The ring of metal, and why the round formula is so simple
A hollow section is the outside area minus the bore. For a round tube that is (π/4)(OD² − ID²), and with ID = OD − 2t the difference of two squares factorises to something much friendlier: A = π × t × (OD − t). No squaring, no subtraction of large similar numbers, and no chance of the cancellation error that trips people up when the wall is thin relative to the diameter.
Multiply by 12 and by the density of carbon steel and you get the pipefitter's rule: lb/ft = 10.69 × (OD − t) × t with dimensions in inches. The metric twin is kg/m = 0.02466 × (OD − t) × t with dimensions in millimetres. Both come from exactly the same algebra.
Square and rectangular sections work the same way. The sharp-corner area is 2t(B + H − 2t), which for a square reduces to 4t(B − t). Real hollow structural sections have substantial radii on the outside corners, conventionally taken as twice the wall thickness, and rounding the corners removes metal. Working the geometry through, the outside corners remove (4 − π)(2t)² of area while the inside corners give back (4 − π)t², so the net correction is exactly 3(4 − π)t². That is the term this calculator subtracts when you choose the HSS corner style, and it is what makes the answer match an AISC catalogue rather than sitting a few percent high.
Nominal pipe size, schedules and design wall
Three naming conventions cause most of the errors on this page, and none of them is arithmetic.
Nominal pipe size is not a dimension. Below NPS 14, the nominal size is a legacy label and the outside diameter is something else entirely: NPS 1/2 is 0.840 in OD, NPS 2 is 2.375 in, NPS 4 is 4.500 in. Outside diameter is constant for a given NPS regardless of schedule, and it is the wall that changes. From NPS 14 upward, nominal size and outside diameter finally agree. Always enter the actual outside diameter.
Schedule is a wall thickness designation, not a thickness. Schedule 40 on NPS 2 is 0.154 in; schedule 40 on NPS 6 is 0.280 in. The schedule number was originally an approximation to 1,000 × pressure ÷ allowable stress, so the same schedule gives roughly similar pressure ratings across sizes rather than the same wall. ASME B36.10M tabulates the actual walls.
Structural HSS has two wall thicknesses. ASTM A500 permits a manufacturing tolerance, and AISC design values use a design wall of 0.93 times the nominal wall for electric-resistance-welded sections. Catalogue weights, however, are computed from the nominal wall. So if you want the weight a supplier will invoice, enter the nominal wall; if you want the area AISC uses in a strength check, enter 0.93 times it. Confusing the two produces a 7 percent discrepancy that looks like an arithmetic error but is not.
Fluid capacity uses the bore rather than the metal. Bore area times length gives cubic inches, and dividing by 231 gives US gallons. For NPS 2 schedule 40 the bore is 2.067 in, giving 0.174 gallons per foot — a figure worth knowing when filling or draining a system.
Worked example: a 20 ft length of 2 in schedule 40 pipe
You are pricing and rigging one 20 ft length of NPS 2 schedule 40 carbon steel pipe, which measures 2.375 in outside diameter with a 0.154 in wall.
- Cross-sectional area. π × 0.154 × (2.375 − 0.154) = π × 0.154 × 2.221 = 1.07443 in².
- Weight per foot. 1.07443 × 12 × 0.2836 = 3.6565 lb/ft, matching the published 3.65 lb/ft for this size and schedule.
- Check with the shortcut. 10.69 × 2.221 × 0.154 = 3.6564 lb/ft. The two agree, as they must.
- Weight of the length. 20 ft × 3.6565 = 73.13 lb. One person can carry it; a bundle of ten cannot be moved by hand.
- Metric equivalent. 3.6565 × 1.48816 = 5.4425 kg/m.
- Bore and capacity. ID = 2.375 − 2(0.154) = 2.067 in. Bore area = π/4 × 2.067² = 3.3556 in². Over 240 in that is 805.4 in³, and dividing by 231 gives 3.486 gallons, or 0.174 gal/ft.
Contrast that with a square hollow section of similar weight. HSS 4 × 4 × 1/4 has an area of 2(0.25)(4 + 4 − 0.5) − 3(4 − π)(0.25²) = 3.75 − 0.161 = 3.589 in², giving 12.22 lb/ft. Without the corner correction the same section computes to 3.75 in² and 12.76 lb/ft — 4.5 percent high, and enough to throw out a truckload estimate.
Carbon steel schedule 40 pipe: weight and capacity
| NPS | OD (in) | Wall (in) | ID (in) | Weight (lb/ft) | Weight (kg/m) | Capacity (gal/ft) |
|---|---|---|---|---|---|---|
| 1/2 | 0.840 | 0.109 | 0.622 | 0.852 | 1.268 | 0.0158 |
| 3/4 | 1.050 | 0.113 | 0.824 | 1.132 | 1.685 | 0.0277 |
| 1 | 1.315 | 0.133 | 1.049 | 1.681 | 2.501 | 0.0449 |
| 1-1/4 | 1.660 | 0.140 | 1.380 | 2.275 | 3.386 | 0.0777 |
| 1-1/2 | 1.900 | 0.145 | 1.610 | 2.721 | 4.049 | 0.1058 |
| 2 | 2.375 | 0.154 | 2.067 | 3.657 | 5.442 | 0.1743 |
| 2-1/2 | 2.875 | 0.203 | 2.469 | 5.799 | 8.630 | 0.2487 |
| 3 | 3.500 | 0.216 | 3.068 | 7.584 | 11.286 | 0.3840 |
| 4 | 4.500 | 0.237 | 4.026 | 10.802 | 16.076 | 0.6613 |
| 6 | 6.625 | 0.280 | 6.065 | 18.995 | 28.267 | 1.5008 |
Bare pipe only. Coatings, linings, threads and couplings add weight, and a filled line adds 8.34 lb per gallon of water on top of the steel.
Errors that make a tube weight wrong
- Entering nominal pipe size as outside diameter. NPS 2 pipe is 2.375 in OD, not 2.000. This single error understates a 2 in pipe by about 16 percent and is by far the most common.
- Using sharp-corner formulas for structural HSS. A500 corners are radiused at roughly twice the wall, removing 3(4 − π)t² of area. On a 4 × 4 × 1/4 section that is 4.5 percent of the weight.
- Mixing design wall with nominal wall. AISC design properties use 0.93 of the nominal wall for ERW sections; catalogue weights use nominal. Pick the one that matches what you are doing.
- Computing (OD² − ID²) with rounded numbers. On thin wall the two squares are close, so rounding either one before subtracting destroys precision. Use πt(OD − t), which never subtracts similar quantities.
- Forgetting the contents. A 6 in schedule 40 line weighs 19 lb/ft empty and 31.5 lb/ft full of water, because 1.5 gal/ft at 8.34 lb/gal adds 12.5 lb/ft. Hanger spacing is designed for the filled condition.
- Ignoring coating and lining weight. Galvanising, epoxy lining, insulation and cladding all add mass that the bare-metal calculation does not include, and on small-bore insulated lines the insulation can outweigh the pipe.
- Assuming square tube is the same as pipe of similar size. A 4 in square HSS and a 4 in NPS pipe have similar outside dimensions and very different areas, weights and section properties.
Weight is not the same as strength for a hollow section
The whole point of a tube is that it puts metal far from the neutral axis, so it buys bending and torsional stiffness that a solid bar of equal weight cannot match. Two sections of identical weight per foot can differ substantially in section modulus depending on how the metal is distributed. Use weight for cost, freight and handling; use section properties for capacity.
Where tube weight sits in a job
Pipe weight drives three practical decisions. It sets support spacing, since a hanger has to carry the pipe plus its contents plus insulation and any snow or ice load on outdoor runs. It sets rigging, since a bundle of 20 ft lengths adds up quickly. And it sets cost, since tube and pipe are bought by the pound or by the foot at prices derived from weight.
For solid stock and open shapes, the metal weight calculator covers round, square, hex, flat and angle sections with the same density set. For flat product, the steel plate weight calculator gives pounds per square foot and handles circular blanks and cut-outs. All three use the same density figures, so a mixed bill of materials totals consistently.
In fabrication, tube weight interacts with welding. A heavier wall needs more passes and a larger fillet, and the metallurgy of the joint is set by energy per unit length, which the welding heat input calculator quantifies. Sizing the weld itself is the job of the fillet weld strength calculator, and note that a weld on a hollow section is limited by the wall thickness available at the joint rather than by the overall section size.
One more caution about tube in structural use. Cold-formed hollow sections work-harden in the corners during forming, which raises hardness and strength locally and reduces ductility there. That is why welding across the corner of a cold-formed section is restricted in some codes, and why a hardness traverse across an HSS corner reads noticeably higher than the flat — the hardness conversion calculator turns those readings into approximate strengths. The stress and strain calculator covers the elastic behaviour that governs how much a member deflects under load.
Weight as a corrosion baseline, and why the loss rate isn't linear
Every output on this page assumes the wall thickness entered is the wall today. On new tube that is the mill or catalogue wall; on tube already in service it may be a wall that has already lost material to corrosion or erosion, and weight, cross-sectional area and bore capacity all move with it. For a round section the area is exactly π × t × (OD − t), and because that expression is not linear in t, a given increment of wall loss does not remove the same amount of weight from every section.
Work through the formula and the reason is visible directly: the area gained per additional unit of wall is proportional to (OD − 2t), which is largest when the wall is thin relative to the outside diameter and shrinks toward zero as the wall approaches half the diameter. A thin-wall tube therefore loses weight faster, per thousandth of an inch of corrosion, than a heavy-wall section of the same outside diameter — the opposite of what a straight-line, percent-of-original-weight estimate assumes. Bore capacity moves the other way: the inside dimension is OD − 2t, so it grows steadily as the wall thins, meaning weight and bore capacity always change in opposite directions as a section corrodes, never together.
This is useful in the field beyond the arithmetic. Weighing a cut sample of known length and back-solving this page's own formula for the average wall gives a true cross-sectional average, which a point ultrasonic-thickness reading cannot — a UT gauge measures wall at the specific spot it touches and can miss pitting or uneven loss between readings. Whichever method is used, compare the measured wall against this section's own area curve rather than against a flat percentage of the original catalogue weight.