Wood Moisture Content: the Number Behind Every Kiln Schedule
What MC actually means, why it is not the same as water weight, and how to use it for shipping, drying and inventory decisions.
MC is water weight divided by dry wood weight
Moisture content on the oven-dry basis (the industry default) compares the water in a sample to the wood substance after oven-drying: MC_od = (W_wet − W_od) / W_od × 100. 100 g of oven-dry wood holding 50 g of water sits at 50% MC; 100 g of dry wood holding 150 g of water sits at 150% MC. The wet basis (common in soil science) divides by the wet mass instead — they are interchangeable, just do not mix them in the same calculation. The Moisture content tool accepts both and converts.
Fresh tropical wood regularly exceeds 100% MC
Acacia mangium sapwood at 70–90% MC, eucalyptus 80–120%, and freshly felled pine sapwood 100–180% are routine. A cubic metre of acacia at 73% MC weighs 970 kg, but the dry wood inside is only 560 kg — the other 410 kg is water that will leave during drying. This is why shipping green timber by truck or container is dominated by water weight, not wood.
MC drives the kiln schedule, not the calendar
Kiln operators start from the initial MC of the charge and run the schedule down to a target MC for the in-service climate. Typical targets: furniture for export 8–10% MC, US interior 6–10%, exterior joinery 12%, pallets 19%. The Drying mode of the Moisture content tool returns the final mass, the dry mass (which never changes), and the water mass lost — these are the three numbers you need to log for each charge.
The dry mass is the constant
The Wood Handbook 2021 Eq. 4-2 (W₂ = W₁ × (100+MC₂)/(100+MC₁)) keeps the dry mass constant. 500 kg of wood at 60% MC contains 312.5 kg of dry wood; dried to 12% MC the same 312.5 kg of wood holds 37.5 kg of water, total 350 kg. A common online 'shortcut' uses the inverse ratio (1+MC₁)/(1+MC₂) and gives 714 kg — impossible. If your number goes up while drying, the formula is wrong.
When in doubt, measure
Pin-style resistance meters are reliable between 6 and 30% MC; dielectric (pinless) meters work in 4–30% and are good for finished furniture but need species-and-temperature correction. Calibrate against an oven-dry sample (103 °C until constant mass, usually 24 h for 25 mm boards) at least once per species per kiln. The Moisture content tool handles the math; the meter and the kiln schedule handle reality.