Wood Moisture Content
Red Calc Tool User Guide
What this tool can do for you
You can use this tool for many purposes, usually with help of a moisture meter
(see photo below), including the determination of
average relative humidity. Enter the air temperature and equilibrium moisture
content of a sample piece of wood that has been in the space. The RED tool
will calculate the average relative humidity for you.
Use this tool to decide if new wood flooring has been properly acclimated.
You will need a moisture meter (see photo below) and
this RED tool to determine when the new flooring is at the ideal 6 to 9 percent
equilibrium moisture content (EMC).
If you burn firewood, you can use this tool and your moisture meter to determine
the equilibrium moisture content (EMC) based on the average outdoor temperature
and relative humidity. Dry firewood should have an EMC of 20 percent or less.
Wood rot has been reported to occur at moisture contents above
20 percent. Use this RED tool and a wood moisture meter to find a safe combination
of ambient temperature and relative humidity in order to prevent rot problems.
Relative humidity - average relative humidity of the air
surrounding the sample of wood over the last week or so. This assumes that the wood has
reached equilibrium with its surroundings and is not in the process
of drying (such as firewood) or wetting.
Equilibrium moisture content (EMC) - the moisture content
of a wood sample when the sample is in equilibrium with the
surrounding air (it is neither drying nor wetting). Wood will
eventually reach equilibrium if it remains in air where the
temperature and relative humidity are constant.
Clicking the label for any input or result will cause a
popup help box to appear. This help box includes the
allowed values (for inputs).
The equilibrium moisture content of wood is affected more
significantly by the relative humidity than by the temperature
of the surrounding air.
A wood moisture meter is a useful tool to use with
this RED Calc Wood Moisture Content tool.
Wood dried outdoors, firewood, for example, usually reaches
an equilibrium moisture content of 15 to 20 percent. Freshly-cut (green)
firewood usually has a moisture content around 100 percent, meaning half
the weight of the wood is water.
Kiln dried lumber usually has an equilibrium moisture content
of 6 to 8 percent when removed from the kiln.
When the moisture content of wood is above 20 percent moisture,
it can support the growth of wood-rotting fungi.
Inputs and field measurements
Ambient air temperature - temperature of air around a wood sample,
measured with an ordinary thermometer.
One of the following two metrics in the bottom section of the tool
must be entered as an input. Click the radio button to select the
value you wish to solve; you must enter the other value.
Relative humidity - see "Calculated values" above.
Equilibrium moisture content - see "Calculated values" above.
Wood is hygroscopic, which means its moisture content will change
in relation to the relative humidity (RH) and temperature of the
surrounding air. As relative humidity increases, the moisture
content of wood increases; as temperature increases, the moisture
content of wood decreases. When the wood sample fully acclimates
to the conditions of the surrounding air, it reaches its
Equilibrium Moisture Content (EMC).
The moisture content of a sample is defined as the ratio of
the weight of the water in the sample to the weight of the sample
with all moisture removed (dry weight), expressed as a percentage:
100 x (water weight) / (dry weight).
Thus, if freshly cut firewood
is 100 percent moisture content, half of its weight is water.
Hardwood milled for furniture, flooring, and cabinets is usually
carefully kiln dried to 6 to 8 percent moisture content. Once this
lumber leaves the kiln, it should be properly stored in conditions
that will not result in the regain of moisture. For a 7 percent
wood moisture content, this means 35 percent relative humidity at
70 °F (21 °C). If the surrounding air conditions are
changed to 80 °F (27 °C) and 80 percent relative humidity,
the EMC of the wood will more than double to almost 16 percent.
When installing new wood flooring, it is best if it has an Equilibrium Moisture Content (EMC)
from 6 to 9 percent; this equates to 70 °F (21 °C) at 30 to 50 percent
relative humidity. Make sure you measure the moisture content of the flooring with a moisture meter
when it arrives at the site. The goal is for the new
flooring to reach its equilibrium moisture content based on the surrounding
temperature and relative humidity before installation.
Moisture meters are used to measure the moisture content of wood and other
hygroscopic materials, such as the paper surface of drywall.
For common usage, two types of moisture meters are commercially available, pin and pinless.
Pin-type meters measure the electrical resistance of wood fibers. The resistance becomes
increasingly lower as the moisture content increases. For measurement, two metal
pins are driven into the wood and the electrical resistance is translated
as moisture content. The major advantage of pin-type meters is that they can
be used on rough and curved surfaces, such as unplaned lumber, logs, and textured concrete surfaces.
Pinless-type meters use the dielectric properties of the wood,
requiring only surface contact rather than pins or probes. These meters have the
advantage of not damaging the material with pin marks, but they don't work
on rough or curved surfaces. The area being sampled must be large enough for complete contact with
the sensor pad of the meter.
Most moisture meters are calibrated for a particular material or group of
materials. Douglas fir is the species to which most wood meters are calibrated.
Meter manufacturers provide information for correcting readings for species other
than the one for which the meter is calibrated.
The ability to measure moisture content in wood is important for many reasons,
including acclimation of wood flooring, kiln drying of lumber, seasoning firewood,
preventing rot in wood, and predicting the expansion and contraction of wood. Make
sure you know your moisture meter well before taking measurements.
United States Department of Agriculture (USDA). Wood Moisture Handbook: Wood as an Engineering
Material (Agricultural Handbook No. 72). Print edition, August 1974; Internet edition, April 2010.
Comment: This document
served as our primary resource for this tool. The basic equation we used can be found in
Calculate up to seven moisture metrics -- saturation vapor pressure,
relative humidity, dew point temperature, water vapor density, vapor
pressure, wet-bulb temperature, and humidity ratio -- merely by entering
dry-bulb temperature and one of the previous values. It's like having all
the benefits of a psychrometric chart without the chart complexities.