Illustrative concept: samples and recording materials prepared for a moisture review.
Illustrative weight-planning context. No scale reading, density or measured moisture content is supplied by the image.Dry-basis moisture content compares water mass with oven-dry wood mass. It is not the percentage of the current wet mass that is water.
- 01Oven-dry wood mass
- 02Water mass
- 03Current total mass
The denominator changes the meaning
In the dry-basis convention, moisture content is water mass divided by oven-dry wood mass, multiplied by 100. A hypothetical piece with 10 kg of dry wood and 2 kg of water has 20% dry-basis moisture content and 12 kg total mass. Water is one-sixth of that current mass, not one-fifth.
This distinction matters when estimating the effect of a moisture change. Subtracting the stated moisture percentage directly from the wet mass uses the wrong denominator. The site’s scenario tool first estimates the dry mass, then calculates the target total under a constant-dry-mass assumption.
A product label does not supply a measurement
Green, air-dried and kiln-dried describe conditions or processes, but they do not provide a current measured moisture value for every piece. Request the relevant specification or measurement record when the calculation depends on one. Include the method, date and sampled material.
The USDA handbook describes wood’s exchange of moisture with its environment. That is why a previously reported condition and a current shipping condition may differ. This reference does not predict drying time, equilibrium at your site or installation readiness.
Document the scenario limits
The mass-change tool assumes the same dry material remains present. Cutting, decay, treatment uptake, coatings, packaging changes and material loss can break that assumption. The tool does not calculate shrinkage, drying stresses or future dimensions.
Use the output to explore a stated what-if question. If a final dispatch mass is required, confirm how the material will be weighed or documented rather than treating the moisture scenario as a substitute for that process.
VISUAL FIELD NOTE · WORKED EXAMPLE
Understand moisture without changing the denominator
Both bars contain the same 100 kg of oven-dry material. Only the water portion changes.
Read the numbers and text explanation
- Initial scenario
- 20 kg water ÷ 100 kg dry wood = 20% moisture; total mass 120 kg
- Target scenario
- 10 kg water ÷ 100 kg dry wood = 10% moisture; total mass 110 kg
- Difference
- 10 kg less water and 10 kg less total mass
- The incorrect shortcut
- Subtracting 10% of 120 kg gives 108 kg, which uses the wrong denominator
Wood moisture content uses dry wood as the denominator
In the conventional dry-basis definition, moisture content equals the mass of water divided by oven-dry wood mass, multiplied by 100. It is not the percentage of the current wet mass that is water. Confusing these definitions leads to an incorrect weight correction even when the arithmetic is otherwise tidy.
Take an illustrative piece containing 10 pounds of dry wood and 2 pounds of water. Its total mass is 12 pounds and its dry-basis moisture content is 20%. The water represents only 16.67% of the current total mass. Both percentages can be calculated, but only the first is the wood moisture input expected by this tool.
Convert mass without pretending to predict drying
If the initial total mass and its dry-basis moisture content are known, infer dry mass by dividing the total by 1 + moisture content/100. Multiply that dry mass by 1 + the target moisture content/100 to obtain the second scenario. This assumes the amount of dry material stays constant. It does not account for wood removed by machining, decay, coatings added or packaging changed between measurements.
For a fictional 120-pound lumber-only quantity at 20% moisture, inferred dry mass is 100 pounds. At an assumed 12%, the same dry material would weigh 112 pounds. The reduction is 8 pounds. Subtracting eight percentage points directly from 120 pounds would give the wrong change because the percentages use dry mass as their denominator.
| Quantity in the example | Calculation | Result |
|---|---|---|
| Initial mass | Entered lumber-only mass | 120 lb |
| Inferred dry mass | 120 ÷ 1.20 | 100 lb |
| Target mass | 100 × 1.12 | 112 lb |
| Modeled water-mass difference | 120 − 112 | 8 lb |
Do not apply the wood correction to the pallet
Packaging, straps, hardware and the wood itself need separate scopes. If a scale record includes a pallet, do not treat all of its mass as though it shares the boards' moisture condition. First establish the lumber-only mass or acknowledge that the available gross figure cannot support that correction accurately.
Likewise, a single moisture reading may not describe every board in a mixed pack. The mass model needs a defensible moisture basis for the quantity it represents. If that basis is unavailable, compare explicit scenarios rather than reporting the calculated change as a measured shipment difference.
Mass change and density change are not interchangeable
Wood dimensions can change with moisture. A mass-only scenario deliberately leaves the volume question unanswered. Do not take the resulting target mass and divide it by an old volume, then call the answer a verified density at the new condition. That would add a hidden constant-volume assumption.
The USDA Wood Handbook moisture chapter explains the dry-basis definition and the connection between moisture and physical properties. Use the moisture calculator to inspect the mass arithmetic. It does not predict drying time, prescribe an installation moisture target or determine whether a particular board is ready to use.
Evidence & scope
Wood–water relationships and physical properties. General reference, not a supplied-product weight certificate.