route is through a resolved Font instance:

using System;
using SixLabors.Fonts;

FontCollection collection = new();
FontFamily family = collection.Add("fonts/SourceSans3-Regular.ttf");
Font font = family.CreateFont(16);

FontMetrics metrics = font.FontMetrics;

You can also inspect available faces on a family before you create a Font.

using System;
using SixLabors.Fonts;

FontCollection collection = new();
FontFamily family = collection.Add("fonts/SourceSans3-Regular.ttf");

if (family.TryGetMetrics(FontStyle.Regular, out FontMetrics? metrics))
{
    Console.WriteLine(metrics.Description.FontNameInvariantCulture);
}

Description, units, and scale

The core identity and scaling properties are:

  • Description for the face metadata
  • UnitsPerEm for the design-space resolution of the font
  • ScaleFactor for the face-level unit-to-point scaling used by glyph metrics

UnitsPerEm is the important anchor for understanding almost every other metric on the typeface. Values like ascenders, underline positions, or glyph advances are stored in font units and should be interpreted relative to that em square.

Horizontal and vertical metrics

FontMetrics exposes both HorizontalMetrics and VerticalMetrics.

Both headers provide the same core fields:

  • Ascender
  • Descender
  • LineGap
  • LineHeight
  • AdvanceWidthMax
  • AdvanceHeightMax

The difference is not in the property names. It is in which layout direction those values are meant to describe.

  • HorizontalMetrics describes the face when text is laid out in horizontal modes such as LayoutMode.HorizontalTopBottom and LayoutMode.HorizontalBottomTop.
  • VerticalMetrics describes the face when text is laid out in vertical modes such as LayoutMode.VerticalLeftRight, LayoutMode.VerticalRightLeft, LayoutMode.VerticalMixedLeftRight, and LayoutMode.VerticalMixedRightLeft.

In practical terms:

  • use HorizontalMetrics for normal Latin-style line layout, UI text, paragraphs, and most measurement scenarios
  • use VerticalMetrics for vertical text layout, especially CJK-oriented column flow and vertical glyph advance

What the fields mean

Ascender and Descender define the font's recommended extents above and below the baseline for the layout direction you are inspecting.

LineGap is the additional space the font recommends between lines or columns beyond the ascender and descender space.

LineHeight is the face's typographic line spacing for that metrics header. If you want the font's default line advance, this is usually the most direct value to start from.

AdvanceWidthMax is the maximum glyph advance width in that face.

AdvanceHeightMax is the maximum glyph advance height in that face. This matters most for vertical layout. For fonts that do not provide dedicated vertical metrics, this value falls back to the line height.

When to use HorizontalMetrics

Reach for HorizontalMetrics when you need:

  • default line spacing for ordinary left-to-right or right-to-left text
  • baseline, ascender, and descender values for UI layout or custom renderers
  • a face-level sanity check before measuring or clipping horizontal text
  • maximum advance budgeting for horizontally flowing glyphs
using SixLabors.Fonts;

Font font = SystemFonts.CreateFont("Segoe UI", 16);
FontMetrics metrics = font.FontMetrics;

short ascender = metrics.HorizontalMetrics.Ascender;
short descender = metrics.HorizontalMetrics.Descender;
short lineHeight = metrics.HorizontalMetrics.LineHeight;
short maxAdvanceWidth = metrics.HorizontalMetrics.AdvanceWidthMax;

When to use VerticalMetrics

Reach for VerticalMetrics when you need:

  • default line or column spacing for vertical layout
  • face-level values for custom vertical renderers
  • the maximum advance height budget for vertical glyph flow
  • inspection of whether a font behaves sensibly in vertical layout
using SixLabors.Fonts;

Font font = SystemFonts.CreateFont("Segoe UI", 16);
FontMetrics metrics = font.FontMetrics;

short verticalAscender = metrics.VerticalMetrics.Ascender;
short verticalLineHeight = metrics.VerticalMetrics.LineHeight;
short maxAdvanceHeight = metrics.VerticalMetrics.AdvanceHeightMax;

These values are expressed in font units, not pixels.

Decoration and script-positioning metrics

FontMetrics also exposes the face-level metrics that support decoration and typographic adjustments:

  • UnderlinePosition
  • UnderlineThickness
  • StrikeoutPosition
  • StrikeoutSize
  • SubscriptXSize
  • SubscriptYSize
  • SubscriptXOffset
  • SubscriptYOffset
  • SuperscriptXSize
  • SuperscriptYSize
  • SuperscriptXOffset
  • SuperscriptYOffset
  • ItalicAngle

These are useful when you are building your own renderer, diagnostics, or typography tools and want the font's own recommendations rather than hard-coded values.

Variable-font support

FontMetrics.TryGetVariationAxes(...) lets you inspect the variation axes that the resolved face supports.

using System;
using SixLabors.Fonts;
using SixLabors.Fonts.Tables.AdvancedTypographic.Variations;

FontCollection collection = new();
FontFamily family = collection.Add("fonts/RobotoFlex.ttf");
Font font = family.CreateFont(16);

if (font.FontMetrics.TryGetVariationAxes(out ReadOnlyMemory<VariationAxis> axes))
{
    foreach (VariationAxis axis in axes.Span)
    {
        Console.WriteLine($"{axis.Tag}: {axis.Min}..{axis.Max} (default {axis.Default})");
    }
}

Each VariationAxis gives you:

  • Name
  • Tag
  • Min
  • Max
  • Default

The registered tags in KnownVariationAxes such as wght, wdth, opsz, slnt, and ital are useful when you want to relate those exposed axes back to font creation with FontVariation.

Code-point coverage

Use GetAvailableCodePoints() when you need to know which Unicode scalar values the face can map directly.

using System;
using SixLabors.Fonts;
using SixLabors.Fonts.Unicode;

Font font = SystemFonts.CreateFont("Segoe UI", 16);
ReadOnlyMemory<CodePoint> codePoints = font.FontMetrics.GetAvailableCodePoints();

This is useful for diagnostics, glyph coverage tooling, fallback decisions, and script-support inspection.

Inspect glyph metrics directly

If you need glyph-level inspection without going through full text layout, use TryGetGlyphMetrics(...).

using SixLabors.Fonts;
using SixLabors.Fonts.Unicode;

Font font = SystemFonts.CreateFont("Segoe UI", 16);

if (font.FontMetrics.TryGetGlyphMetrics(
        new CodePoint('A'),
        TextAttributes.None,
        TextDecorations.None,
        LayoutMode.HorizontalTopBottom,
        ColorFontSupport.None,
        out FontGlyphMetrics? glyphMetrics))
{
    float width = glyphMetrics.Width;
    ushort advance = glyphMetrics.AdvanceWidth;
    GlyphType glyphType = glyphMetrics.GlyphType;
}

This is the lower-level face inspection API behind the higher-level Font.TryGetGlyphs(...) helpers.

When to use FontMetrics vs FontDescription

Use FontDescription when you care about names and face identity.

Use FontMetrics when you care about:

  • line and em metrics
  • underline and strikeout placement
  • subscript and superscript recommendations
  • variation-axis availability
  • code-point coverage
  • direct glyph inspection

For face names and other descriptive metadata, see Font Metadata and Inspection. For variable-font usage, see Variable Fonts.

Practical guidance

  • Use font metrics when layout, decoration, glyph coverage, or variation axes matter.
  • Use font descriptions when the question is identity, naming, style, or version metadata.
  • Treat glyph availability as a layout input, not as a guarantee of final script quality.
  • Cache metrics-derived decisions with the font face and variation values that produced them.
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