CIRCLE CALCULATOR
ENTER RADIUS OR DIAMETER
INPUTS
Use NEXT for the answer, then WORKING for the full steps.
CIRCLE ANSWER
RADIUS • DIAMETER • CIRCUMFERENCE • AREA
FULL WORKING
CONVERT • FORMULAS • ANSWERS
Use PREVIOUS for answer, or NEXT for formula help.
CIRCLE FORMULAS
RADIUS • DIAMETER
Diameter: d = 2r.
Radius: r = d ÷ 2.
The radius goes from the centre to the edge.
The diameter goes all the way across through the centre.
AREA + CIRCUMFERENCE
MAIN CIRCLE FORMULAS
Circumference: C = 2πr.
Circumference: C = πd.
Area: A = πr².
Use radius, not diameter, in the area formula.
WHAT THIS TOOL DOES
ONE VALUE GIVES FOUR VALUES
This circle calculator finds radius, diameter, circumference and area.
Enter either radius or diameter as the known measurement.
The calculator fills in the missing circle values automatically.
It is built for GCSE geometry and quick homework checks.
HOW TO USE
CHOOSE GIVEN • ENTER VALUE • SOLVE
Select whether the known value is radius or diameter.
Enter the value, then choose the rounding level.
Press SOLVE or Enter to calculate all circle values.
Use PREV and NEXT to read help without scrolling.
WORKED EXAMPLE
RADIUS GIVEN
If r = 4, then d = 2 × 4 = 8.
C = 2πr = 2π × 4 ≈ 25.13.
A = πr² = π × 4² ≈ 50.27.
Circumference uses length units; area uses square units.
WORKED EXAMPLE
DIAMETER GIVEN
If d = 10, then r = 10 ÷ 2 = 5.
C = πd = π × 10 ≈ 31.42.
A = πr² = π × 5² ≈ 78.54.
The calculator uses built-in π for consistent results.
UNITS
LENGTH UNITS AND SQUARE UNITS
Radius, diameter and circumference use ordinary length units.
If the input is in centimetres, circumference is in centimetres.
Area uses square units, such as cm², m² or mm².
Keep the same unit throughout each calculation.
COMMON SEARCHES
SEARCH INTENT COVERAGE
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WHO USES IT
STUDENTS • TEACHERS • PARENTS
Students use it for GCSE circle questions and homework checking.
Teachers use it to show formula substitution quickly.
Tutors use it for circumference and area revision.
Parents can use it to support maths homework with π calculations.
RELATED TOOLS
GEOMETRY AND SHAPE LINKS
Circle calculations connect with area, perimeter and surface area tools.
FAQ
CIRCLE QUESTIONS
What is circumference? The distance around a circle.
What is radius? Centre to edge.
What is diameter? Full width through the centre.
Why is area squared? Area measures flat space.
SEARCH TERMS
READABLE FINAL SEO PAGE
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