Inside the published range.
Planning estimate. We confirm the final quantity in writing against your cargo moisture, packing and trade lane before you order.
Most online desiccant calculators apply one number to every container. That number is a useful starting point and a poor answer, because the water a container has to deal with is not fixed. This tool keeps the published industry range visible as a cross-check and then adjusts the water load for the conditions you actually have. Every step is shown, so you can disagree with a specific input rather than with the whole result.
The published shorthand — 4–6 kg of desiccant for a 20ft, 8–12 kg for a 40ft — is applied to calcium chloride, which is dimensioned at 150% of its own weight. Taking the midpoints gives 0.2273 kg of water per m³ of container air. At baseline conditions this calculator therefore reproduces the published range rather than contradicting it.
The baseline is multiplied by how humid the loading port is, how long the voyage runs, how hygroscopic the cargo is and whether the pallet wood is genuinely dry. Voyage length is treated as close to linear in time, and the humidity figures are standard psychrometric values at the loading temperature.
The water load is divided by the working capacity of the desiccant you chose: 1.5 kg of water per kg for calcium chloride, 0.30 kg per kg for container-grade silica gel. That is why the two chemistries give answers five times apart on the same shipment.
| Variable | Effect on the dose |
|---|---|
| Container volume | A 40ft holds about twice the sealed air of a 20ft, so it carries roughly twice the water vapour before the voyage even starts. |
| Climate at loading | Tropical or monsoon loading seals in far more water than a cool dry inland loading point — the single largest factor you control. |
| Voyage duration | Door gaskets leak about 0.6% of the container volume every day. A 45-day voyage carries roughly twice the ingress of a 21-day one. |
| Cargo type | Hygroscopic cargo — textiles, leather, paper, food, wood — carries its own moisture and releases it as the box heats and cools each day. |
| Pallets and dunnage | Air-dried or unknown wood is the classic hidden source. An ISPM 15 HT stamp certifies pest treatment, not dryness. |
| Desiccant chemistry | Calcium chloride and silica gel differ by roughly five times in the mass needed for the same duty. |
Use this if JavaScript is unavailable, or as a sanity check on the figure above. It assumes a warm humid loading port, a 30-day voyage, non-hygroscopic cargo and dry pallets.
| Container | Air volume | Water load | Calcium chloride | 1 kg strips |
|---|---|---|---|---|
| 20ft | 33 m³ | 7.5 kg | 5.0 kg | 5 |
| 40ft | 67 m³ | 15.2 kg | 10.2 kg | 11 |
| 40ft high cube | 76 m³ | 17.3 kg | 11.5 kg | 12 |
This is the same checklist we use internally. With these answers we can state a confirmed quantity and price instead of a range — and tell you if your shipment is one where a liner, not more desiccant, is the answer.
Send the eight points above. We supply container desiccants and food and pharmaceutical packaging desiccants direct from our own production, from one tonne with OEM and ODM available.
On the published industry shorthand, 4 to 6 kg of calcium chloride desiccant for a 20ft container and 8 to 12 kg for a 40ft. That shorthand assumes a standard route, a standard climate and inert cargo. Short dry routes with non-hygroscopic freight sit at the bottom of the range; long tropical voyages with hygroscopic cargo and air-dried pallets go above it.
Because the rule of thumb assumes conditions that rarely hold. It ignores how hygroscopic the cargo is, the dew point shift between loading and discharge port, and how much humidity was sealed in when the doors closed. This calculator keeps the published range visible as a cross-check, then adjusts the water load for the six variables that actually decide it.
Calcium chloride for container duty. Silica gel adsorbs roughly 40% of its own weight; calcium chloride absorbs up to 300%, and although about 150% is the conservative figure used for dimensioning, that still means silica gel needs about five times the mass for the same job. Silica gel remains the right answer inside individual cartons and for short low-humidity shipments.
Hang the strips evenly along the ceiling line and upper corrugations, not clustered in one spot. Condensation forms on the roof, so the desiccant belongs above the cargo. Floor bags suit doses above about 10 kg, part-loaded containers, or stows where nothing can hang. Never lay strips flat on top of cartons, and load the desiccant last, just before the doors close.
Ten 1 kg hanging strips. Container-grade strips are normally supplied at 1 kg or 2 kg each, so 10 kg is ten 1 kg strips or five 2 kg strips. Distribute them along the ceiling rather than bunching them by the door.
Beyond roughly 50 days at sea, or on a near-50-day voyage with hygroscopic cargo and wet pallet wood, daily leakage through the door gaskets outruns any practical desiccant load. Those shipments need a sealed container liner, with desiccant used as backup rather than as the primary moisture barrier.
Our guide to container desiccant dosage explains the condensation cycle behind these numbers, and the calcium chloride page covers strip formats and capacities.
