Schedule 40 vs Schedule 80 Pipe — Wall, Bore & When to Use Each
Schedule 40 vs Schedule 80 pipe compared: wall thickness, inside diameter, flow area, pressure capacity, and which schedule to specify — per ASME B36.10M.
Updated August 20, 2026
Schedule 40 and Schedule 80 pipe of the same nominal size have exactly the same outside diameter — a 2-inch pipe is 2.375 in OD in both. The schedule number only changes how far the wall grows inward: Schedule 80 trades bore for wall thickness, which is why fittings interchange between the two but flow capacity and pressure capacity do not.
That single geometric fact drives every practical difference. The thicker Schedule 80 wall means a smaller inside diameter (about 12% less flow area at 2″ NPS), more steel — so more weight and cost per foot — and more metal left under a cut thread or a corrosion allowance.
Every dimensional value in the table below is mapped in code from the same ASME B36.10M constants that drive the pipe schedule reference chart, with inside diameters computed as ID = OD − 2 × wall; a cross-check test holds each cell equal to that verified export.
Side by Side
| Spec | Schedule 40 | Schedule 80 |
|---|---|---|
| Outside diameter — 2″ NPS (in)The OD is fixed by the nominal size for every schedule, so fittings interchange dimensionally. | 2.375 | 2.375 |
| Wall thickness — 1/2″ NPS steel (in) | 0.109 | 0.147 |
| Wall thickness — 1″ NPS steel (in) | 0.133 | 0.179 |
| Wall thickness — 2″ NPS steel (in) | 0.154 | 0.218 |
| Wall thickness — 4″ NPS steel (in) | 0.237 | 0.337 |
| Inside diameter — 1/2″ NPS steel (in) | 0.622 | 0.546 |
| Inside diameter — 1″ NPS steel (in) | 1.049 | 0.957 |
| Inside diameter — 2″ NPS steel (in) | 2.067 | 1.939 |
| Inside diameter — 4″ NPS steel (in) | 4.026 | 3.826 |
| Flow area — 2″ NPSComputed from the two inside diameters above: area scales with ID². | Baseline bore | ≈12% less flow area |
| Pressure capacity (same material & size)At a fixed OD, capacity comes from wall thickness; the rating itself depends on material, temperature, and joining method. | Lower — thinner wall at the same OD | Higher — thicker wall at the same OD |
| Weight & material per foot | Less steel — lighter and cheaper per foot | More steel — heavier and costlier per foot |
| Typical uses | General plumbing, drainage, vents, low-pressure water and air | Higher-pressure service, threaded joints, pump and process piping |
Which One for the Job
Residential water supply, drains, and vent stacks
Schedule 40
Ordinary building services run nowhere near the pressure a thicker wall exists for, and the larger bore moves more water at the same nominal size for less weight and money per foot.
Threaded connections on pressurized lines
Schedule 80
Cutting a thread removes wall material at the joint — the thinnest, most stressed point of the run. Starting from the thicker Schedule 80 wall leaves more metal under the thread root at the same outside diameter.
Tying into an existing run or reusing fittings
Either
Both schedules share one outside diameter per nominal size, so sockets, flanges, and threads mate across them. Match the schedule of the existing line where pressure matters; the joint itself fits either way.
Gravity drainage where flow capacity is the whole job
Schedule 40
The pipe sees essentially no pressure, so the Schedule 80 wall buys nothing — and at 2″ NPS it costs about 12% of the flow area. Take the bigger bore.
Frequently Asked Questions
Do Schedule 40 fittings fit Schedule 80 pipe?
Dimensionally, yes: the outside diameter of pipe is fixed by its nominal size, so a 2-inch socket or thread engages a 2-inch pipe of either schedule. The joint, however, is only as strong as its thinnest member — on a pressurized line the fittings should carry the same schedule (and rating) as the pipe.
Does Schedule 80 pipe carry more water than Schedule 40?
Less, not more. The schedules share one outside diameter, so the thicker Schedule 80 wall grows inward and shrinks the bore — at 2″ NPS the inside diameter drops from 2.067 in to 1.939 in, roughly a 12% loss of flow area. Schedule 80 buys pressure capacity and thread depth, never flow.
How can I tell Schedule 40 from Schedule 80 pipe on site?
Not from the outside dimensions — the OD is identical by design. Read the print line: pipe product standards (ASTM D1785 for PVC, for example) require the schedule to be marked along the pipe. Failing a legible print line, measure the wall at a cut end and compare it to the schedule tables.
Is Schedule 80 always the safer choice if the budget allows?
No. Where pressure or threading does not demand the extra wall, Schedule 80 just costs you bore, weight, and money: the same nominal size delivers less flow, and the heavier pipe is harder to hang and cut. Specify the wall the service actually needs and spend the difference on the sizes themselves.
Try the Calculators
Sources
- ASME B36.10M — Welded and Seamless Wrought Steel Pipe (ODs and nominal walls; verified across two independent tabulations, as cited by the pipe schedule reference chart)
- ASTM D1785-12 — PVC Plastic Pipe, Schedules 40, 80, and 120, Tables 1 & 2 (iron-pipe-size ODs and schedule walls; marking requirements)