Firewood volumes, weight and kWh
A price per stacked cubic metre and one per tonne cannot be compared by eye. This brings them to the same measure, with species and moisture doing most of the work, and tells you how much a season takes.
Enter how much firewood you are being sold and in which unit. The tool turns it into real cubic metres of wood, kilos and kWh, and if you add the price it tells you what the energy costs.
Why a stacked cubic metre is not a cubic metre of wood
A stacked cubic metre is a cubic metre of space taken up by the pile, not of wood. Air sits between the logs, and how much of it depends on the wood: about 30% with split wood stacked properly (0.70 m³ of wood per stacked m³ with 1 m logs, 0.65 with 25-50 cm pieces), about 40% with round unsplit pieces, and more than 55% when the load is simply tipped into a crate or a bulk bag (0.45 m³ of wood). The real reference is the solid cubic metre, the one you measure on whole logs. In practice one solid cubic metre becomes roughly 1.5 stacked cubic metres and 2.2 loose ones, so comparing a stacked price with a loose price without converting puts you 44% off. For readers used to cords, a full cord of 128 cubic feet is 3.62 stacked cubic metres, while a face cord means nothing until you know the log length. The factors here are averages, and you can type your own.
Moisture is paid for twice
Bone dry wood has almost the same calorific value in every species, about 5.14 kWh per kilo (5.33 for conifers, which carry resin). What changes from load to load is how much water comes with it. The formula used here is the standard one for solid biofuels, NCV = 5.14 × (1 − M) − 0.68 × M, where M is moisture on the total weight and 0.68 kWh/kg is the heat spent boiling that water off. At 20% it gives 3.98 kWh/kg, the famous 4 kWh; at 50% it drops to 2.23 kWh/kg, nearly half. Buying by weight you pay for the water as if it were wood and then pay again in the firebox. Buying by volume you lose far less, under 10%, because a stacked cubic metre of damp wood simply holds more kilos. The practical problem stays either way: wet wood needs a year or two before it burns properly, and burning it wet coats the flue with creosote.
How two quotes really compare
The honest comparison is on the cost of the energy, not on the price of the measuring unit. The tool brings everything to a cost per kWh and then rewrites both quotes in the same unit, which is how you see that 120 € per stacked cubic metre of seasoned beech and 180 € per tonne of wood at 40% are not the same spend. What stays outside the figures is what the quote usually does not say and you should ask: the log length (a 50 cm pile will not go into a 33 cm stove), whether the price includes delivery or collection, whether they tip it in the street or carry it into the yard, and above all the declared moisture. The word dry with no number attached means nothing.
How much you need for a season
The second tab starts from the heat you need. Type the kWh if you know them, otherwise they are estimated from the stove, rated output in kW times hours a day times days in the season. Then it is divided by the efficiency, because part of the heat goes up the chimney: an open fireplace recovers about 20%, a closed insert 65%, a modern stove 75-80%, a wood boiler a little more. The result is how many kWh the wood has to hold, and from there the stacked cubic metres, the kilos and the metres of woodshed. It stays an estimate, since real burning hours and how cold the winter turns out matter more than any decimal.
The numbers used and their limits
Densities are averages for air dry wood at 20% moisture, per solid cubic metre: beech 700 kg, oak 700, black locust 740, ash 690, sweet chestnut 570, birch 640, spruce 450, poplar 420, olive around 950. Within one species a lowland tree and a mountain tree differ easily by 10%, so if your supplier gives you a figure you can type it instead of ours. Wood shrinks by a few per cent while it dries and that is ignored here, so a stack built today with fresh wood will be slightly under a stacked cubic metre two years from now. One last trap that ruins a lot of arithmetic: moisture here is always on the total weight (wet basis), as in the solid biofuel trade, while some meters report it on dry weight. A meter reading 25% dry basis means 20% wet basis, because M = u ÷ (1 + u).