Firewood & Heating Cost Calculator

Firewood is sold in units almost nobody can picture, in species that differ enormously in how much heat they hold, at a moisture content that can quietly halve what you paid for. Two loads at the same price can be worth very different amounts. This guide walks through the three questions that decide whether a delivery is a fair deal: how much wood is actually there, how much heat is actually in it, and how that compares with the other ways of heating the same room.

One cord of oak, followed all the way through

Volume comes first. A cord is fixed at 3.624556 cubic metres, the metric size of 128 cubic feet. A stacked cubic foot is 0.02831685 cubic metres. A stacked cubic yard is 0.7645549. A face cord has no fixed size, so it is built from its own shape: 4 feet by 8 feet by the log length. In metres that is 1.2192 by 2.4384 by the logs. With 16 inch logs it lands on 1.208 cubic metres. That is exactly a third of a cord. With 24 inch logs it is exactly half.

Tipped loads are divided by a swell factor, which starts at 1.4. A dumpy bag counts as 0.7 loose cubic metres and a crate as 1.2. So at the opening factor, a bag is half a stacked cubic metre and a crate is 0.86. The stacked figure is then cut by the solid share, which starts at 75 per cent. What is left is real wood.

Each wood carries just two numbers: oven-dry kilograms in a solid cubic metre, and megajoules of heat in each dry kilogram. Oak is 670 kg and 18.6 MJ. Spruce is 400 kg and 19.2 MJ. Poplar and willow are the lightest here at 390 kg. Eucalyptus is the heaviest at 780 kg. See how little the heat figure shifts. Most woods on the list sit at 18.6, and the resinous ones reach 19.2. Weight does nearly all the work.

Then the water is charged for. Boiling a kilogram of water out of the fuel takes 2.443 MJ, the figure from EN 14918. Per dry kilogram the sum is the heat value of the wood, minus 2.443 times M divided by one minus M. M is the water share of the delivered weight.

Here is the opening set of values on the page, run through that chain. One cord of oak at a price of 320, 20 per cent moisture on a wet basis, a 75 per cent solid share, and a modern clean-burn stove rated at 78 per cent.

The other fuels get the same last step, and the starting prices are set up for it. Gas at 0.07 a kWh in a 90 per cent boiler works out at 0.078. Oil at 0.75 a litre holds 10.35 kWh a litre, and 88 per cent of that is 9.108, so 0.082. Electricity at 0.26 loses nothing, so it stays at 0.260. Pellets at 330 a tonne hold 4,800 kWh a tonne, and 85 per cent of that is 4,080, so 0.081. On those figures the logs are clearly the cheapest heat in the room.

The winter sum runs backwards from all this. Ask for 12,000 kWh a year with the wood covering 60 per cent, and you want 7,200 kWh. Divide by the 7,099 kWh a cord gives you, and the answer is 1.01 cords. That is 3.7 cubic metres of stack and about 325 of spending.

• Stacked size: 1 times 3.624556, so 3.62 cubic metres.
• Real wood in it: 3.624556 times 0.75, so 2.72 cubic metres.
• Dry weight: 2.7184 times 670, so 1,821 kg.
• Weight on the truck: 1,821 divided by 0.8, so 2,277 kg.
• Heat per dry kilo: 18.6 minus 2.443 times 0.25, so 17.989 MJ.
• Heat in the load: 1,821 times 17.989 divided by 3.6, so 9,101 kWh.
• Heat reaching the room: 9,101 times 0.78, so 7,099 kWh.
• Price of that heat: 320 divided by 7,099, so 0.045 for each useful kWh.

The damp rule, the guard rails, and the bills it never sees

Wet wood is charged for twice, and the two charges are very different sizes. The fuel side is small. At 35 per cent moisture each dry kilo holds 17.285 MJ, against 17.989 MJ at 20 per cent. That is under 4 per cent gone. The fire itself is where the real loss sits.

So a second rule handles the fire. Below 20 per cent moisture nothing happens at all. Above it, every extra point of water takes 0.9 of a point off the rating you set. At 35 per cent the multiplier is 0.865, so a 78 per cent stove is treated as a 67 per cent one. The code calls this a rule of thumb, and so does the page. It is not a reading off your own stove. The reason behind it is real, though. Steam cools the firebox until the smoke stops burning and leaves as tar.

Typed figures are held inside limits, so a slip of the finger cannot wreck the answer.

Because moisture stops at 70 per cent, the damp multiplier never drops below 0.55, even though the code allows a floor of 0.5. That floor is dead wood in the file. Worth knowing, if you were hoping to model a truly sodden load.

Buying by the tonne turns the chain around. There is no volume to start from. So the page takes the delivered weight, strips the water out to get dry weight, divides by the density of the wood for solid volume, then divides by the solid share for a stacked one. Every cubic metre you read after that leans on the species you picked being right. A tonne of oak at 20 per cent becomes 1.19 cubic metres of wood, 1.59 stacked, and 0.44 of a cord. Pick beech instead and those change. The heat barely moves, because heat follows the dry weight, and the dry weight was weighed rather than guessed.

Plenty of money sits outside all this. You enter a price for each unit, and that is the only cost the page knows. Delivery, splitting, stacking, sweeping the flue and carrying out ash are all missing. So is time. It will not tell you how long a wet load needs before it burns well, and it never checks that the moisture you typed is one you could reach. The final table column prices every fuel against the full year of heat. That keeps fuels honest against each other, but it is not a plan for a house warmed by two of them at once.

Rounding deserves a look before you argue with a supplier. Volumes show two decimals. Weights and kilowatt-hours show none. The cost of useful heat shows three. A very cheap fuel can therefore read as zero on screen. The saved spreadsheet keeps four decimals, so lean on that file when two fuels are close.

One last warning about the species table. It keeps your solid volume still and swaps the wood, so it answers a question about volume, not about weight. At the starting values, eucalyptus tops it with 8,402 useful kWh for the cord. Oak reaches 84 per cent of that, spruce 52 per cent and poplar 49. The densities behind that are typical seasoned figures, and the code says as much. A slow-grown oak and a fast-grown one can differ by more than oak differs from beech. Read the ranking as a hint about which offers deserve a phone call, not as a promise about the load in your driveway.

• Moisture: 0 to 70 per cent.
• Solid share of a stack: 40 to 95 per cent.
• Swell factor for tipped wood: 1 to 3.
• Log length: 6 to 48 inches.
• Rating for the wood appliance: 5 to 95 per cent.
• Rating for the other fuels: 5 to 500 per cent, which leaves room for a heat pump.
• Winter heat wanted: up to five million kWh.

The measuring problem nobody warns you about

Wood is sold by volume, and volume is where the confusion starts. A full cord is a stack measuring eight feet long, four feet high and four feet deep, which comes to 128 cubic feet of stacked wood and air. A face cord, sometimes called a rick, is the same eight by four face but only as deep as the logs are long. If the logs are sixteen inches, a face cord is a third of a cord. If they are twelve inches, it is a quarter.

That single fact is responsible for most firewood disputes. Somebody advertises a cord, delivers a face cord, and the buyer has no easy way to tell until it is stacked. Ask what the log length is before agreeing a price, because a face cord of twelve-inch logs and a face cord of twenty-inch logs differ by two thirds and the seller may not think the distinction is worth mentioning.

Elsewhere the stere is used, which is simply a cubic metre of stacked wood. It is a cleaner unit because it does not depend on the log length, and it converts to about 0.276 of a cord. Loose volumes are different again, and they are covered below.

Stacked, loose, and solid are three different things

A stack of split logs is not solid wood. Air fills the gaps, and how much air depends on how neatly it is stacked and how straight the pieces are. A tidy stack of split wood is usually around seventy to eighty per cent solid; a stack of round, knotty pieces is worse.

Wood tipped loose off a truck is worse again, because the pieces land at random angles and lock together with much bigger voids. A loose cubic metre typically holds around forty to fifty per cent solid wood, which means a loose load looks far more impressive than the same wood stacked. Sellers who deliver loose and price by the apparent volume are not always being dishonest, but the buyer is paying for a lot of air.

This matters for the calculation because heat comes from the solid wood, not the stack. Converting a stacked or loose volume to a solid volume before doing anything else is the step that makes different offers comparable.

Species: why the same volume can differ by half

Dry wood of any species holds roughly the same energy per kilogram. What differs enormously is how many kilograms fit in a given space. Dense hardwoods like oak, beech, ash and hornbeam pack far more mass into a cubic metre than light softwoods like spruce, poplar or pine.

The practical result is that a cord of oak can hold roughly twice the heat of a cord of spruce. If softwood is offered at half the price of hardwood it may be exactly fair; if it is offered at three quarters of the price it is poor value, even though the stack looks the same size.

Softwood is not worthless. It lights easily, burns hot and fast, and is genuinely useful for getting a fire going or for a short evening fire. But for overnight burning or for heating a house through a cold week, density is what you are buying.

Moisture is the expensive part

Freshly felled wood can be half water by weight. Burning it does not just waste the space that water occupies: the fire has to boil that water off before the wood can burn properly, and the energy for that comes out of the fire. Wet wood therefore delivers far less useful heat, burns cooler, and deposits far more tar in the flue.

Well seasoned firewood should be around fifteen to twenty per cent moisture. Getting from freshly cut to that point usually takes a year for softwood and up to two for dense hardwood, split, stacked off the ground and under cover with the sides open.

Moisture can be quoted two ways, which is a trap. Wet basis expresses water as a fraction of the total weight; dry basis expresses it as a fraction of the oven-dry wood weight. The same wood is twenty per cent on a wet basis and twenty-five per cent on a dry basis. Meters usually read wet basis. If a supplier quotes an unusually good number, it is worth asking which basis they mean.

From volume to useful heat

Putting those together gives a chain: the quantity and unit give a stacked or loose volume, the stacking factor gives solid volume, the species gives a mass, the moisture content gives the net heat that mass will actually release, and the appliance efficiency gives what reaches the room.

That last step is easy to forget and it is large. An open fireplace may put only a small fraction of the fire's heat into the room, most of it going up the chimney along with warm air drawn out of the house. A modern closed stove does far better, and a modern boiler stove better still. Comparing fuels without allowing for the appliance is comparing the wrong thing.

The number that comes out of this chain is a cost per useful kilowatt-hour: what it costs to actually get a unit of heat into the room. That is the only number that can be compared fairly across fuels.

Comparing with gas, oil, electricity and pellets

Other fuels are sold by energy content already, which makes them easier. Gas and electricity are billed per kilowatt-hour, oil by volume with a known energy content, pellets by weight. Each has its own appliance efficiency, and a condensing gas boiler is very efficient indeed.

Once every fuel is expressed as cost per useful kilowatt-hour, the comparison is honest. Sometimes wood wins comfortably, particularly where it is cheap locally or free. Sometimes it does not, particularly where hardwood is expensive and delivered wet, and where the alternative is a modern condensing boiler.

There is a cost the arithmetic cannot capture: the work. Wood has to be stacked, carried, loaded and cleared out as ash, and the flue has to be swept. Some people enjoy all of that and some regard it as the price of a real fire. Neither position is wrong, but it should be a decision made knowingly rather than because the sums looked better than they were.

How much a winter takes

Working backwards from a heat demand is the way to answer how much to order. If you know roughly how many kilowatt-hours of heating the house needs over the season, and you know the useful heat per unit of wood from the chain above, the number of loads follows.

Estimates of seasonal demand vary enormously with insulation, climate and how warm you like the house, so treat the result as a planning figure rather than a promise. It is far better to be roughly right in advance than to discover in February that the woodshed is empty.

Buying more than one winter's worth at a time is often sensible, because wood bought early can be seasoned properly at home rather than trusting a supplier's description. A two-year rotation, with this year's wood already dry and next year's drying, is the arrangement that removes the moisture problem entirely.

Judging a delivery when it arrives

Stack it and measure it. A tidy stack against a known length of wall is the only reliable check, and it takes fifteen minutes. If the delivery is well short, that conversation is much easier to have while the wood is still in the driveway.

Check the moisture with a meter on a freshly split face, not on the end grain or the weathered outside, both of which read low and flatter the wood. Split a couple of pieces from the middle of the load rather than the top.

Look at the piece size too. Very large rounds may be cheap per volume but need splitting, and very short pieces reduce how much fits in the stove. Neither is a fault, but both affect what the load is worth to you.

What this tool does not tell you

The figures here are typical values for species density, energy content and appliance efficiency. Real wood varies with where it grew, how it was stored and which part of the tree it came from, and real appliances vary with how they are run. Treat every result as an estimate good enough to compare offers, not as a laboratory measurement.

It also says nothing about air quality or local rules. Many towns and cities restrict what may be burned, and in smoke control areas only approved appliances and fuels are legal. Wet wood in particular produces a great deal of particulate pollution, which is why several countries have moved to restrict its sale. Burning dry wood in a modern appliance is much cleaner than burning wet wood in an old one, but it is not free of consequence, and the rules where you live come before any calculation on this page.