Circumference to Diameter Calculator
This circumference to diameter calculator turns any measurement around a circle into the distance straight across it. Enter inches, feet, cm or mm to get the diameter, radius, area and the nearest 1/16 inch.
Updated September 2026
Circumference to Diameter Chart (Inches)
This circumference to diameter chart uses diameter = circumference ÷ π, with every result rounded to 2 decimals and to the nearest 1/16 inch so you can read it straight off a tape measure.
The inch sizes people look up most
Values between the rows work the same way. For shop work that stays in imperial units, open the dedicated circumference to diameter in inches tool.
| Circumference (in) | Diameter (in) | Nearest 1/16 in | Radius (in) | Diameter (cm) |
|---|---|---|---|---|
| 1 | 0.32 | 5/16 | 0.16 | 0.81 |
| 2 | 0.64 | 5/8 | 0.32 | 1.62 |
| 3 | 0.95 | 15/16 | 0.48 | 2.43 |
| 4 | 1.27 | 1 1/4 | 0.64 | 3.23 |
| 4.5 | 1.43 | 1 7/16 | 0.72 | 3.64 |
| 4.7 | 1.50 | 1 1/2 | 0.75 | 3.80 |
| 4.8 | 1.53 | 1 1/2 | 0.76 | 3.88 |
| 5 | 1.59 | 1 9/16 | 0.80 | 4.04 |
| 5.2 | 1.66 | 1 5/8 | 0.83 | 4.20 |
| 5.3 | 1.69 | 1 11/16 | 0.84 | 4.29 |
| 5.75 | 1.83 | 1 13/16 | 0.92 | 4.65 |
| 5.9 | 1.88 | 1 7/8 | 0.94 | 4.77 |
| 6 | 1.91 | 1 15/16 | 0.95 | 4.85 |
| 6.2 | 1.97 | 2 | 0.99 | 5.01 |
| 6.3 | 2.01 | 2 | 1.00 | 5.09 |
| 6.5 | 2.07 | 2 1/16 | 1.03 | 5.26 |
| 6.7 | 2.13 | 2 1/8 | 1.07 | 5.42 |
| 7 | 2.23 | 2 1/4 | 1.11 | 5.66 |
| 7.4 | 2.36 | 2 3/8 | 1.18 | 5.98 |
| 8 | 2.55 | 2 9/16 | 1.27 | 6.47 |
| 9 | 2.86 | 2 7/8 | 1.43 | 7.28 |
| 10 | 3.18 | 3 3/16 | 1.59 | 8.09 |
| 10.25 | 3.26 | 3 1/4 | 1.63 | 8.29 |
| 11 | 3.50 | 3 1/2 | 1.75 | 8.89 |
| 12 | 3.82 | 3 13/16 | 1.91 | 9.70 |
| 13 | 4.14 | 4 1/8 | 2.07 | 10.51 |
| 14 | 4.46 | 4 7/16 | 2.23 | 11.32 |
| 14.5 | 4.62 | 4 5/8 | 2.31 | 11.72 |
| 15 | 4.77 | 4 3/4 | 2.39 | 12.13 |
| 16 | 5.09 | 5 1/16 | 2.55 | 12.94 |
| 17 | 5.41 | 5 7/16 | 2.71 | 13.74 |
| 18 | 5.73 | 5 3/4 | 2.86 | 14.55 |
| 19 | 6.05 | 6 1/16 | 3.02 | 15.36 |
| 20 | 6.37 | 6 3/8 | 3.18 | 16.17 |
| 21 | 6.68 | 6 11/16 | 3.34 | 16.98 |
| 22 | 7.00 | 7 | 3.50 | 17.79 |
| 23 | 7.32 | 7 5/16 | 3.66 | 18.60 |
| 24 | 7.64 | 7 5/8 | 3.82 | 19.40 |
| 25 | 7.96 | 7 15/16 | 3.98 | 20.21 |
| 26 | 8.28 | 8 1/4 | 4.14 | 21.02 |
| 27 | 8.59 | 8 5/8 | 4.30 | 21.83 |
| 28 | 8.91 | 8 15/16 | 4.46 | 22.64 |
| 29 | 9.23 | 9 1/4 | 4.62 | 23.45 |
| 30 | 9.55 | 9 9/16 | 4.77 | 24.26 |
Diameter = circumference ÷ π • Radius = diameter ÷ 2 • 1 in = 2.54 cm • Rows marked in yellow are the most searched sizes
Which column to use
Use the decimal column when the number feeds another calculation, such as an area, a bore size or a CNC setting. Decimals keep the precision you started with.
Use the 1/16 inch column when you are about to mark or cut something, because that is what a standard tape measure and most rules are graduated in. A diameter of 2.23 in reads as 2 1/4 in on the tape, which is the mark you can actually hit.
The centimeter column is there for parts and fittings that are specified in metric even though you measured in inches. Nothing is converted twice, so the rounding stays predictable.
How to Find Diameter from Circumference
Measure around the object, divide that number by π, and the answer is the diameter in the same unit you started with. Three steps, no unit conversion.
Step by step
- 1
Measure the circumference
Run a string or a flexible tape all the way around. Pull it snug, keep it flat, and measure at the widest point.
- 2
Divide by π (3.14159)
Use the formula d = C ÷ π. The unit never changes: inches in, inches out.
- 3
Round to something you can use
Two decimals is enough for most jobs. For a tape measure, round to the nearest 1/16 inch instead.
Worked example in inches
A 7 inch circumference
7 ÷ 3.14159 = 2.23 in
about 2 1/4 in on a tape measure
The radius is half of that, 1.11 in. Swap 7 for your own measurement and the steps are identical.
By hand
Dividing by 3.14 instead of the full value of π is close enough for practical work: 7 ÷ 3.14 = 2.23 in, the same answer to 2 decimals. The gap only shows up in the third decimal place.
Going the other way
C = π × d
Multiply instead of divide with the diameter to circumference calculator.
What throws the measurement off
The division is exact, so any error in your answer came from the tape. Three things cause most of it.
A twisted tape
If the tape rolls onto its edge anywhere along the loop, it travels further than the true circumference and the diameter comes out too large. Run a finger around the loop before reading it.
Measuring on a slant
The tape has to sit in a plane square to the axis of the object. On a pipe, a loop that spirals even slightly reads long.
Squeezing soft material
Rope, insulation and foam compress under tension. Pull the tape snug enough to remove slack, then stop.
A useful check: divide by π, then multiply the answer back by π. If it does not return your original measurement, the arithmetic slipped rather than the tape.
Circumference to Width and Girth to Diameter
On a round object the width straight across is the diameter, so converting circumference to width is the same division: width = circumference ÷ π.
Girth is the everyday and trade word for the distance around something. A girth to diameter conversion is therefore the same calculation under a different name, whether the tape went around a rope, a post or a wrist.
The one thing to watch is shape. The formula assumes a true circle.
Pipe
A pipe that measures 10 in around is 3.18 in wide, close to 3 3/16 in. That is the number to match against a fitting size.
Tree trunk or post
A trunk with a 30 in girth is 9.55 in across, near 9 9/16 in. Foresters record this as diameter, not girth.
Wrist or bangle
A 6.5 in wrist measurement gives 2.07 in, about 2 1/16 in, which is the inside width a bangle needs to clear.
For an oval or flattened object, this gives an average width rather than an exact one, so measure the narrow and wide axes directly when the fit is tight.
That distinction matters more often than it sounds. A wrist is closer to an ellipse than a circle, a length of used pipe can be dented, and a tree rarely grows perfectly round. In each case the division still gives you a good working number, just treat it as the average width and leave clearance rather than machining to it.
Circumference to Diameter in Feet
Divide a circumference in feet by π and the diameter comes back in feet, which is the quickest way to size a tank, a pool or a fire pit from a tape run around the outside.
| Circumference | Diameter (ft) | Diameter (ft + in) |
|---|---|---|
| 8 ft | 2.55 ft | 2 ft 6.6 in |
| 10 ft | 3.18 ft | 3 ft 2.2 in |
| 16 ft | 5.09 ft | 5 ft 1.1 in |
| 20 ft | 6.37 ft | 6 ft 4.4 in |
Diameter = circumference ÷ π • Multiply the leftover decimal by 12 for the inches column
Turning the decimal into inches
A diameter of 5.09 ft is not 5 ft 9 in. The digits after the point are a fraction of a foot, so multiply them by 12 to get inches: 0.09 × 12 = 1.1, which makes 5 ft 1.1 in.
The same step handles the rest of the table. A 20 ft circumference gives 6.37 ft, and 0.37 × 12 = 4.4, so the diameter is 6 ft 4.4 in. An 8 ft circumference gives 2.55 ft, which is 2 ft 6.6 in.
If you would rather skip the second step, measure in inches from the start and read the answer off the inch chart above. Working out a diameter from circumference in a single unit is always less error-prone than converting afterwards.
Set the unit selector to ft in the circumference to diameter calculator above for any other value, or leave it on inches if your tape never left inches.
Circumference to Diameter in CM and MM
Metric works the same way, since dividing by π never changes the unit: d = C ÷ π.
Millimeters follow the same rule: a 25 mm circumference gives a 7.96 mm diameter, and 65 mm gives 20.69 mm. Use the circumference to diameter in MM tool for engineering-grade precision.
For centimeter work to 4 decimals, open the circumference to diameter in CM page, or let the metric calculator switch between mm, cm and m for you.
Circumference vs Diameter vs Radius
Three numbers describe every circle: the distance around it, the distance across it, and half of that distance across. Any one of them gives you the other two.
Drag to resize the circle
Drag the slider and watch C and D climb at a fixed rate. Double the diameter and the circumference doubles too, which is exactly why a single constant covers circles of any size.
Why π shows up every time
Take any circle, divide the distance around it by the distance across it, and the answer lands on 3.14159. A bottle cap and a running track both return the same figure. That fixed result is π, and it is the only reason one formula works at every scale.
Because the ratio never moves, going backwards is just as reliable: d = C ÷ π. Read more in our guide to the circumference to diameter of a circle.
The mistake to avoid
The diameter is not half the circumference. It is roughly 1/3.14 of it, a little under a third. A 20 in circumference gives a 6.37 in diameter, not 10 in.
Halving belongs one step later: the radius is half the diameter. Get the radius on its own with the circumference to radius calculator, reverse it with radius to circumference, or halve a known diameter with the diameter to radius calculator.
| Property | Circumference | Diameter |
|---|---|---|
| What it measures | Distance around the circle | Distance across the circle |
| Type of line | Curved | Straight |
| Passes through center? | No, runs along the edge | Yes, always |
| Formula | C = π × d | d = C ÷ π |
| How they relate | The circumference is always π (≈ 3.14159) times the diameter | |
Quick Conversions
The area follows from the radius, so one measurement around an object is enough to get all four. See how it plays out in our circle area calculator guide.