How this instrument works
A cylinder is generated by sweeping a circle along a straight axis, and its diameter belongs entirely to that circular cross-section — the width measured straight across the flat end, through the center, equal to two radii placed end to end. Because the sweep only stretches the shape along the axis, the diameter never depends on how far that axis runs: a coin-thin washer and a length of drainpipe can share the identical radius, and therefore the identical diameter, d = 2r, no matter how their heights differ.
That independence holds even when the cylinder leans. An oblique cylinder — one whose axis tilts instead of standing perpendicular to its base — still has a circular cross-section with the same radius and the same diameter as an upright cylinder built from the identical circle; only the slant height changes, not d = 2r. The formula describes the base alone, so tilting the solid never touches it.
Machinists call this figure the bore, one half of the pair — bore and stroke — that sets an engine cylinder's displacement; stroke measures the piston's travel along the height, a completely separate number from the bore's diameter. Spec sheets, gaskets, and piston rings are all dimensioned by that bore diameter directly, which is why reading a radius off a drawing and forgetting to double it is a genuinely common fitting error.
- Enter the cylinder's radius into the Radius field — any length unit works, as long as you read the result in that same unit.
- Diameter updates immediately beside it, computed as exactly twice the value you entered, d = 2r.
- Use that Diameter figure wherever a bore chart, gasket size, or tank spec calls for the width straight across the circular end, not the distance from the center.
- The cylinder's height or length plays no part in this figure — reuse the same Diameter reading for a short cylinder or a long one, as long as the radius matches.
Worked example — a drum with a 3-unit radius
A cylindrical shipping drum is built with a radius of exactly 3 units for its circular base, so its diameter is d = 2 × 3 = 6.0 units — the exact width a lid, gasket ring, or pallet cutout must span, since round stock is always specified by that diameter rather than the radius used to derive it. This 6.0-unit figure holds regardless of whether the drum stands ankle-high or floor-to-ceiling, because diameter and height measure two independent dimensions of the same cylinder.
The same doubling works at any radius: a narrower cylinder with r = 2.5 units carries a diameter of exactly 5.0 units, and halving that 5.0 back returns the original 2.5 with nothing lost. At the opposite extreme, a radius of 0 collapses the circular base to a single point, so the diameter is 0 as well — the cylinder degenerates into a bare line segment along its axis, the lower boundary this formula can describe.
Questions
What does the diameter of a cylinder actually measure?
It is the width straight across the cylinder's circular base, through the center, equal to twice the radius: d = 2r. It says nothing about how far the cylinder runs along its axis — that is a separate measurement entirely, usually called the height or the length.
Does a cylinder's diameter change with its height?
No. A cylinder is a circle swept along an axis, and the diameter belongs entirely to that circle — two cylinders with the same radius share the same diameter whether one is a coin-thin washer or a metre-long pipe. Only the height is free to vary independently.
How do I find a cylinder's diameter from its circumference instead of its radius?
Divide the circumference by π: d = C ⁄ π, since C = πd for any circle, cylinder base included. A base whose taped circumference comes out to exactly 6π, about 18.850 units, has a diameter of exactly 6 — the same number this calculator reaches directly from a radius of 3.
Why do machinists call this measurement the 'bore'?
Bore is the shop term for a cylinder's inside diameter, most familiar from engine cylinders, where bore pairs with stroke — the piston's travel along the height — to set displacement. Bore and stroke are independent figures; this calculator handles the bore side of that pair, d = 2r.
Does tilting a cylinder change its diameter?
No. An oblique cylinder, one whose axis leans instead of standing perpendicular to its base, still has a circular cross-section with the same radius and diameter as an upright cylinder built from that same circle. Tilting changes the slant height, not d = 2r, because the formula describes the base circle alone.
What's a common mistake when reading radius and diameter off a spec sheet?
Treating the two as interchangeable. A drawing that lists a 6-unit diameter describes a 3-unit radius, and plugging that 6 into a formula expecting r doubles the true diameter to 12 — a frequent error when a spec sheet is dimensioned one way and a formula expects the other. Always check which one is labeled before doubling or halving.