What is the actual inside diameter of 6 inch Schedule 40 pipe?

Nominal 6 in. Schedule 40 pipe has an outside diameter of 6.625 in., a wall thickness of 0.280 in., and an inside diameter of 6.065 in. In Schedule 80 the same 6.625 in. OD carries a 0.432 in. wall, giving an inside diameter of 5.761 in. Outside diameter is fixed by the nominal size; the schedule changes only the wall, and therefore the ID.

SDR is the standard dimension ratio, the outside diameter divided by the minimum wall thickness. Because it is a ratio, one SDR gives the same pressure rating at every diameter. Inside diameter follows directly: ID = OD × (1 − 2/SDR). A lower SDR means a thicker wall, a higher pressure rating, and a smaller bore.

What pressure class is DR 18 PVC?

For AWWA C900 PVC, pressure class equals 2 times the 2,000 psi hydrostatic design basis divided by (DR − 1). DR 18 gives 235 psi. The full series is DR 51 at 80 psi, DR 41 at 100, DR 32.5 at 125, DR 25 at 165, DR 21 at 200, DR 18 at 235 and DR 14 at 305 psi.

What are RCP D-load classes?

ASTM C76 rates reinforced concrete pipe by three-edge-bearing D-load in pounds per linear foot per foot of inside diameter. Class II is 1,000 lb at the 0.01 in. crack and 1,500 ultimate; Class III is 1,350 and 2,000; Class IV is 2,000 and 3,000; Class V is 3,000 and 3,750. Class III is the common default for storm drain under normal cover.

Does ductile iron pipe have the same OD as steel pipe?

No. Ductile iron uses cast-iron outside diameters, which are larger than iron pipe size: 6 in. DIP is 6.90 in. OD versus 6.625 in. for 6 in. steel, and 12 in. DIP is 13.20 in. versus 12.75 in. AWWA C900 PVC and DIPS-sized HDPE use the ductile iron OD so they can share the same fittings and restraints; IPS-sized HDPE and PVC use the steel OD. Mixing the two families is a common and expensive field surprise.

How do you find pipe flow area from diameter?

Area equals pi times diameter squared divided by four. With diameter in inches and area wanted in square feet, A = 0.005454 × D². A 24 in. pipe has 3.14 sq ft of area, a 36 in. pipe 7.07 sq ft, and a 48 in. pipe 12.57 sq ft. Always use the actual inside diameter, not the nominal size.

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Pipe Sizes & Actual Inside Diameter — Reference

By Michael Flynn, PE · water resources & dam safety engineer

The number on the plan sheet is a name, not a diameter. Every hydraulic calculation on this site — Manning's, Darcy-Weisbach, Hazen-Williams, full-flow capacity — wants the actual inside diameter, which depends on material, schedule, pressure class and lining. This card is the translation table.

Steel & IPS-Dimensioned Pipe (ASME B36.10)

PVC and CPVC Schedule 40 and 80 share these same outside diameters and wall thicknesses. OD is fixed by the nominal size; the schedule changes only the wall.

NPS	OD (in)	Sch 40 wall	Sch 40 ID	Sch 80 wall	Sch 80 ID
½	0.840	0.109	0.622	0.147	0.546
¾	1.050	0.113	0.824	0.154	0.742
1	1.315	0.133	1.049	0.179	0.957
1¼	1.660	0.140	1.380	0.191	1.278
1½	1.900	0.145	1.610	0.200	1.500
2	2.375	0.154	2.067	0.218	1.939
2½	2.875	0.203	2.469	0.276	2.323
3	3.500	0.216	3.068	0.300	2.900
4	4.500	0.237	4.026	0.337	3.826
6	6.625	0.280	6.065	0.432	5.761
8	8.625	0.322	7.981	0.500	7.625
10	10.750	0.365	10.020	0.594	9.562
12	12.750	0.406	12.000	0.688	11.374
14	14.000	0.438	13.124	0.750	12.500
16	16.000	0.500	15.000	0.844	14.312
18	18.000	0.562	16.876	0.938	16.124
20	20.000	0.594	18.812	1.031	17.938
24	24.000	0.688	22.624	1.219	21.562

Above NPS 12 the nominal size is the outside diameter. Below it, OD exceeds the nominal — and 12 in. Sch 40 is the one happy coincidence where the ID comes out at exactly 12.000 in.

SDR / DR Pipe — the Ratio Does the Work

SDR = OD / t_min ID = OD × (1 − 2 / SDR)
Pressure rating = 2 · HDS / (SDR − 1)

SDR / DR	ID factor (× OD)	PVC, HDS 2,000 psi (psi @ 73°F)	HDPE PE4710, HDS 1,000 psi (psi @ 73°F)
51	0.9608	80	—
41	0.9512	100	—
32.5	0.9385	125	63
26	0.9231	160	80
25	0.9200	165	—
21	0.9048	200	100
18	0.8889	235	—
17	0.8824	250	125
14	0.8571	305	—
13.5	0.8519	315	160
11	0.8182	—	200
9	0.7778	—	250
7.3	0.7260	—	320

The SDR 41 / 32.5 / 26 / 21 / 17 / 13.5 series is ASTM D2241 PVC on IPS outside diameters. The DR 51 / 41 / 32.5 / 25 / 21 / 18 / 14 series is AWWA C900 PVC on ductile-iron outside diameters. HDPE per ASTM F714 / AWWA C906 comes in both IPS and DIPS sizing — specify which. HDPE pressure ratings are for water at 73°F and must be derated for temperature and for surge.

Worked example — how much bore a class change costs

A “6-inch” IPS PVC main, OD = 6.625 in.: SDR 41 → ID 6.30 in. · SDR 26 → 6.12 · SDR 21 → 6.00 · SDR 17 → 5.85. That is a 14% spread in flow area between the thinnest and thickest class, and roughly a 20% spread in Hazen-Williams head loss at a fixed flow — enough to change a pump selection.

Ductile Iron Pipe (AWWA C151) — Outside Diameters

Nominal	3	4	6	8	10	12	14	16	18	20	24	30	36
OD (in)	3.96	4.80	6.90	9.05	11.10	13.20	15.30	17.40	19.50	21.60	25.80	32.00	38.30

Wall thickness varies by pressure class (PC 150 through PC 350); 4 in. through 12 in. carry a 0.25 in. minimum wall in every class. Standard cement-mortar lining (AWWA C104) takes roughly another ⅛ in. off the diameter, double-thickness lining about ¼ in. Take the bare ID as OD − 2t and subtract the lining, or read the manufacturer's table — for hydraulics, the lined ID is the one that counts. DIP is normally analyzed with Hazen-Williams C = 140 new, 130 design, dropping with age and tuberculation.

Reinforced Concrete Pipe (ASTM C76)

Standard inside diameters (in)	12, 15, 18, 21, 24, 27, 30, 33, 36, 42, 48, 54, 60, 66, 72, 78, 84, 90, 96, 102, 108
Wall A	t = D/12 (in., D in in.) — thinnest
Wall B	t = D/12 + 1 — the usual default
Wall C	t = D/12 + 1.75 — heaviest

Class	D-load at 0.01 in. crack (lb/ft/ft of dia.)	Ultimate D-load	Typical use
I	800	1,200	Large diameter only (60 in. and up), shallow cover
II	1,000	1,500	Light cover, no traffic
III	1,350	2,000	The common storm-drain default
IV	2,000	3,000	Deep fill or heavy traffic
V	3,000	3,750	Very deep fill, industrial loading

D-load is a three-edge-bearing test result in pounds per linear foot per foot of inside diameter. Required D-load = (earth load + live load) × factor of safety ÷ bedding factor ÷ D — so the bedding class specified on the detail is doing as much work as the pipe class.

Corrugated Pipe

Product	Sizes	Manning's n	Notes
CMP, 2⅔ × ½ corrugation (AASHTO M36)	12–120 in.	0.024	Annular; helical in small diameters runs lower, ~0.012–0.022
CMP, 3 × 1 corrugation	36–144 in.	0.027	Larger diameters and structural plate
CMP, 5 × 1 corrugation	48–144 in.	0.025–0.026
Corrugated HDPE, dual wall (AASHTO M294)	12–60 in.	0.012	Smooth interior liner; n applies to the liner, not the corrugation
Corrugated HDPE, single wall	3–24 in.	0.020–0.025	Drainage / underdrain, not a storm main

CMP gauge is the sheet thickness: 16 ga = 0.064 in., 14 ga = 0.079, 12 ga = 0.109, 10 ga = 0.138, 8 ga = 0.168. For a corrugated pipe the nominal diameter is the inside diameter measured across the corrugation crests, and it is what you use for hydraulics.

Flow Area by Actual Inside Diameter

A (ft²) = πD²/4 = 0.005454 · D² (D in inches)

D (in)	4	6	8	10	12	15	18	24	30	36	42	48	60	72
A (ft²)	0.087	0.196	0.349	0.545	0.785	1.227	1.767	3.142	4.909	7.069	9.621	12.57	19.63	28.27

Two OD families, and they do not mix. IPS (iron pipe size, the steel ODs at the top of this card) and DIPS/CIOD (the ductile-iron ODs) are different pipes with the same names. AWWA C900 PVC and DIPS HDPE are built to the ductile-iron OD so they take ductile-iron fittings, restraints and tapping sleeves; ASTM D2241 PVC and IPS HDPE are not. Call out the OD family on the plans and in the specification, not just the nominal size and the class — the mismatch is usually discovered in the trench.

Sources: ASME B36.10M (welded and seamless wrought steel pipe); ASTM D1785 and D2241 (PVC); AWWA C900 (PVC pressure pipe), C905, C906 and ASTM F714 (PE); AWWA C151/A21.51 and C104 (ductile iron and cement-mortar lining); ASTM C76 (RCP); AASHTO M36 (CMP) and M294 (corrugated PE). Pressure ratings are for water at 73°F before surge allowance and temperature derating; confirm against the current standard and the manufacturer's submittal.

Have the real ID? Darcy-Weisbach head loss → · Hazen-Williams · Manning's full/partial flow.

Related cheat sheets and tools

Feed the actual ID into Darcy-Weisbach with ε from the absolute roughness card, into Hazen-Williams with C from the C-value card, or into Manning's with n from the roughness card. Add fittings from the minor loss K card or the equivalent-length card, and check the layout against the storm sewer criteria card. For a full network, see HydroComplete.