Underfloor-heating pipe is the textbook "light and bulky" cargo: weight almost never limits the load, volume always does. Containers are chosen by internal cubic capacity, so the buyer's job is to work out how many metres fit into a 20- or 40-foot box in order to plan the order size and the freight cost per metre.
Usable volume of standard containers
The volume you can actually fill is lower than the nominal figure: gaps between coils, dunnage posts, dead space by the doors and an incomplete top row all eat into it.
- 20GP — about 28 m³ usable.
- 40GP — about 58 m³.
- 40HQ — about 68 m³.
These figures are approximate and for estimation only; confirm actual capacity from the supplier's packing list.
The method
The package volume of one coil depends on the coiled outer diameter and the coil length — bigger pipe and longer coils mean a bigger "box". Metres per container are therefore computed as:
The 0.8 stowage factor covers the voids between round coils, pallet height and the unusable top layer. For palletised rectangular stacking you can use 0.85–0.9; for loose round coils, closer to 0.75.
Illustrative example
Say a 16×2 mm, 120 m coil in its packaging occupies about 0.30 m³. For a 40HQ: 68 × 0.8 = 54.4 m³ of effective load; 54.4 ÷ 0.30 ≈ 181 coils; 181 × 120 m ≈ 21 700 m of pipe. This is an order of magnitude, not a contractual number.
| Pipe size | Coil length | Coil volume (est.) | Coils per 40HQ | Approx. metres per 40HQ |
|---|---|---|---|---|
| 16×2 mm | 120 m | ~0.30 m³ | ~180 | ~21 500 m |
| 16×2 mm | 240 m | ~0.55 m³ | ~98 | ~23 500 m |
| 20×2 mm | 120 m | ~0.38 m³ | ~143 | ~17 100 m |
| 20×2.3 mm | 200 m | ~0.75 m³ | ~72 | ~14 400 m |
| 26×3 mm | 300 m | ~1.60 m³ | ~34 | ~10 200 m |
Note the pattern: the longer coil carries more metres per cubic metre (fewer packages, fewer voids), but it needs a forklift or crane and a strong pallet. The table is an example for estimation — real package dimensions vary with coil-winding style and must always be confirmed against the factory's packing list.
Reverse calculation: from metres to containers
What a buyer usually needs is not "how much fits" but "how many boxes my quantity takes". Flip the formula:
Sixty thousand metres of 16×2 mm at 21 500 m per 40HQ comes to three containers. If the order is well below one 40HQ, compare two routes: consolidating into a 20GP or shipping LCL (groupage). Below roughly 15–18 m³, groupage is often cheaper, but slower and harder on the coils, which get crushed in transit warehouses.
Weight, pallets and MOQ
The net weight of 16×2 mm PE-RT or PEX coil is about 0.11 kg per metre, so even 20 000 m is only around 2.2 t — far below the ~26 t payload of a 40HQ. Volume is the binding constraint, which is why long coils and tight stacking pay off. On pallets the coils are held with strapping and corner protectors; loose-stacked rows lose cubic capacity to leaning and to the incomplete top layer. One more practical point: each size carries its own MOQ, usually a multiple of whole coils, so a mixed load solves two problems at once — it fills the cubes and tops up the slow-moving sizes without dedicating a container to them.
One 40HQ or two 20GP: which is cheaper per metre
Compare freight per metre of pipe, not the headline price of the box:
A single 40HQ is almost always better value than two 20GP: the ocean rate for a 40-foot container is roughly 1.6–1.8 times that of a 20-foot one while carrying double the volume, and the shipment attracts one set of local charges — clearance, inspection, terminal-to-warehouse trucking — instead of two. The exception is an urgent order spread across several slow-moving sizes destined for different sites, where two smaller boxes give more flexibility.
Check the internal height as well: 2.70 m in a 40HQ against 2.35 m in a 40GP. If the coils can take a fourth tier, that tier alone is where the 10–15 % saving on freight per metre comes from.
Once you know the metres per container, you can convert an FOB price into freight-per-metre and compare one 40HQ against two 20GP. The next step is customs and duty — covered in article 09.