Most moisture damage inside a carton is not caused by a wet product, a leak or a bad supplier. It is caused by desiccant sizing that was done by habit rather than by arithmetic, with a sachet chosen because it was the size already in the storeroom. Cartons are a harder case than containers, because the volume is small, the packaging is thin, the transit can involve several climate zones, and the margin for error is measured in a couple of grams of silica gel. This guide sets out a practical desiccant sizing method built on four inputs, gives worked rules of thumb that can be applied at a packing bench without laboratory data, and explains the field factors that most often make a correct calculation go wrong.
Why Carton Desiccant Sizing Differs From Container Sizing
Container desiccant sizing starts with a large enclosed volume and a long voyage, and it is dominated by the container breathing cycle that produces condensation on the roof. The water load is large, the answer is usually expressed in kilograms, and the format is a pole or a blanket.
Carton desiccant sizing starts from the opposite end. The enclosed volume is a fraction of a cubic metre, the sachet weighs a few grams, and the dominant water sources are the product itself, the corrugated board and the labels rather than the air. Corrugated carton board at equilibrium in a fifty percent relative humidity warehouse can hold six to ten percent moisture by weight, and a one-kilogram carton may therefore contain fifty to eighty grams of water locked in the paper. That single number can exceed the entire capacity of a small sachet several times over, which is why desiccant sizing by air volume alone produces failures that appear inexplicable.
The correct approach is to size the desiccant against the water that will actually be released into the sealed cavity over the transit and storage period, and to accept that a carton is never truly sealed. Every desiccant sizing exercise is therefore a calculation plus a decision about how much of the load the desiccant is expected to handle.
The Four Inputs of Every Desiccant Sizing Calculation
Every defensible desiccant sizing starts with four numbers. Skip a step in that desiccant sizing and the result is a guess with a decimal point.
The first input is enclosed volume, measured inside the closed carton including the void around the product, not the outer carton dimensions. Poly bags, liners and internal dividers reduce the free volume and should be accounted for.
The second input is the exposure period: how many days from packing to unpacking, including warehouse dwell time at both ends, which is frequently longer than the sea transit itself.
The third input is the climate the carton will experience. The relevant figure is the maximum temperature the package will reach, because warm air holds far more moisture than cool air and drives the vapour pressure difference across the carton wall.
The fourth input is the moisture content of everything inside the carton at the moment of sealing. This is where most calculations are wrong, because the product, the board, the foam and the labels all carry water, and paper based materials carry the most.
Water Load From the Enclosed Air
The air inside a carton contributes less than most people assume, and it is worth calculating so that it can be set aside as the smaller part of the problem.
Take a carton with thirty litres of free volume. At twenty-five degrees Celsius and eighty percent relative humidity, saturated air holds roughly twenty-three grams of water per cubic metre, so the enclosed air contains about 0.55 grams of water. At the target interior condition of fifty percent relative humidity, the amount the desiccant must remove from the air to reach that point is roughly 0.2 grams. For a small carton in a moderate climate, the air is not the problem.
Change the climate and the numbers move, but not by orders of magnitude. Even a large carton in a hot humid port will hold only a few grams of water in its air. The lesson is that a desiccant sizing exercise that only considers air will systematically recommend a sachet that is far too small.
Water Load From the Product and the Packaging
In carton desiccant sizing the dominant term is almost always the moisture held in the materials.
Corrugated board is the largest single contributor in most cartons. A standard double-wall carton may weigh around seven hundred grams and hold between forty and seventy grams of water at typical warehouse humidity. Foam inserts, paper labels, moulded pulp trays and textile products add more. Food powders, timber and natural fibres can hold far more again.
Not all of that water will be released during transit, because equilibrium is slow. A reasonable field approximation is that between ten and thirty percent of the material moisture will migrate into the enclosed air during a typical thirty to sixty day cycle, and the desiccant should be sized to absorb that fraction while still holding the headspace below the target relative humidity.
That approximation is why a barrier liner changes the desiccant sizing so dramatically. Place the product inside a sealed polyethylene or foil liner and the water held in the outer carton is no longer part of the enclosed load, because it is outside the barrier. Adding a liner is often cheaper than multiplying the desiccant, and it also removes the daily exchange of vapour as the carton warms and cools.
| Carton type | Free volume | Board and inserts | Air at 25 C and 80 percent RH | Sizing priority |
|---|---|---|---|---|
| Small electronics carton | 5 litres | 0.3 kilograms | Under 1 gram | Liner plus 5 gram silica gel |
| Apparel carton | 30 litres | 0.8 kilograms | About 1 gram | 30 to 50 grams clay or silica gel |
| Machinery part carton | 60 litres | 1.5 kilograms plus foam | 2 to 3 grams | Liner plus 100 grams silica gel |
| Furniture carton | 200 litres | 4 kilograms | 8 to 10 grams | Liner plus 250 grams or a small pole |
Transit Days, Temperature and Altitude
Transit duration and temperature swing the answer more than packaging does at the margin.
A carton moving from a humid port into a cold climate experiences a falling temperature inside, which raises the relative humidity of the enclosed air and drives condensation onto the coldest surface, usually the inner face of the carton wall or a metal component. That is the same mechanism as container rain at a smaller scale, and it is the reason a carton that looks dry on arrival can contain a rusted bracket.
Transit through high altitude adds a subtle effect. Air freight holds are typically depressurised, and the reduced pressure reduces the absolute moisture content of the enclosed air, but it also increases the rate at which vapour migrates through the carton wall while the pressure difference exists. For air freight of sensitive goods, a liner and a slightly larger desiccant is the standard answer.
Long dwell periods are the most commonly underestimated factor. A carton that sails for twenty days and then sits for sixty days in a distribution centre has an eighty day exposure, not a twenty day one, and the desiccant sizing must reflect the full period.
A Field Method for Desiccant Sizing Without Lab Data
Where there is no time or equipment for measurement, a simple field method gets close enough for most general cargo.
Begin the desiccant sizing by estimating the free volume in litres. Estimate the water load as ten grams per kilogram of paper based packaging material inside the carton, plus twenty grams per kilogram of moisture holding product, plus one gram per thirty litres of air. Convert to the target material capacity: silica gel holds roughly twenty to thirty percent of its weight under these conditions, and clay holds roughly fifteen to twenty percent. Apply a margin of at least fifty percent, and round up to the next standard sachet size.
Then check the result against a sanity rule. For a sealed carton in a mild climate on a short route, one gram of silica gel per litre of free volume is a serviceable starting point. For a long ocean route with temperature swings, double it, or add a liner and keep the original figure.
The field method for desiccant sizing is deliberately conservative. Underestimating gives a failed shipment; overestimating costs a few cents.
Checklist Before You Seal the Carton
- Measure free volume inside the closed carton, not the outer dimensions.
- Count the full exposure period from packing to unpacking, including dwell time.
- Identify the maximum temperature the shipment will reach, not the average.
- Estimate the moisture carried by board, foam, labels and the product itself.
- Decide whether a barrier liner can replace part of the desiccant load.
- Choose the chemistry that matches the cargo, and confirm food contact requirements where they apply.
- Apply a margin of fifty percent or more and round up to a standard size.
- Confirm the sachet is intact, in date and has not been left open at the packing bench.
- Record the desiccant size, quantity and card reading for traceability.
- Place the sachet where vapour will reach it, not crushed under the product.
FAQ
Q1: How much desiccant does a carton need?
Start with one gram of silica gel per litre of free volume for a short, mild journey and scale up for longer routes, warmer climates and moisture holding products. A barrier liner reduces the requirement substantially.
Q2: Does the corrugated carton itself release moisture?
Yes, and it is usually the largest single source. Board at warehouse humidity can hold several percent of its own weight in water, which is why desiccant sizing must include packaging materials.
Q3: Is it better to add more desiccant or to add a liner?
A liner is usually the better investment, because it stops new moisture entering as well as reducing the initial load.
Q4: How does desiccant sizing change for air freight?
The lower cabin pressure increases vapour migration during the flight, so add margin and use a barrier liner for sensitive goods.
Q5: Can I use clay instead of silica gel for cartons?
Clay is often the cheaper and perfectly adequate choice for general merchandise in cartons, provided the quantity is sized on its lower capacity per unit weight.
Q6: Why did a carton fail even though desiccant sizing was done?
The usual causes are desiccant sizing on air volume alone, ignoring board moisture, a sixty day dwell period that was not counted, or a sachet that was crushed or already partly spent.
Q7: Should desiccant sizing be reviewed when the process changes?
Yes. A new carton supplier, a different foam, a longer route or a slower distribution network all change the water load and should trigger a fresh desiccant sizing review.
Conclusion
Desiccant sizing for a carton is a small calculation with a large consequence. Measure the free volume, count the real exposure days, find the peak temperature, and above all account for the water already sitting in the board, the foam and the product. Add a liner where the cargo justifies it, apply a sensible margin, and record what you did so the next shipment starts from data rather than from habit.
Dongguan Dingxing Industry Co., Ltd. manufactures silica gel, clay and calcium chloride desiccants in sachet, pole and blanket formats, together with oxygen absorbers and ethylene absorbers. Send us your carton dimensions, product description, route and transit duration and our team will complete the desiccant sizing and recommend a specification.
