How this instrument works
A concrete test cylinder is a small, precisely sized sample molded from a fresh concrete batch and cured under controlled conditions, then crushed in a compression-testing machine to measure the mix's compressive strength. ASTM C39, the standard governing that compression test, and ASTM C31, the companion standard for making and curing specimens in the field, both specify exact cylinder sizes — most commonly 6 inches in diameter by 12 inches tall, with a 4 inch by 8 inch size also permitted for mixes with a smaller maximum aggregate size.
This instrument applies the same cylinder-volume formula used for full-scale columns — pi times radius squared times height — but scaled for small, standard test-specimen dimensions entered in inches rather than a mix of inches and feet. The diameter divides by 24 to convert it into a radius in feet, the height divides by 12 to convert it into feet as well, and the result comes back in cubic feet rather than cubic yards, since a single test cylinder's volume is a small fraction of a cubic yard.
Knowing a cylinder's exact volume matters for quality-control record-keeping and for estimating how much sample concrete a testing lab needs to pull from a batch to fill however many cylinders a project's sampling frequency requires. A single 6x12 in cylinder holds roughly 0.196 cubic feet of concrete — small on its own, but it adds up quickly across the multiple cylinders a typical structural pour requires for early-break, standard-cure, and reserve testing.
This is a different calculation from the ones this site's concrete-column, concrete-slab, and concrete-driveway-cost instruments perform, even though all of them ultimately multiply a cross-sectional area by a height. Those instruments size full-scale placed concrete in cubic yards for ordering purposes; this one sizes a small standardized test specimen in cubic feet for quality-control and lab purposes — a genuinely different job despite the shared underlying geometry.
- Enter the mold's diameter into Cylinder diameter (in) — 6 for the standard ASTM C39 cylinder, or 4 for the smaller permitted size.
- Enter the mold's height into Cylinder height (in) — 12 for the standard cylinder, or 8 for the smaller size, keeping the 2:1 height-to-diameter ratio ASTM requires.
- Read Volume (cubic feet) beneath the inputs — the concrete volume a single cylinder mold holds.
- Multiply the result by the number of cylinders a project's sampling plan requires to estimate the total sample volume needed from a batch.
Worked example — a standard 6 in × 12 in ASTM cylinder
Enter 6 into Cylinder diameter (in) and 12 into Cylinder height (in) for the standard ASTM C39 compression-test cylinder size. The instrument converts the radius to 6/24 = 0.25 ft and the height to 12/12 = 1 ft, then computes pi x 0.25² x 1 = pi x 0.0625 ≈ 0.19635. Volume (cubic feet) reads 0.19635 ft³.
That 0.19635 ft³ figure is the volume of concrete a single standard cylinder mold holds — a testing lab pulling a sample to fill, say, four cylinders for one batch's quality-control set needs roughly 0.785 ft³ of concrete (4 x 0.19635), well before accounting for any material lost to mixing, spillage, or consolidation during molding.
Questions
What size is a standard ASTM concrete test cylinder?
6 inches in diameter by 12 inches tall is the standard size specified under ASTM C39 (the compression test) and ASTM C31 (making and curing specimens in the field). A smaller 4 inch by 8 inch cylinder is also permitted, typically for concrete mixes with a smaller maximum aggregate size, and both sizes keep the same 2:1 height-to-diameter ratio the standards require.
Why does the smaller 4x8 cylinder keep the 2:1 ratio?
Because the 2:1 height-to-diameter ratio, not the absolute size, is what the compression test's stress calculations and correction factors are built around. Both the 6x12 and 4x8 sizes satisfy that ratio (12/6 = 2 and 8/4 = 2), which is why ASTM permits either one, choosing the smaller size mainly when the mix's maximum aggregate size doesn't require the larger mold.
How much concrete do I need to fill several cylinders?
Multiply the single-cylinder volume this instrument returns by the number of cylinders the project's sampling plan calls for, then add a margin for material lost to mixing, transfer, and consolidation during molding — labs typically pull noticeably more sample concrete than the cylinders' combined theoretical volume to account for that loss.
Is this the same calculation as the concrete column instrument?
The underlying cylinder-volume geometry is identical, but the two serve different purposes. This instrument sizes a small ASTM-standard test specimen, in inches, reported in cubic feet, used to check a concrete mix's compressive strength. This site's concrete column instrument sizes a full-scale structural column, in feet, reported in cubic yards, used to order ready-mix concrete for an actual structural pour.
What standard covers cylinder size versus the test itself?
ASTM C31, Standard Practice for Making and Curing Concrete Test Specimens in the Field, governs how the cylinders are molded, cured, and handled before testing. ASTM C39, Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens, governs the compression test itself, including the cylinder dimensions this calculator computes the volume for.
Does the cylinder's volume affect its measured strength?
Not directly — compressive strength is reported as a stress (load divided by cross-sectional area), which normalizes out the specimen's size to a degree, though ASTM does apply specific correction factors when cylinder dimensions deviate from the standard 2:1 ratio or aren't perfectly formed. This instrument computes volume for sampling and quality-control purposes; the strength calculation itself is a separate step performed by the compression-testing machine and lab.