Figure

Shapes and geometry

Circle measurements

Relate diameter, radius, circumference, and area, including multiplication by pi.

Diameter crosses the whole circle through its center; radius reaches from the center to the edge. Circumference is the distance around it, and area is the surface enclosed. Use the measurement at the surface relevant to the job.

If d is diameter and r is radius, r = d ÷ 2, circumference = π × d, and area = π × r². Circumference has length units; area has square units. For a pipe wrapper, use the diameter at the surface whose distance around you need.

A 12 in diameter gives a circumference of 12π ≈ 37.699 in. The pipe example displays this as 37 11/16 in at its selected fraction precision. That fraction is a rounded display, not an exact value of π.

Find the distance around a pipe

Find the distance around a round pipe or duct from its outside diameter.

Why pi?

The distance around a circle is about 3.14159 times its diameter. π supplies that number: diameter × π = circumference.

A round pipe has a 12 inch outside diameter measured through its center. The circumference is the distance around the outside edge.12 in.Around the outside
Schematic, not to scale.

Try it · a 12 in. diameter

Use 2nd if Inch is above its key.

  1. Start fresh

    Press Clear entry, then Clear entry, then Select length units.

  2. Outside diameter

    Press 12, then inches, then Multiply.

  3. Multiply by pi

    Press Second, then Pi, then Equals.
  1. Circumference · rounded to 1/16 in.37 and 11/16 inches

About 37.7 inches. Your fraction setting controls rounding.

Also useful for

Pipe insulation wraps, round duct, circular edging, tank bands and the outside distance around a post. Use the diameter at the surface you are measuring.

Remember

This is the distance around the pipe. Add any seam or overlap allowance separately.

From a diameter

Tap Diameter for the full width across the circle.1

Start a new calculation

Use 2nd if a unit is above its button.

  1. Start fresh

    Press Clear entry, then Clear entry.

  2. Press the keys

    Press Select length units.

Open Circle

Skip if this mode is already open. Skip Close if no mode is open.

  1. Press Close calculation mode, then Open Circle.

1. Find circumference

  1. Diameter

    Press 11, then inches, then Store diameter.

  2. Complete the value

    Press Equals.
  1. Example · Circumference34 and 9/16 inches

2. Read area

  1. Press Equals.

Results in orderPress = to continue

  1. Example · Circle area95.03318 square inches
  2. Radius5 and 1/2 inches
  3. Circumference again34 and 9/16 inches
  4. Circle area95.03318 square inches

Keep in mind

The displayed fractions are rounded; calculations retain the underlying value.

From a radius

Tap Radius for half the width, then = to read diameter.1

Start a new calculation

Use 2nd if a unit is above its button.

  1. Start fresh

    Press Clear entry, then Clear entry.

  2. Press the keys

    Press Select length units.

Open Circle

Skip if this mode is already open. Skip Close if no mode is open.

  1. Press Close calculation mode, then Open Circle.

1. Find diameter

  1. Radius

    Press 8, then feet, then 4, then inches, then Store radius.

  2. Complete the value

    Press Equals.
  1. Example · Diameter16 feet 8 inches

2. Read circumference

  1. Press Equals.
  1. Example · Circumference52 feet 4 and 5/16 inches

3. Read area

  1. Press Equals.

Results in orderPress = to continue

  1. Example · Circle area218.1662 square feet
  2. Diameter again16 feet 8 inches
  3. Circumference52 feet 4 and 5/16 inches
  4. Circle area218.1662 square feet

Keep in mind

The displayed fractions are rounded; calculations retain the underlying value.

Use pi in arithmetic

Multiply diameter by π to find circumference.2

Start a new calculation

Use 2nd if a unit is above its button.

  1. Start fresh

    Press Clear entry, then Clear entry.

  2. Press the keys

    Press Select length units.

Calculate

  1. Diameter

    Press 2, then feet, then Multiply.

  2. Enter pi

    Press Second, then Pi.

  3. Complete the value

    Press Equals.
  1. Example result6 feet 3 and 3/8 inches

Display assumptions

  1. Fraction precision: 1/16″; Fraction format: Simplified; Area units: Automatic. ↩1↩2

  2. Fraction precision: 1/16″; Fraction format: Simplified. ↩

Keep the guide beside your calculation.

These examples are built into Figure. Open Help while you calculate, then return to the same guide and reading position.

See Help in Figure →