Formula
Volume is the inside circle times the length. Gallons are cubic inches divided by 231. The inside diameter is the outside diameter minus twice the wall.
ID = OD − 2 × wall
gallons = π × (ID / 2)² × length in inches / 231
Steel sizes are ASME B36.10. Schedule 40 and Schedule 80 are listed separately so a 12 inch Schedule 40 is not silently treated as STD. Copper is ASTM B88, one outside diameter and three walls. PEX is ASTM F876 SDR-9, wall = OD / 9. The number stamped on the tube wins if it disagrees. These bores are also what the friction loss calculator should use. The tank volume calculator is the vessel, not the line.
Worked examples
100 ft of 2 inch Schedule 40. ID 2.067 in. The cross-section is π × (1.0335)² = 3.356 in². Over 1,200 inches that is 4,027 in³, or 17.43 gallons.
The same length of 2 inch Schedule 80. The wall is thicker, the ID is 1.939 in, and the volume drops.
1/2 inch PEX, SDR-9. OD 0.625. Wall 0.625 / 9. ID is 7/9 of the OD.
Reference
A short slice of the table. The selector has the rest. ID = OD − 2 × wall.
| Pipe | OD, in | Wall, in | ID, in |
|---|---|---|---|
| Steel sch 40 1/2 in | 0.840 | 0.109 | 0.622 |
| Steel sch 40 2 in | 2.375 | 0.154 | 2.067 |
| Steel sch 40 3 in | 3.500 | 0.216 | 3.068 |
| Steel sch 40 12 in | 12.750 | 0.406 | 11.938 |
| Steel sch 80 2 in | 2.375 | 0.218 | 1.939 |
| Copper L 3/4 in | 0.875 | 0.045 | 0.785 |
| PEX SDR-9 1/2 in | 0.625 | 0.069 | 0.486 |
Questions
Is Schedule 40 the same as STD?
Through NPS 10, yes. At NPS 12 and larger, STD is a 3/8 inch wall and Schedule 40 is thicker. This page uses the Schedule 40 wall, so 12 inch Schedule 40 is 0.406 inch, ID 11.938, not the STD bore.
Were these read from the ASME PDF?
Schedule 40 outside diameters and walls are the commonly published ASME B36.10 table, including the small sizes checked against published reprints. Schedule 80, and the ASTM B88 copper walls, are the commonly published nominals and were not re-read line by line from the standards PDFs for this page. The stamp on the pipe wins.
How is PEX sized?
ASTM F876 SDR-9 copper-tube size. The wall is the outside diameter divided by 9, and the outside diameter matches type copper. A 1/2 inch tube is 0.625 inch OD.
Sources
- ASME B36.10 welded and seamless steel pipe. Schedule 40 dimensions here are the commonly published table. NPS 12 Schedule 40 wall is 0.406 in, not the 0.375 in STD wall used above NPS 10. Schedule 80 walls are the commonly published reprints and were not re-read from the ASME PDF for this page.
- ASTM B88 copper water tube. K, L, and M share an outside diameter. The walls are commonly published nominals, not a line-by-line read of the ASTM PDF. The tube stamp wins.
- ASTM F876 SDR-9 PEX. Wall = outside diameter / 9. CTS outside diameters match copper tube size.