We all sometimes wonder how big a tree is. We can guess the height and measure the circumference, but we really need to know more for two reasons:
- we don't want to be ripped off even if we're only selling firewood;
- and the Forestry Commission only allows us to fell trees, without a licence, up to a maximum of 2 cubic metres of wood per 3 month period if the wood is being sold, or 5 cubic metres if it's for our own use *.
Measuring a standing tree The most important dimension is its diameter at 1.3 M up the trunk. This position is known the world over as 'breast height' and the diameter as 'diameter breast height' or DBH for short. Foresters use tapes specially marked off to read diameter when placed around the trunk, but an ordinary tape measure is fine, just remember to divide the circumference (girth) by pi (3.142).
Now although the shape of a tree is wretchedly awkward, the relationship between DBH and volume or weight of wood is a good one. And the size to try and remember is that a tree of 40 cm DBH has, very roughly, one cubic metre or one ton of wet wood in it. One of 20 cm DBH will have about a quarter of this and one of 60 cm about 3 -4 times. It's all on page 83 of Badgers, Beeches and Blisters. What we don't mention in the book is a nice rule of thumb: if you can hug a tree and your fingers just overlap it will be about 40 cm DBH and have about a ton of wood in it! So you can fell two huggable trees every 3 months without needing a licence!!
Measuring a log on the ground
This is easy provided you can get your measuring tape underneath the log at its mid point! Measure the length of the log, determine its mid point, i.e. exactly half way along, and measure its diameter there. Use this diameter to calculate the cross-sectional area (the area of the circle). Multiply cross-sectional area by length and that will give you approximate volume.
Measuring a stack of logs
Assuming the stack is reasonably tidy, simply measure its length, its average height and multiply these dimensions by the log/billet lengths in the stack. (The logs/billets will normally have been cut to a standard length.) Multiplying these three figures gives us a the solid volume of the stack. To take account of all the gaps or airspace because logs are round, knobbly, bent etc. multiply the solid volume by 0.6 to estimate the approximate amount (volume) of actual wood. This proportion of 60% or 0.6 varies somewhat, but by using it you will get a safe and usually conservative estimate.
* There are one or two other exemptions - see Badgers, Beeches and Blisters pp 44-45.
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