Desiccant Calculator: How Much Desiccant Do You Need?
A unit of desiccant is the amount that adsorbs at least 3 grams of water vapor at 20 percent relative humidity and 6 grams at 40 percent, at 77°F, under the MIL-D-3464 standard. For a sealed barrier bag, MIL-STD-2073 sizes the charge as 0.011 units per square inch of bag surface plus 8 units for every pound of paper, cardboard or wood inside. For a rigid sealed container it is 0.0007 units per cubic inch, about 1.2 units per cubic foot, plus the same allowance for packing material. Enter your package below.
| Packet | Packets by listed coverage |
|---|
Packet counts divide the package volume by each packet's listed coverage. MIL-STD units are the stricter number for sealed military-style packs with dunnage inside.
The desiccant formulas
MIL-STD-2073-1, the US military packaging standard, gives two formulas for the units of desiccant a sealed pack needs. U is units of desiccant and D is pounds of packing material inside the seal.
Sealed barrier bag or other flexible enclosure: U = 0.011 × bag area in square inches + dunnage allowance
Rigid sealed metal container: U = 0.0007 × volume in cubic inches + dunnage allowance
| Material inside | Units per pound |
|---|---|
| Paper, cardboard, wood and other cellulose | 8.0 |
| Bound fibers, such as rubberized hair or fiber pads | 3.6 |
| Glass fiber | 2.0 |
| Synthetic foam and rubber | 0.5 |
The bag area is the total surface of the enclosure: two times length times width, plus two times length times height, plus two times width times height. The rigid-container formula applies to sealed metal containers and to other rigid containers with a comparably low vapor transmission rate.
Desiccant units for common packages
| Package | Size | Cardboard inside | Units | Silica gel |
|---|---|---|---|---|
| Sealed barrier bag | 12 x 9 x 4 in | 0 lb | 5 | 130 g |
| Sealed barrier bag | 12 x 12 x 12 in | 0 lb | 10 | 260 g |
| Sealed barrier bag | 12 x 12 x 12 in | 2 lb | 26 | 676 g |
| Sealed barrier bag | 24 x 18 x 12 in | 0 lb | 21 | 546 g |
| Sealed barrier bag | 24 x 18 x 12 in | 5 lb | 61 | 1,586 g |
| Rigid sealed container | 12 x 12 x 12 in | 0 lb | 2 | 52 g |
| Rigid sealed container | 24 x 18 x 18 in | 0 lb | 6 | 156 g |
| Rigid sealed container | 48 x 40 x 40 in | 0 lb | 54 | 1,404 g |
The cardboard rows show why packing material matters: two pounds of corrugated inside a sealed 12 inch bag more than doubles the desiccant it needs, because paper fiber stores moisture and releases it into the sealed air.
How many grams is a unit of desiccant?
The standard defines a unit by what it adsorbs, not by weight. In practice one unit is about 26 grams of silica gel and about one ounce of clay. A unit must weigh no more than 50 grams.
Desiccant packets by coverage
Each stocked packet lists the volume it protects. For a closed carton or bin, divide the inside volume by the packet's coverage and round up.
| Packet | Coverage per packet | Packets per case |
|---|---|---|
| Silica gel, 5/8 x 1 9/32 in | 24 cu in, 0.01 cu ft | 6,000 |
| Silica gel, 7/8 x 1 1/2 in | 47 cu in, 0.03 cu ft | 5,000 |
| Silica gel, 7/8 x 2 1/8 in | 94 cu in, 0.05 cu ft | 3,000 |
| Silica gel, 1 1/16 x 2 1/2 in | 141 cu in, 0.08 cu ft | 3,000 |
| Clay, 1 x 2 1/2 in kraft | 242 cu in, 0.14 cu ft | 1,200 |
| Clay, 1 x 3 1/2 in Tyvek | 484 cu in, 0.28 cu ft | 700 |
| Clay, 3 x 3 in kraft | 726 cu in, 0.42 cu ft | 550 |
| Clay, 3 x 4 in Tyvek | 1,434 cu in, 0.83 cu ft | 300 |
| Clay, 3 x 6 in Tyvek | 2,886 cu in, 1.67 cu ft | 150 |
Get more from desiccant
- Seal the package. Desiccant only works in a closed space. In an open box it pulls moisture from the room until it is saturated.
- Keep packets sealed until use and close the pail or bag right after taking what you need.
- Cut the paper inside the seal. Cardboard inserts and paper fill add 8 units per pound; foam adds 0.5.
- Check with a humidity indicator card from the desiccant collection; when the spot changes color, the charge is spent.
- Use a moisture barrier. The moisture protection guide covers liners, VCI and sealing.
Frequently asked questions
How much desiccant do I need?
For a sealed barrier bag, about 0.011 units per square inch of bag surface plus 8 units for every pound of paper or cardboard inside. For a rigid sealed container, about 1.2 units per cubic foot plus the same packing allowance. The calculator does both.
How much desiccant per cubic foot?
About 1.2 units per cubic foot for a rigid sealed container under MIL-STD-2073, which is roughly 31 grams of silica gel. Flexible bags are sized by surface area instead, and packing material inside adds more.
What is a unit of desiccant?
The amount that adsorbs at least 3 grams of water vapor at 20 percent relative humidity and 6 grams at 40 percent, at 77°F, as defined in MIL-D-3464. That is about 26 grams of silica gel or one ounce of clay.
Silica gel or clay desiccant: which is better?
Both are rated in the same units. Clay is the economical choice for cartons and general storage; silica gel is common in small packets for food, pharmaceuticals and electronics. Check the packet for food-contact approval when it goes near food.
Does desiccant work in an open box?
Not for long. Desiccant only controls humidity in a sealed space; in an open box it adsorbs moisture from the room until it is saturated.
How many silica gel packets do I need for a box?
Divide the box's inside volume by the packet's listed coverage. A 12 x 12 x 12 inch box is 1,728 cubic inches, so it takes 19 packets rated for 94 cubic inches, or 13 packets rated for 141 cubic inches.
Related tools and charts
- How to protect packages from moisture
- VCI packaging guide
- Desiccant packs and humidity indicators
- Cubic feet calculator
- All packaging calculators and tools
- What is a desiccant? Silica gel packets explained
Formulas from MIL-STD-2073-1E, paragraph 5.2.3.7. Unit definition from MIL-D-3464E. Grams per unit are typical manufacturer figures. Packet coverage is each product's listed specification.