Geometry
Using the Diameter Instead of the Radius in a Circle Formula
Plugging the diameter into a circle formula that calls for the radius.
Quick check
Why students make it
Problems often give the diameter and it is the most visible measurement, so it gets used directly without halving.
The rule done right
The radius is half the diameter. Convert first with , then substitute into the formula.
Related resources
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Circles in the Coordinate Plane Infographic -
Equation of a Circle Infographic -
Circle Parts & Vocabulary Infographic -
Math: Arc Length and Sector Area Infographic -
Geometry: Tangent Line Properties Infographic - Circle circumference and area formulas Memory Aid Cheat Sheet
- Circles Cheat Sheet
- Circle Properties & Arc Measures Cheat Sheet
- Circle Theorems Cheat Sheet
- Coordinate Geometry Formulas Card Cheat Sheet
- Circle Circumference and Area Formulas Practice Worksheet
- Math: Circles: Radius, Diameter, Circumference Worksheet
- Geometry: Circles (Middle School) Worksheet
- Circle Theorems and Arc Length Worksheet
- Area & Perimeter Explorer Tool
- Coordinate Geometry Calculator Tool
- Circle Theorems Explorer Tool
- Conic Sections Explorer Tool
- Geometry Proof Helper Tool
- Circle Theorems & Tangency Lab Lab
- Coordinate Geometry & Conics Lab Lab
- Shape Classifier Lab Lab