WMO-No. 8 Annex 4.B

Container desiccant calculator

Stop container rain before it wets the bags.

Enter the container, the stuffing climate, the coldest temperature and the pallets and packaging to get the water load in kilograms and the desiccant capacity it needs.

Last updated 2026-09-28. Math runs in your browser, no data leaves your computer.

Work out the water first and the desiccant second. The answer is a water load in kilograms, which you then divide by the uptake your own desiccant supplier states, so the result holds for any brand of bag, strip or pole.

The pallets usually decide the answer. Weigh one and check its moisture content with a pin meter before stuffing, because that number moves the result more than anything else on this form.

Units

Internal volumes are the ONE dry container load capacities, and other carriers' boxes differ by well under one cubic meter.

Use the stowed volume from the packing list, since only the air left around the cargo carries water vapour.

Take the temperature inside the warehouse or on the dock when the doors close, since that air is what gets sealed in.

A monsoon season dock in South China often reads far higher than an air conditioned warehouse, so measure it on the day if you can.

Use the lowest night or destination temperature the box may see, because the steel roof cools to about that and the water condenses there.

Leave 100 to size against condensation only, or enter the lower limit your product spec gives for caking or clumping.

Add the packaging next. Enter zero weight for any line that does not apply, and take the equilibrium moisture content from your pallet or carton supplier, or from a published wood table for the humidity you expect inside the box.

Weigh one pallet and multiply, since pallet weight swings widely with timber and build.

Read it with a pin meter on a few boards, because the stamp on the pallet says nothing about how dry the wood is today.

This is where the wood settles in the container air, and the gap to the loading figure is the water it gives up.

Count only the paper and board, not the product, since the bag contents are covered by the product's own moisture spec.

Ask the bag or carton maker for the moisture content at delivery, because board stored on an open dock takes water up fast.

Use the maker's figure at the humidity you expect in the box, since paper settles lower in drier air.

Leave at zero unless you have a measured rate, because TIS says outside air usually adds no moisture to a closed box.

Door to door days, which only count against the leak rate above, so they matter only when that rate is above zero.

Take it from the supplier datasheet at the temperature and humidity you expect, since uptake falls sharply in drier air.

Before the booking, screen the name on the invoice. Shipment pre-check is on the paid plans, from USD 59 a month.

General guidance only. Verify against the cited primary sources before you commit to a shipment, declaration, or contract.

How the water load is worked out

Saturation vapour pressure over water is ew(t) = 6.112 exp[17.62 t / (243.12 + t)] hPa, and over ice below 0 °C it is 6.112 exp[22.46 t / (272.62 + t)], the formulae in WMO-No. 8 Annex 4.B (2008 edition, copy hosted by the National Weather Service; the current edition is on the WMO library). The moist air factor f(p) is applied at 1,013.25 hPa, and the ideal gas law with 461.5 J/(kg·K) for water vapour turns vapour pressure into grams per cubic metre.

Those figures match the TIS climate table to one decimal, for example 24.3 g/m³ at 30 °C and 80 percent humidity and 6.8 g/m³ at 5 °C and saturation. The air term is the free air volume times the gap between the stuffing air and what the air can hold at the coldest temperature and your humidity limit.

Wood and paper release dry mass × (moisture at loading − equilibrium moisture), with dry mass = weight / (1 + moisture at loading). That is the full drying from one state to the other, which is the worst case for a voyage. Leak air adds changes per day × days × free air volume × the same humidity gap, using the stuffing air as the outside air.

The TIS page lists the cargo, its packaging, the wooden floor and dunnage as the main water sources, and a hygroscopic product also trades water with the air. The product and the floor are not modelled, so leave headroom when either is damp.

Worked example. Desiccant sized for the air, not the pallets

The booking. A 40 ft high cube of bagged hygroscopic salt leaves Shanghai in August, stuffed at 30 °C and 80 percent humidity, with the cargo taking 40 m³ and a winter arrival where the box drops to 5 °C. The air holds 24.3 g/m³ at stuffing and 6.8 g/m³ at 5 °C, so the 36.2 m³ of free air carries 36.2 × 17.5 = 634 g to condense, and the shipper hangs 2 kg of desiccant, which looks fine on paper.

The failure. The 20 pallets weigh 500 kg in all and the meter reads 20 percent moisture, against an equilibrium of 14 percent assumed for this example. Their dry mass is 500 / 1.20 = 417 kg, so they can give up 417 × 0.06 = 25 kg of water, forty times the air. The choice is the usual one: dry the pallets, or hang far more desiccant.

The fix. The shipper swaps to pallets dried to 14 percent, so the wood term drops to zero, and sizes for the air with room to spare. At a supplier uptake of 100 percent of own weight the 634 g needs about 0.6 kg of desiccant, and hanging 3 kg covers the paper bags and a damp floor with margin.

Checking what weight the same box may carry on the truck at each end? The container road weight limit calculator works out the legal cargo for US Interstates and Australian roads.

Frequently asked

How much desiccant do I need for a shipping container?

Enough absorbent capacity to hold the water that the air, the pallets, the packaging and any leaks can release on the voyage, measured at the conditions inside the box. This tool adds those sources up in kilograms of water. Divide that by the uptake your desiccant supplier states for the expected temperature and humidity, and you have the kilograms of desiccant to hang, whatever brand you buy.

What causes container rain?

Warm, moist air inside the box meets a roof or wall that has cooled below the air's dew point, often overnight or on a cold ocean leg, and the water condenses and drips onto the cargo. The TIS container climate page from the German insurers names the cargo, its packaging, the wooden floor and dunnage as the largest sources of that water in a closed container.

How is the water in the container air worked out?

From the temperature and relative humidity at stuffing. The saturation vapour pressure comes from the Magnus type formula in WMO-No. 8 Annex 4.B, the ideal gas law turns vapour pressure into grams of water per cubic metre, and the same sum at the coldest temperature gives what the air can still hold. The difference, times the free air volume, is the water that would condense.

Why do wooden pallets matter so much?

Wood holds water as a share of its own dry weight and gives it up as it dries towards equilibrium with the air around it. A load of pallets at a high moisture content can hold tens of kilograms of water that it is ready to release, which is far more than the few hundred grams in the container air. Measuring the pallets with a moisture meter before stuffing is the cheapest check in the whole calculation.

Does a longer voyage need more desiccant?

Not by itself, in this model. The TIS page says incoming outside air usually has no negative impact, because the inside of the box is generally warmer than outside. Longer voyages give the packaging more time to reach equilibrium and more day and night cycles, which is why the tool assumes every source releases all it can. Leak air only counts if you enter an air change rate for a box you know is not tight.

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Sourzi shows what official files say. It is not customs or legal advice. For a binding answer, ask a licensed customs broker, a CBP ruling or an Australian Border Force Tariff Advice.

File dates and links are on Data sources.