About the B6 Tariff
Introduction
The B6 tariff is a relatively cheap way of assessing standing timber volume, by applying a pre-determined volume-basal area ratio (VBAR) to measured basal area (BA). This works, in theory, due to the strong linear relationship which exists between between BA and standing volume in a given stand. However, its weakness, in practice, is the fact that VBAR is not measured locally, but applied based on a national model, the confidence intervals of which are not known (Matthews and Mackie, 2006). The B6 tariff is specific to the UK.
To carry out a B6 tariff, foresters measure average BA/tree and stems/ha using a representative sample of fixed radius plots. They also measure top height, which is used to select a 'tariff number'. This, in turn, is used to derive a VBAR, and VBAR is applied to the BA to estimate volume.
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If the above doesn't quite make sense, try using the slider on the chart below to adjust tariff number. This visualises the formula that is used to calculate volume in a B6 tariff, based on tariff number and basal area (V = a1 + a2 x BA). ​​The blue line shows the standing volume plotted against basal area, and the red line shows the volume-basal area ratio.
Important caveats
The B6 tariff is a clever way of estimating standing volume whilst keeping time consuming tree height measurements to a minimum, but it has some important caveats.
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Using a B6 tariff, VBAR is not measured locally, it is based on a national model (originally developed in the 1950s) which maps ‘tariff numbers’ to stand top height (as shown in the above figure), and this is likely to be the largest source of error in the workflow. ​The confidence intervals of these estimates are not known. Error may be significant and vary greatly from one stand to another.
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Further, the sample plots which support the tariff tables were composed of un-improved genetic material. The actual VBAR in a stand of improved Sitka spruce is likely to be different from the VBAR of a wild seed stand with the same top height, but this is not accounted for in the tariff tables.
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More generally, Matthews and Mackie (2006) stress that an important assumption of the tariff number models is that stands are managed according to a relatively uniform silvicultural regime. If measured stands are managed differently to the original reference stands, bias may be introduced.
Abbreviated tariffing using VBARs was developed in the UK in the 1950s. This report charts its development.
When to use a B6 tariff, and when not to
The B6 tariff may still be the strongest method of estimating volume whilst limiting the number of trees measured for height to 1 per hundredth of a hectare.
However, even with this in mind, whether it is worth the cost-benefit of measuring (rather than going with the ‘eyeball’ estimate of an experienced forester) is a question that’s worth asking - particularly in genetically improved stands - but as of today, it’s not one we can answer.
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For relatively cheap assessments of volume, it may still be appropriate, but it should not be a default prescription (in my experience, it often is). ​It should certainly not be used where high levels of accuracy, and error propagation are important, such as for permanent sample plots.
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Alternative approaches
Many books have been written on forest inventory, and there are too many well established methods of assessing volume to list here.
One alternative which is worthy of mention is to measure local VBAR and apply it to BA only plots. This may be appropriate where higher levels of accuracy are required, but costs still need to be kept under control.
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Using this approach, a proportion of plots can be measured for volume and BA using single-tree equations to derive local VBAR. Then, this VBAR can be applied to BA only plots, where no height measurements need to be taken and BA could be measured with a prism sweep.
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Further reading
The 'blue-book' (Matthews and Mackie, 2006) provides rich context, and is of course the first port of call to learn more about the B6 tariff.
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Hummel (1955) is also an excellent read if you are interested in the historical rationale, measurements, and computations which support the method, but it is undoubtedly more advanced reading. This report would have set you back 9 shillings when it was first published, but it has since been generously published online by Forest Research.
The Hummel report contains wonderful quotes like "the laborious computations, which would have been necessary in order to eliminate the bias [...], were not carried out, because it was thought preferable to search for a simpler method". This quote comes at the end of a long section justifying why non-linear models for Scot's pine and larch would not be tested. These computations were deemed too complex in the 1950s, but would be trivial with the compute resources available just a few decades later.
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Foresters may also be surprised to learn about the number of permanent sample plots which originally supported the tariff tables. For Sitka spruce, just 13 plots (each with 4 sub-plots) were measured across 8 properties in England and Scotland.
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Finally, we should not be afraid to look further afield for forest inventory inspiration. Timber measurement protocols used in other territories are widely documented, and having an awareness of these helps us to understand UK methods in a more informed way. A Sampler of Inventory Topics by Dr Kim Iles (reviewed here) is a great place to start. ​
References
Hummel, FC. (1955) "The Volume-basal area line: A study in forest mensuration" HMSO: London. Available at: https://cdn.forestresearch.gov.uk/1955/04/fcbu024.pdf [Accessed 15/12/2025]
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Iles, K. (2014) A sampler of inventory topics: a practical discussion for resource samplers, concentrating on forest inventory techniques. Kim Iles & Associates Ltd: British Columbia.
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Matthews, R. Mackie, E. (2006) Forest Mensuration: A handbook for practitioners. Forestry Commission: Edinburgh.