Two weights, and why both matter
Pipe has an empty weight and a filled weight, and different people on the same job need different ones. The rigger lifting a 40 ft joint into a rack cares about the empty weight. The structural engineer checking the beam cares about the filled weight, because the pipe will spend its life full. The estimator pricing freight cares about empty; the hydrostatic test engineer cares about filled, because the pipe is heaviest during the test.
The gap between them is not small on large pipe. NPS 6 schedule 40 steel weighs 18.97 lb/ft empty and 31.48 lb/ft full of water - the water adds about two thirds again on top of the steel. On plastic pipe the ratio inverts entirely: a 6 in SDR 26 PVC line has a 0.255 in wall and works out at 3.07 lb/ft empty and 15.80 lb/ft full, so more than 80% of what the hangers carry is the water. That is why plastic pipe needs closer support than steel despite being lighter.
Neither figure includes the fittings, valves, insulation or the hangers themselves. A gate valve on a 6 in line can weigh more than 10 ft of the pipe it sits in, and a concentrated load like that belongs directly over a support, not mid-span. Insulation adds weight and increases the effective diameter for wind and snow loading on outdoor runs.
If you want the water volume rather than its weight - for flushing, chlorination or system capacity - the pipe volume calculator gives gallons directly.
Where 10.69 and 0.3403 come from
The metal weight is a cross-sectional area times a length times a density. The annulus between outside and inside diameter has area π/4 × (OD² − ID²) square inches; multiply by 12 inches to get the volume in a foot of pipe, then by the density in pounds per cubic inch.
Substituting ID = OD − 2t and simplifying gives the shop formula every pipefitter knows: the annulus area becomes π × t × (OD − t), so weight per foot is 12πρ × t × (OD − t). For carbon steel at 0.2836 lb/in³, 12π × 0.2836 = 10.69, which is the constant in W = 10.69 t (OD − t). The same substitution gives 10.90 for stainless, 12.18 for copper and 1.89 for PVC - the constant is nothing more than 37.699 times the density.
The contents follow the same logic on the bore alone. A foot of pipe holds π/4 × ID² × 12 cubic inches; water at 62.4 lb/ft³ is 0.03611 lb/in³, and the product of those constants is 0.3403. So W_water = 0.3403 × ID² pounds per foot, multiplied by specific gravity for anything other than water.
One trap worth naming: copper tube is sized by nominal size, and the outside diameter is 1/8 inch larger. Nominal 1 in Type L copper has an outside diameter of 1.125 in and a wall of 0.050 in. Enter 1.125, not 1.000. Steel pipe is the opposite convention - NPS 2 has an outside diameter of 2.375 in, larger than nominal, and the nominal number is close to the bore instead. Both conventions catch people out, and both are worth checking against the dimension table before you trust a weight.
Worked example: 100 ft of NPS 2 schedule 40 steel, water filled
NPS 2 schedule 40 carbon steel has an outside diameter of 2.375 in and a wall of 0.154 in, from the ASME B36.10M dimension table. The run is 100 ft, full of water, on hangers every 10 ft.
- Inside diameter. 2.375 − 2 × 0.154 = 2.067 in.
- Metal area. π/4 × (2.375² − 2.067²) = π/4 × (5.6406 − 4.2725) = π/4 × 1.3681 = 1.0745 in².
- Weight per foot empty. 1.0745 × 12 × 0.2836 = 3.657 lb/ft. The published figure is 3.65 lb/ft, and the shop formula confirms it: 10.69 × 0.154 × (2.375 − 0.154) = 3.657.
- Water inside. 0.3403 × 2.067² = 0.3403 × 4.2725 = 1.454 lb/ft.
- Filled weight per foot. 3.657 + 1.454 = 5.111 lb/ft.
- The run. 100 ft × 3.657 = 366 lb empty, 100 × 5.111 = 511 lb full - which is 232 kg.
- Each hanger. 10 ft of span × 5.111 = 51 lb per hanger, before allowing for a valve or a fitting near any one of them.
In metric, 3.657 lb/ft × 1.48816 = 5.44 kg/m empty. The conversion is worth memorising: one pound per foot is 1.488 kilograms per metre.
Using the number: hangers, structure and lifting
The hanger load is filled weight per foot times the span each hanger carries. That is the load on the rod, the strut and the structure above it, and it is only the starting point - a support next to a valve or a flange carries the concentrated weight of that item as well, and seismic bracing where required adds lateral demand that has nothing to do with the dead weight at all.
Maximum support spacing is a code question, not a strength calculation you make yourself. IPC Table 308.5 sets the maximum horizontal spacing by material: 12 ft for steel, 10 ft for copper 1-1/2 in and larger, 6 ft for smaller copper, 4 ft for PVC, and as little as 32 in for PEX. Those limits are governed by deflection and sag rather than by rupture, which is why the plastics are so much tighter even though they weigh less.
For lifting, use the empty weight and remember the shape. A 21 ft joint of NPS 6 schedule 40 is only about 400 lb, which two people can theoretically manage and should not - the length makes it unwieldy and the rigging point matters more than the weight. For estimating freight, multiply the empty weight per foot by the total footage; suppliers price steel pipe by the hundredweight and the numbers should agree closely with the published tables.
The filled figure is also what a hydrostatic test imposes. A long run that has never been full is at its heaviest during the test, and temporary supports fitted for construction are frequently sized for empty pipe only. If you are testing before the permanent hangers are complete, check the temporary ones against the filled figure.
Why empty pipe floats before it is backfilled
Weight is only half the story when a run sits in a flooded trench or is drained for a repair below the water table: the water the pipe displaces is pushing back. A submerged length floats the moment its empty weight per foot drops below the weight of the water it displaces, and that has nothing to do with contents - it is a comparison this calculator does not make for you.
The displaced-water force uses the same constant as the contents formula, 0.3403, but applied to the outside diameter instead of the bore: buoyant force per foot ≈ 0.3403 × OD². For NPS 6 schedule 40 steel, that is 0.3403 × 6.625² = 14.94 lb/ft against an empty weight of 18.99 lb/ft - the steel wins by a comfortable margin and stays down.
Plastic pipe is a different case. Dividing the two constants shows the crossover directly: a material floats when empty once its wall-to-diameter ratio falls below 0.3403 divided by its own weight constant - 0.03184 for steel's 10.69, but 0.1799 for PVC's 1.89. That is a wall-to-diameter ratio above roughly 1-in-5.6, expressed as SDR it means any SDR rating above about 5.6 is light enough to float. Every standard PVC and CPVC schedule (SDR 11 through SDR 41) is well past that line, so a drained plastic line in a wet trench floats essentially every time, regardless of its diameter.
The practical fix is procedural, not arithmetic: keep a plastic run either full of water or backfilled promptly, and never leave it empty and unrestrained in a trench with standing water. If you need the actual displaced-water figure for a specific pipe, compute 0.3403 × OD² yourself using the outside diameter you entered above and compare it to the empty-weight output.
Schedule 40 carbon steel: weight per foot empty and water filled
| NPS | OD (in) | Wall (in) | ID (in) | Empty (lb/ft) | Water (lb/ft) | Filled (lb/ft) | Empty (kg/m) |
|---|---|---|---|---|---|---|---|
| 1/2 | 0.840 | 0.109 | 0.622 | 0.85 | 0.13 | 0.98 | 1.27 |
| 3/4 | 1.050 | 0.113 | 0.824 | 1.13 | 0.23 | 1.36 | 1.68 |
| 1 | 1.315 | 0.133 | 1.049 | 1.68 | 0.37 | 2.06 | 2.50 |
| 1-1/4 | 1.660 | 0.140 | 1.380 | 2.28 | 0.65 | 2.92 | 3.39 |
| 1-1/2 | 1.900 | 0.145 | 1.610 | 2.72 | 0.88 | 3.60 | 4.05 |
| 2 | 2.375 | 0.154 | 2.067 | 3.66 | 1.45 | 5.11 | 5.44 |
| 2-1/2 | 2.875 | 0.203 | 2.469 | 5.80 | 2.07 | 7.87 | 8.63 |
| 3 | 3.500 | 0.216 | 3.068 | 7.58 | 3.20 | 10.79 | 11.29 |
| 4 | 4.500 | 0.237 | 4.026 | 10.80 | 5.52 | 16.32 | 16.08 |
| 6 | 6.625 | 0.280 | 6.065 | 18.99 | 12.52 | 31.51 | 28.27 |
| 8 | 8.625 | 0.322 | 7.981 | 28.58 | 21.68 | 50.26 | 42.54 |
Dimensions from ASME B36.10M. The empty column is 10.69 x t x (OD - t) and the water column is 0.3403 x ID squared, both computed with the same expressions this calculator uses.
Mistakes that produce the wrong weight
- Entering nominal size as the outside diameter. NPS 2 steel is 2.375 in across, and nominal 2 in copper is 2.125 in. Neither is 2.000 in, and using 2.000 understates a steel pipe's weight by about 20%.
- Using the wrong schedule. NPS 4 goes from 10.79 lb/ft in schedule 40 to 14.98 lb/ft in schedule 80 with no change in outside diameter. The wall is the whole difference.
- Forgetting the contents on a plastic system. Water is typically two to three times the weight of the pipe on plastics. Sizing supports on empty weight there is a serious error.
- Ignoring valves, flanges and fittings. These are concentrated loads and should sit at or immediately beside a support, not mid-span.
- Leaving out insulation and cladding. On a large insulated line these can add several pounds per foot, and outdoors they also collect snow and catch wind.
- Using plain-end weight for threaded and coupled pipe. The published plain-end figure excludes couplings; threaded and coupled pipe is slightly heavier per foot as delivered.
- Applying steel spacing rules to plastic. Sag, not strength, governs plastic support spacing, and the permitted spans are a fraction of steel's.
Where the dimensions come from
Outside diameters and wall thicknesses for steel pipe are set by ASME B36.10M for welded and seamless wrought steel pipe, and by ASME B36.19M for stainless steel pipe. Copper tube dimensions come from ASTM B88 in types K, L and M. Plastic pipe is dimensioned by schedule or by standard dimension ratio - in an SDR system the wall is a fixed fraction of the outside diameter, so a 6 in SDR 26 pipe has a wall of 6.625/26 = 0.255 in. Support spacing for plumbing systems is set by IPC Table 308.5 or the equivalent UPC table. This calculator computes weight from whatever dimensions you enter; it does not know which schedule you meant, so check the dimension table first.
Key terms
- Plain-end weight
- The weight per foot of pipe with square-cut ends and no couplings, as published in the ASME dimension tables. It is the figure this calculator produces.
- NPS
- Nominal pipe size. A designation, not a measurement: NPS 2 steel pipe has an outside diameter of 2.375 in and a bore near 2 in.
- SDR
- Standard dimension ratio - outside diameter divided by wall thickness. A constant SDR gives a constant pressure rating across sizes.
- Specific gravity
- Density of a liquid relative to water. Multiply the water content weight by it to get the weight of any other fluid.
- Hanger load
- The weight one support carries: filled weight per foot multiplied by the span, plus any concentrated items near it.