Firewood numbers

There are a lot of numbers associated with firewood and I’ve tried to collect best estimates relevant to small woodlands, here in one place, along with enough context to use them. They’re not a substitute for what you actually see in your own circumstances, but they’re the kind of thing you need if you’re putting together a woodland management plan, prior notification for a drying barn, a business case, or even deciding roughly what you can do.

I’ve organised it in the same order as the firewood processing sequence: how much grows per year, what lengths to cut, when to split, how drying works, how much heat different species produce, loose vs stacked, and bag sizes.

As part of their Biomass Energy Centre pages, Forestry Research has a practice note from 2009 “Establishment and Management of Broadleaved Coppice Plantations for energy”, with a lot of advice about how to create coppices for firewood. It gives estimates of between 1 and 6 “oven dried tonnes” of firewood per hectare per year from broadleaf coppice averaged over the cycle of rotation, which is between 10 and 15 years.  In appendix 1, is a table listing common firewood species, with their yields and notes about the conditions they require. For woods not managed as coppice, the yields are of course upper limits on what can be taken.

The basic pieces of equipment involved in firewood production from a small woodland are a saw, a splitter, and something to move the wood around with.

A chainsaw can cope with any firewood job at that scale, and enthusiastic woodlanders might use hand saws instead. But what lengths to cut logs into? There’s a standard 16 inch length derived from the “face cord”, which I’ll cover later, but many wood stoves specify 12 inch / one foot / 30 cm. In practice, a bit shorter can be more convenient for stacking inside the stove, and 10 inch / 25 cm lengths are specified in the Solid Biomass Assurance Scheme. However, the shorter lengths you cut the wood into, the more pieces you have to move and stack, and so you might find it easier to cut the wood into, say, 48 inch / four foot / 120cm lengths, to start with, and then move them to where the wood will be stacked, before cutting it up further.

Splitting wood by hand. Used with permission.

Firewood dries quicker and better, and lights more easily, when in smaller pieces. So ideally you would cut it to the final length before stacking. For the same reasons, you would split it lengthways then too. Some kind of splitting maul or axe, or an electric or petrol splitting machine are really needed if you’re doing more than a few logs.

Drying firewood is really important for a bunch of reasons: you get more heat when burning each log, you’re not transporting water around, less smoke and particulates are produced, and less creosote coating the chimney. In 2020, new firewood regulations for the supply of firewood in England have been passed and will be phased in between May 2021 and May 2022. It will be illegal to sell “wet” firewood in loads of less than 2 cubic metres, based on the assumption that people buying larger loads are going to dry it themselves. Wet in this case means a moisture content of more than 20% – that is, the weight of water as a fraction of the total weight is 20%, which is called the “wet basis” moisture.

But what’s actually happening as wood dries? First of all, there are two different types of water in freshly cut wood. Between the cells is “free water”, mostly in the channels which carry water from the roots and from the leaves. This is the first to be removed by drying, and once the free water is gone, the wood is down to between 25% and 30% water content. This is called the “fibre saturation point”. Next is “bound water”, either embedded in the structure of the cell walls or in the liquid inside the cells (the cytoplasm). Further drying starts to remove the water from the cell walls, causing shrinkage and cracking, and making the wood harder and less flexible.

Eventually the drying stops as the moisture in the wood is matched by the relative humidity of the surrounding air. This is the “equilibrium moisture content” or EMC. The relative humidity is the amount of water present compared to the maximum the air can take. In the UK, it’s between about 60% and 90% most of the year, with lowest numbers in the summer. As you can see from this graph, you may eventually get down around 15% moisture in the wood (EMC) just by seasoning the wood outside. Higher temperatures, air flowing across the surfaces of the wood, and lower humidities accelerate the process, but after a summer or two, the wood gets as dry as it’s going to get.

Equilibrium moisture content of wood versus humidity and temperature, according to the Hailwood-Horrobin equation. Used with permission.

As the humidity rises again in autumn and winter, or if rain is allowed on to the wood, it starts to take up moisture and get wetter again. Keeping the wood under some kind of roof helps with this, and you can go further by drying it in a barn, which creates a microclimate with lower humidity, maintains higher temperatures, and allows less chance of rain getting at the logs. You really need something like a barn to reliably produce firewood with these lower moisture levels. However, it’s important that it still allows air to circulate and carry the moisture away from the surfaces of the logs.

Some commercial producers use kiln drying, which can get wood down to 15% moisture in a few days. This saves on their storage costs as they don’t need to have stacks of wood seasoning for months, but firewood at 15% is really the same whether kiln dried or seasoned naturally. Furthermore, wood at 15% will start to take up moisture from the air again during most of the year. If gas, oil, or more firewood has been used to power a kiln, that’s energy which isn’t available for heating homes, and gas and oil themselves always have net carbon emissions.

Wood stacked outdoors. Used with permission.

You can buy cheap moisture meters with metal spikes which use electrical resistance to estimate how much moisture is still in the wood. This should be done by pushing the spikes into a freshly split log to measure the moisture content in the body of the logs. This means you can check on the progress of the drying, and also do a final check before using or selling the firewood. They work best once the moisture level is below the fibre saturation point of about 25% to 30%. One thing to note is that electronic moisture meters are usually calibrated on a “dry basis” and give the water as a percentage of the dry weight, not as a percentage of the total weight, the “wet basis” figures which are used in firewood calculations. A 25% reading on most moisture meters will in fact correspond to 20% moisture on the wet basis.

It also possible to measure the water content above the “oven dried” level, by taking a sample piece of the wood, weighing it and then drying it in an oven until it stops losing weight. The difference between the lowest, driest weight and the original weight is a very good measure of the amount of water originally present, and dividing the water weight by the original total weight gives the “wet basis” moisture percentage. Forestry Research has a guide “Testing moisture content (simple method)” covering this process in more detail. An “Oven Dried Tonne” is a unit of dried firewood based on this idea. To convert from an oven dried weight to the weight of the same wood allowed to reach 20% moisture, multiply by 1.25.

So how much heat energy do you get from firewood? That depends on what species it is, and how dry we’ve got it. We also need to agree how to measure the amount of wood we’re talking about.

Forestry Research publish another handy guide “Wood as Fuel – Technical Supplement for Fuel Suppliers” which has a chart of the amount of heat produced by one oven dried tonne of different species. It ranges from just over 5000 kWH/tonne for Birch up to about 5500 kWH/tonne for European Larch, with Sitka and hardwoods in the lower half, and the other softwoods in the upper half of that range. That’s only about a 10% difference between species, and that’s because the cellular structures of different trees are very similar. For seasoned wood with a moisture content of 20% the average figure is about 4250 kWh.

Green or undried wood does vary a lot in wetness though, from Ash at 32% moisture when cut, up to 64% for Poplar and the wetter softwoods. These numbers also reflect how hygroscopic the wood is: how easily it gets damp again if you give it the chance. They are the origin of the idea that you can burn Ash when green (you can, but shouldn’t: 32% moisture is still bad for your chimney and for air quality); and that Poplar is a bad firewood (if you dry it properly and keep it dry, it’s fine.)

But weight is only half the picture. Wood is usually sold by volume rather than weight, because people don’t want to be paying for water in undried wood, and because volume is easier to measure. As I’ve said, the amount of heat produced by a tonne of wood varies by about 10% for different species. The size of a tonne of oven dry wood varies a lot more: Sitka is at the low end with a density of 0.40 tonne/m3, and oak at the high end with 0.63 tonne/m3 – a difference of 60%. There’s a 15% difference between most of the hardwoods, with elm and poplar unusually low in density. This means the heat energy per cubic metre varies a lot too. (Forestry Research have a page with more on this and a spreadsheet showing many wood species’ densities.)

This is all about the wood itself. When firewood is delivered, there are gaps between the logs: least if stacked neatly, most if just heaped randomly. So when we talk about a cubic metre of logs, we need to take the packing into account too. For perfectly cylindrical logs, all the same size and arranged in a hexagonal pattern, 91% of the space will be occupied by wood and 9% by air. Forestry Research give 70% as a real world figure for well-stacked logs, dropping to 40% or less for loose logs in a random heap. Some very open and airy stacking systems for seasoning firewood can have a figure as low as 10%.

You see firewood advertised as “stacked” or “loose”, with stacked understood to mean well-stacked logs occupying a minimum of space and loose to be logs randomly thrown into a bag or trailer.

Historically firewood was measured by the “cord”, equal to an 8 foot wide, 4 foot deep, and 4 foot high stack. That adds up to 128 cubic feet whatever the arrangement, or 3.6 cubic metres. A “face cord” is a third of that, equal to an 8 by 4 foot stack that is only 16 inches deep, and corresponds to the 16 inch convention for larger firewood logs.

In Britain, firewood is often sold to householders in cubic metres, in builders one tonne bags, or in smaller “garage forecourt” bags and nets.

A cubic metre, either stacked or loose, is straightforward, and you can buy one cubic metre firewood bags, often with ventilation panels, which makes measuring them easier too.

Builders “one tonne bags” are trickier. The “one tonne” or 1000 kg refers to how much sand or gravel or whatever can safely be in the bag when you lift it with a crane by the straps. In fact, the Builders Merchant Federation have an agreement that these bulk bags should have a minimum of 800kg of aggregate. Since aggregates are so dense, these bags hold about 0.8m x 0.8m x 0.8m = 0.5 cubic metres. Sometimes people make the mistake of thinking that since a cubic metre of water weighs a tonne, then somehow a “one tonne bag” will hold a cubic metre of firewood.

So putting that all together, a one tonne builders bag of loose oak logs might contain about 0.5 x 40% = 0.2 cubic metres of solid wood, weighing only 130kg if oven-dry or 160kg if at 20% moisture. Those 160kg of seasoned logs would give up 0.160 x 4250 = 680kWh when burned.

Smaller bags and nets are even more variable, but are usually sold in person rather than ordered by phone or online, and so at least the customer can judge the size of them in advance.

Overall, there are a lot details to be followed up to work out the numbers for your particular circumstances, but hopefully this post gives you the pointers. The various estimates and averages aren’t a substitute for what you measure yourself on the ground, but they are a start. They’re also independent numbers which you’ll need to write something formal like a business case.

3 Replies to “Firewood numbers”

  1. Helpful article Iain. Thank you. I’m glad for the information on the ‘dumpy’ bag volume. Quite a difference between the idea of a tonne bag and 130kg!
    Personally I don’t support the move to charge small firewood producers to accredit their timber as ‘dry’ if sold in loads less than 2m3. Could this be the knock on from the wood fuel implementation plan recommendations back in 2011 which recognised that there was a risk that expanding market potential for woodland owners might ‘divert material from existing wood processing businesses which could achieve higher carbon saving benefits’. I wonder what study is required to show the difference between selling locally (we only supply within 7miles of our woods) and burning this in a fairly efficient wood-burner (backboiler etc) compared to transporting timber across the country to a drying facility for use in a bigger process, to chip or make pellets & then re-distribute. Or worse still, importing kiln dried wood from abroad.

  2. Thanks Iain for some useful info. For all small scale hand splitters best tip I ever got was fixing an old deep tyre to the block, walk around splitting multiple small or single large logs and never have to bend over to pick them up! (especially if you prefer a relatively high block as I do)

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