Streamline Spacing with FontForge Metrics Wizard

Proper character spacing and kerning define the readability and professional finish of a typeface, but setting side bearings manually for hundreds of glyphs is time-consuming. FontForge provides automated metrics tools—often referred to collectively as the Metrics Wizard (encompassing the Auto Width and Auto Kern features)—to calculate optical spacing programmatically. This guide walks you through preparing your glyphs, configuring automated width and kerning parameters, and fine-tuning the results to achieve balanced typography in a fraction of the time.

1. Prepare Your Glyphs

Before running any automated spacing routines, ensure all glyph contours are finalized, correctly oriented, and free of overlap errors:

  • Select all glyphs (Ctrl + A or Cmd + A).
  • Run Element > Correct Direction to verify path winding.
  • Run Element > Overlap > Remove Overlap to avoid erroneous bounding box calculations.

2. Configure Auto Width (Side Bearings)

The Auto Width utility automatically calculates the Left Side Bearing (LSB) and Right Side Bearing (RSB) by analyzing the optical mass and edges of each glyph.

  1. In the Font View window, select the glyphs you want to space (it is best to start with standard Latin lowercase letters a-z, then uppercase A-Z).
  2. Navigate to Metrics > Auto Width... from the top menu.
  3. In the dialog box, specify the target spacing parameters:
    • Desired Spacing / Guess: Enter the desired optical distance between characters (measured in font units, commonly scaled relative to your UPM, such as 100 to 200 units on a 1000-UPM font).
    • Minimum Bearing: Set a lower limit so glyphs with steep curves do not collapse into adjacent letters.
  4. Click OK. FontForge will compute the optical margins and adjust the LSB and RSB for each selected glyph.

3. Generate Kerning Pairs with Auto Kern

Once side bearings are set, use the Auto Kern tool to catch problematic pairs with extreme overhangs or diagonal shapes (such as AV, To, and Wa).

  1. Select the glyph set you want to test for pair collisions or gaps.
  2. Go to Metrics > Auto Kern....
  3. Choose the target kerning subtable or allow FontForge to generate a new GPOS lookup table.
  4. Set your kerning parameters:
    • Desired Optical Distance: This should match or closely approximate the visual spacing established during the Auto Width step.
    • Threshold / Minimum Kern: Define the minimum adjustment value required before a kerning pair is created (e.g., 5 or 10 units) to prevent bloat in your final font file.
    • Maximum Kern Value: Set a ceiling to avoid overly aggressive negative adjustments.
  5. Click OK to run the calculation.

4. Review and Refine in the Metrics Window

Automated metrics provide an estimated baseline, but human optical verification is required for high-quality results.

  1. Open the Metrics Window by selecting Metrics > Open Metrics Window (or pressing Ctrl + K / Cmd + K).
  2. Type standard spacing strings to inspect rhythm and harmony:
    • For lowercase: nnanbnooppnqqnrn or hhahbhchoohphqh
    • For uppercase: HHAMBORGEVONS
  3. Click and drag between characters in the preview area to adjust kerning values interactively, or edit the numeric LSB, RSB, and Kern fields at the top of the window.