What is moisture content and how is it calculated?

Date: June 17, 2022

As I’m sure you all know, it is important that the moisture content of the wood you burn is low, and ideally below the magic number of 20%. If you burn wood with a high moisture content, your fire will be smoky, produce less heat and release more tar. Burning wet wood means your chimney will need sweeping more regularly, or even in extreme cases, increase the risk of a chimney fire.

Here at Pentland Biomass, we kiln dry our logs so you can be sure that they have a moisture content of under 20%, so you can be sure that the logs you get from us are nice and dry, but what does this mean and how is it calculated? What exactly is a moisture content of 20%?

Moisture content is a ratio, not a percentage

While moisture content is usually expressed as a percentage, it’s easier to think of it as the ratio of water to dry wood.  20% can be expressed as fraction, 1/5, or a ratio of 1:5.

To put it another way, 6kg of wood with a moisture content of 20% will consist of 5kg of dry wood and 1kg of water.

How do you work out the moisture content of wood?

There are very effective moisture meters that use the electric fields to calculate the moisture content. These are quick and easy to use, however it’s easier to understand what the number actually means if you measure it the old-fashioned way.

Traditionally, the moisture content was calculated by completely drying out the sample you are testing, and comparing the mass of the dry sample to the mass of the sample before it was completely dried.

First you weigh the sample piece of wood you want to measure the moisture content of.

Once you have done this, you need to dry the piece of wood completely. This can be done by placing the sample in a kiln for 24 hours at 150 degrees C. You can check if the sample is dry by measuring its mass twice 1 hour apart and seeing if its mass has changed. If it changes, the sample is still losing water.  

Once you have 2 readings that are the same, you can be confident that your sample of wood is fully dry.

Once you know the original mass of the wood, and the dry mass, you can work out the moisture content by plugging the numbers into this formula.

There is a handy online calculator here, which will calculate this for you.

You can have a moisture content of over 100%

Since moisture content is a ratio of water to dry wood, it is possible to have a moisture content of over 100%.

This initially sounds impossible until you understand that moisture content is a ratio. A 100% moisture content of 100% means that the wet wood contains 1kg of water for every 1kg of dry wood fibre, or that half the total weight of the wet wood is water.

Some species of tree contain so much water that there is more than 1kg of water for every kg of dry wood in the freshly felled logs – these can have a moisture content of 110% or even 120%

Our logs are kiln dried to a moisture content of less than 20%. If you want to try  them out, you can visitor our log shop here!

© Pentland Biomass is a trading name of Pentland Plants Limited.
Company Number SC259669, registered in Scotland.
Our registered office is 6 Pentland Nurseries, Loanhead, EH20 9QG