Diameter of a Cylinder Calculator
Enter any two known cylinder measurements to calculate the diameter instantly. Choose your solve mode: use volume and height, a known radius, or the outer circumference. All results update as you type, with full step-by-step working shown below. Switch between metric and imperial units at any time.
Formula
Worked example
A cylinder with volume 785.4 cm3 and height 10 cm: V / (pi x h) = 785.4 / 31.416 = 25.0, sqrt(25.0) = 5.0 cm (radius), diameter = 2 x 5 = 10 cm.
Three ways to find the diameter of a cylinder
You can solve for the diameter of a cylinder from three different starting points. If you know the volume and height, use d = 2 x sqrt(V / (pi x h)). This is the most common engineering scenario: you have a capacity requirement and a length constraint and need to know how wide the cylinder must be. If you already know the radius, the diameter is simply d = 2r. If you have measured the outer circumference with a tape, use d = C / pi. All three modes are available in the calculator above.
How the diameter relates to the rest of the cylinder geometry
The diameter is the starting point for all other cylinder measurements. The radius r = d / 2 feeds into every area and volume formula. Volume equals pi x r^2 x h. Lateral (side) surface area equals 2 x pi x r x h, which is simply the circumference of the base times the height - imagine unrolling the curved wall into a flat rectangle. Total surface area adds two circular bases: 2 x pi x r^2. Because r is squared in the volume and area formulas, a small change in diameter has a larger effect on capacity than the same change in height. A cylinder that is 10% wider holds about 21% more volume at the same height.
Practical uses for cylinder diameter calculations
Engineers and trades workers calculate cylinder diameter to size pipes, tanks, pressure vessels, columns, and round structural members. A common task is back-calculating the pipe bore needed to carry a given flow volume in a given length of run. Machinists need the diameter of a turned part when only a finished volume specification is given. HVAC technicians size round duct from airflow capacity. Packaging designers determine the diameter of a cylindrical container from a required fill volume and a shelf-height constraint. The circumference method is used in manufacturing and field work where wrapping a tape around an existing cylinder is easier than measuring across it directly.
Metric and imperial units
This calculator works in whatever units you enter. In metric mode the length fields are in centimetres and volume in cubic centimetres (cm3, equal to millilitres). In imperial mode lengths are in inches and volume in cubic inches. To convert cubic inches to US gallons divide by 231; to convert cubic centimetres to litres divide by 1000. The underlying mathematics is identical in both systems - only the scale changes.
Common cylinder diameter reference values
| Object | Approximate diameter | Notes |
|---|---|---|
| Standard drinking straw | 0.6 cm (0.24 in) | Inner bore, plastic straw |
| AA battery | 1.4 cm (0.55 in) | IEC standard |
| Tennis ball | 6.7 cm (2.63 in) | ITF specification |
| Standard dinner plate rim | 27 cm (10.6 in) | Restaurant standard |
| 50-gallon drum (US) | 57 cm (22.5 in) | Nominal outer diameter |
| Standard 55-gallon oil drum | 58 cm (22.9 in) | Nominal outer diameter |
| Residential water heater tank | 45-51 cm (18-20 in) | Outer shell, 40-50 gal |
Typical diameters for everyday cylindrical objects, for quick sanity-checking your results.
Frequently asked questions
What is the formula for the diameter of a cylinder from volume and height?
Rearrange the volume formula V = pi x r^2 x h for r, then double it: r = sqrt(V / (pi x h)), so d = 2 x sqrt(V / (pi x h)). For example, a cylinder with volume 785.4 cm3 and height 10 cm gives r = sqrt(785.4 / 31.416) = sqrt(25) = 5 cm, and d = 10 cm.
How do I find the diameter from the circumference?
Divide the circumference by pi: d = C / pi. If the circumference is 31.416 cm, the diameter is 31.416 / 3.14159 = 10 cm. This is useful in the field when you can wrap a measuring tape around a pipe or tank but cannot measure straight across.
Is the diameter the same as the bore of a pipe?
Not always. The bore of a pipe refers to the inner diameter (the clear opening fluid passes through), while the outer diameter also includes the pipe wall thickness. This calculator computes whichever diameter the inputs describe - if you enter the inner volume of the pipe, you get the inner diameter; if you measure the outer circumference, you get the outer diameter.
What is the difference between diameter and radius?
The radius is the distance from the center of the circle to any point on its edge. The diameter is a straight line passing through the center from one edge to the other, so it is exactly twice the radius: d = 2r. In practice, diameter is easier to measure directly on a physical object with calipers or a ruler.
How does changing the diameter affect volume?
Volume scales with the square of the radius, which is a quarter of the square of the diameter. Doubling the diameter gives four times the volume at the same height. A 10% increase in diameter raises volume by about 21% (1.1 squared = 1.21). This non-linear relationship is why small errors in diameter measurements have a larger impact on volume than the same error in height.