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Radius / Diameter Constraints

In one sentence: Fix the size of a circle or arc by specifying its radius or diameter, in either driving or reference mode.

Overview

Workbench: Sketcher · Menu: Sketch → Sketcher constraints → Radius / Diameter / Radius/Diameter
Toolbar: Sketcher Constraints · Shortcut: K, R (Radius) · K, O (Diameter) · R (auto Radiam)

Tool Shortcut Description
Radius K, R Set the radius of a circle or arc
Diameter K, O Set the diameter of a circle (shown as Ø)
Radius/Diameter (Radiam) R Automatically applies Radius to arcs and Diameter to full circles

The Radiam tool (R) is the recommended default: it follows the drafting convention that arcs are dimensioned by radius and full circles by diameter.

Intuition

A circle has one degree of freedom for its size — you can shrink or grow it freely until you specify its radius or diameter. These three constraints pin that single DoF. Radius and Diameter are two ways of expressing the same measurement: Ø = 2R.

The Radiam tool removes the decision: for a full circle it shows a diameter (Ø) label; for an arc it shows a radius (R) label. This matches standard engineering drawing practice.

When to use them

  • Any time you draw a circle or arc and need to fix its size.
  • Constraining the corner arcs of a Rounded Rectangle to a specific radius.
  • Setting the slot end-cap radius.
  • Ensuring all holes in a pattern have the same diameter (apply Radiam to one, then Equal to the others).

When NOT to use them

  • You need to constrain a B-Spline weight circle — Radiam/Radius are not applicable to B-Spline knot circles; constrain the control point positions instead.
  • You want the circle's position — Radius/Diameter only constrains size; use Lock Position or Coincident for position.

Step-by-step walkthrough

  1. Select a circle or arc.
  2. Press R (Radiam) or K, R (Radius) or K, O (Diameter).
  3. Click to place the dimension label (drag it away from the geometry for readability).
  4. Enter the value in the dialog and click OK.

Tip

After typing a value, press Enter to confirm without clicking OK. This is faster for batch constraining multiple circles.

Parameters

Parameter Type Default Description
Value Distance (mm) Current radius The radius (or diameter) to set. Must be positive.
Mode Driving / Reference Driving See Dimension → Driving vs Reference.

Radius, Diameter, and Radiam in depth

All three tools write the same underlying constraint type (Radius in the solver) but differ in how the value is interpreted and displayed:

Radius (K, R)

Displays an R label. The value entered is the radius directly. Applied to both circles and arcs.

Use Radius when: - You are dimensioning an arc (standard drafting practice). - The design spec provides a radius value.

Diameter (K, O)

Displays an Ø label. The value entered is the diameter (2 × radius). Applied to circles; also works on arcs but is unusual.

Use Diameter when: - The design spec provides a diameter (common for holes, shafts, bosses). - You need to match a nominal size from a standard (M6 bolt = Ø6 mm).

Radius/Diameter — Radiam (R)

The smart default: shows R for arcs, Ø for full circles.

Use Radiam: - Always, unless you need to override the convention (e.g. you want a diameter label on an arc for some reason).

Watch out for: When cycling constraint types with the unified Dimension tool (D + M), Radiam may flip between R and Ø display; check the label before confirming to avoid entering the wrong scale.

Common mistakes and pitfalls

Constraint applied but circle size does not change

Cause: The circle was already constrained to a different radius by an Equal constraint to another geometry. The new constraint conflicts.
Fix: Remove the Equal constraint or remove the old Radius constraint; keep only one that directly controls the size.

Diameter value entered when radius was expected

Cause: You applied Radius (K, R) but entered the diameter value, resulting in a circle twice the intended size.
Fix: Delete the constraint, reapply with the correct value, or switch to Diameter (K, O) and re-enter. Always check the label: R 10 = radius 10 mm (diameter 20 mm); Ø 10 = diameter 10 mm (radius 5 mm).

Python API

Minimal example

import FreeCAD as App
import Sketcher
import Part

doc = App.activeDocument()
sketch = doc.getObject("Sketch")

circle = sketch.addGeometry(
    Part.Circle(App.Vector(0, 0, 0), App.Vector(0, 0, 1), 1), False)

# Set radius to 8 mm
sketch.addConstraint(Sketcher.Constraint("Radius", circle, 8.0))
doc.recompute()

Method signature

sketch.addConstraint(Sketcher.Constraint("Radius", geoId, value))
# 'value' is always the radius regardless of whether you display as Ø.
# To display as diameter in the UI, the constraint display mode is a UI-only setting.

See also