How Many Desiccant Bags per Container? A 20ft & 40ft Dosage Guide

"How many desiccant bags do I need for a 20ft or 40ft container?" is the single most common question we receive from importers shipping to the Gulf. There is no single fixed number — the right quantity depends on the cargo, the voyage length, the season and the destination climate. This guide gives you the working formula, indicative quantities, and the specification points that decide whether your desiccant system actually works.

Why the Quantity Is Not a Fixed Number

A desiccant only protects cargo if it removes water vapour faster than the cargo and container release it. So the required capacity is driven by three variables:

• Cargo moisture content — hygroscopic goods (wood, cardboard, textiles, leather, food, paper) release far more moisture than metal parts or plastic.
• Voyage duration — a 12-day Asia–Europe run needs roughly half the capacity of a 35-day passage with long port dwell times.
• Climate swing — a container loaded at 20 °C that arrives at a 45 °C Gulf terminal in high humidity is the worst case; the internal dew point depends on the coldest surface the air touches.

The industry rule of thumb for a well-specified calcium chloride container desiccant is 1 kg of desiccant per 1–2 cubic metres of container air for a standard voyage, but the cargo-derived moisture load often dominates — which is why we recommend sizing from the cargo, not just the container volume.

Indicative Quantities for 20ft and 40ft Containers

The table below gives practical starting quantities for calcium chloride container desiccants (the standard choice for ocean freight). Use it as a baseline, then adjust for voyage length and cargo type.

These figures assume a typical 20–30 day voyage in temperate-to-tropical conditions. For peak-season Gulf arrivals (Jun–Sep), or for very damp cargo such as green timber or leather, move to the upper end or above.

How to Size More Precisely

For a defensible specification, estimate the total water load and match it to desiccant capacity:

1. Cargo moisture: multiply cargo weight by its moisture content (e.g. 20 t of wood at 15% = 3,000 kg of water that can be released).
2. Air and packaging moisture: allow roughly 1 kg of water per 10–15 m³ of container volume for packaging and pallets.
3. Desiccant capacity: a good calcium chloride unit absorbs 100–200% of its own weight in water vapour over a long voyage. Divide the total water load by the per-unit capacity to get the minimum number of units.
4. Safety margin: add 20–30% for weather variability and port delays.

Placement Matters as Much as Quantity

Putting all the desiccant in one corner will not protect a full container. To work, the units should be distributed so they act on the air where condensation actually forms:

• Hang units from the side wall lashing rings, spaced evenly along the length of the load, at roughly knee-to-chest height.
• Keep desiccants clear of the cargo by 10–15 cm so air can circulate; never bury them inside cartons.
• Do not block the container's door area — leave one unit near the doors, since the doors are the coldest surface on arrival.
• Seal the container as tightly as possible; the desiccant can only hold humidity down if outside air is not constantly entering.

Calcium Chloride or Silica Gel?

For container shipping, calcium chloride is the default: it holds a very high capacity and keeps absorbing even at high relative humidity. Silica gel is better suited to sealed packaging (electronics, pharma, food) where humidity is moderate and the pack is air-tight. Many exporters use both — bagged silica gel inside cartons, container desiccants on the walls — for layered protection.

Common Mistakes That Cause Claims

• Under-dosing on the cheapest unit count — the most expensive error; a claim costs far more than a few extra kilos of desiccant.
• Ignoring cargo moisture — sizing purely by container volume underestimates damp cargo.
• Poor placement — units piled at one end or buried in the load.
• Re-used or expired desiccant — calcium chloride that has already absorbed water is spent.