Pipe Size Chart: Nominal Size, OD, ID & Flow (Sch 40/80)

How the pipe size chart works

Pipe is sold by nominal size, and the nominal size is not a measurement of anything on the pipe: a 1 inch pipe is not 1 inch across. What is fixed is the outside diameter (OD), which is the same for every schedule of a given nominal size — a 1 inch pipe is always 1.315 inches outside, whether it is Schedule 40 or Schedule 80. What changes with the schedule is the wall thickness: Schedule 80 walls are thicker, so the inside diameter (ID) is smaller, the bore is smaller, and the pipe carries less water at the same velocity. The inside diameter is simply OD minus twice the wall thickness, and the flow comes from the bore area times the velocity.

At the defaults — a 1 inch Schedule 40 pipe — the OD is 1.315 inches and the wall is 0.133 inches, so the ID is 1.315 − 2 × 0.133 = 1.049 inches. The cross-section area is π/4 × 1.049² = 0.864 square inches, and at 5 feet per second that bore carries 5 × (0.864 / 144) × 448.831 = 13.47 gallons per minute. The 448.831 is the conversion from cubic feet per second to gallons per minute, and the 144 converts square inches to square feet. All dimensions on this page follow ASME B36.10M, the standard for welded and seamless wrought steel pipe.

Schedule 40 dimensions above are from ASME B36.10M. Flow is the maximum at 5 feet per second, the usual design velocity for water supply; faster water is noisier and wears fittings out.

Schedule 80 dimensions above are from ASME B36.10M. The outside diameter is the same as Schedule 40 for every size; only the wall is thicker, so every ID, area, and flow is smaller.

The worked example, line by line

Setup: nominal size 1 inch; schedule Schedule 40; required flow 50 GPM.

Line 1 — the outside diameter. A 1 inch pipe is 1.315 inches outside, whatever the schedule. Nominal size is a name, not a measurement.

Line 2 — the inside diameter. ID = OD − 2 × wall = 1.315 − 2 × 0.133 = 1.049 inches.

Line 3 — the cross-section area. Area = π/4 × ID² = 0.785398 × 1.049² = 0.864 square inches.

Line 4 — the flow. Flow = 5 ft/s × (0.8643 / 144) ft² × 448.831 = 13.47 GPM. The 144 converts square inches to square feet; 448.831 converts cubic feet per second to gallons per minute.

Line 5 — the reverse. A 1-1/2 inch Schedule 40 pipe carries only 31.73 GPM at 5 ft/s, which is short of 50 GPM, so the smallest pipe that meets the requirement is a 2 inch Schedule 40 pipe, which carries 52.30 GPM.

Every branch, checked

1/2 inch Schedule 40: OD 0.840 inches, wall 0.109 inches, ID 0.840 − 2 × 0.109 = 0.622 inches, area π/4 × 0.622² = 0.304 square inches, flow 4.74 GPM at 5 ft/s. A half-inch line feeds a single fixture, not a house.

2 inch Schedule 40: OD 2.375 inches, wall 0.154 inches, ID 2.067 inches, area 3.356 square inches, flow 52.30 GPM at 5 ft/s. Going from 1-1/2 inch to 2 inch nearly doubles the flow, because area grows with the square of the diameter.

4 inch Schedule 40: OD 4.500 inches, wall 0.237 inches, ID 4.026 inches, area 12.730 square inches, flow 198.39 GPM at 5 ft/s. A 4 inch main carries almost four times what a 2 inch line carries.

1 inch Schedule 80: the wall is 0.179 inches instead of 0.133, so the ID is 1.315 − 2 × 0.179 = 0.957 inches, the area is 0.719 square inches, and the flow is 11.21 GPM at 5 ft/s — about 17% less than the same nominal size in Schedule 40, the price of the thicker wall.

Reverse — 50 GPM: the smallest Schedule 40 pipe that carries 50 GPM at 5 ft/s is the 2 inch pipe at 52.30 GPM. The 1-1/2 inch pipe manages only 31.73 GPM, so it is not close.

What the defaults assume

Dimensions are nominal ASME B36.10M values for steel pipe, in inches; copper, PVC, and PEX use different sizing systems, so do not read this chart against a copper or plastic pipe — see the copper pipe size chart and the PVC pipe size chart for those. Flow is the maximum at 5 feet per second with a full, unobstructed bore: fittings, elbows, valves, and scale all reduce real flow, and water over about 5 ft/s is noisy and wears fittings, so 5 ft/s is a design limit, not a target to exceed. The reverse lookup uses Schedule 40 only, because that is the usual water-supply schedule; a Schedule 80 pipe of the same nominal size always carries less. Rounding convention: diameters and walls to three decimals, areas to three decimals, flows to two decimals, all computed from unrounded values.

Output · formula · meaning

Who uses this

• The homeowner sizing a water line: needs 50 GPM for the house, looks it up, and learns a 1-1/2 inch pipe only carries 31.73 GPM — so a 2 inch Schedule 40 line is the smallest that works, at 52.30 GPM.
• The DIY plumber buying fittings: measures a pipe at 1.315 inches outside and learns it is a 1 inch nominal pipe — so they buy 1 inch fittings, not 1-5/16 inch ones, because fittings are sold by nominal size too.
• The contractor checking a Schedule 80 run: is told to use Schedule 80 for a high-pressure line, and learns a 1 inch Schedule 80 pipe only carries 11.21 GPM against 13.47 for Schedule 40 — so the thicker wall costs flow, and the run may need upsizing.
• The student learning pipe sizing: sees that doubling the nominal size from 1 inch to 2 inch takes the flow from 13.47 GPM to 52.30 GPM — nearly four times, not twice — because flow follows the bore area, which grows with the square of the diameter.

Accuracy & limitations

The dimensions are ASME B36.10M nominal values: OD and wall are exact to the thousandths of an inch shown, ID is OD minus twice the wall, area is π/4 times ID squared, and flow is 5 ft/s times the bore area in square feet times 448.831 gallons per minute per cubic foot per second. What the chart cannot do is replace a hydraulic calculation: real flow depends on pressure, pipe length, fittings, elevation, and water temperature, and the 5 ft/s figure is a design velocity, not a guarantee. It also covers steel pipe only: copper, PVC, CPVC, and PEX have their own sizing tables, and a 1 inch copper pipe is not 1.315 inches outside. Read the NPT pipe thread chart for the threads that match these sizes.