Firewood Moisture Content Calculator: Under 20% Yet?

Two numbers decide whether a log is worth burning: how much of it is water, and how much heat is left once that water has been boiled off. This calculator works out both, either from a moisture meter reading or from a pair of kitchen scales if you have no meter. The insight most people miss is that the 20% limit, and the firewood meters sold to check it, work on the wet basis, the water as a share of the log's whole weight, while forestry papers and many timber meters quote the dry basis. The same log is 20% on one scale and 25% on the other, so a figure copied from the wrong source can send perfectly good wood back to the stack for another year.

How to read the answer

The headline figure is the wet basis percentage, because that is the basis the 20% Ready to Burn threshold uses and the one firewood meters are normally calibrated to. It is the weight of water divided by the total weight of the log. Timber and joinery meters sometimes report dry basis instead, so if a log you know is well seasoned reads oddly high, check which basis your meter's manual quotes before you blame the wood. That ceiling matters: on the wet basis, wood can never reach 100% moisture, because at 100% there would be no wood left. Dry basis divides the same water by the weight of the wood alone, so it climbs past 100% for freshly felled timber and is the figure you will meet in forestry papers and kiln specifications. The calculator prints both, so you can match whichever number your source is using.

The heat figures come from the same arithmetic your fire performs. Bone dry wood gives up roughly 5.3 kWh of usable heat per kilogram whatever the species, and each kilogram of water riding along with it takes about 0.68 kWh to heat and boil away, heat that goes up the flue as steam instead of into the room. Put those together and a kilogram of wood at 20% moisture yields about 4.1 kWh, while the same kilogram at 50% yields about 2.3. That is the real cost of burning wet logs: you are paying to evaporate water, and the steam cools the firebox enough to stop the smoke burning cleanly, which is what tars the glass and lines the chimney.

Notice what the calculator does with a weight. Drying does not destroy any wood, it only removes water, so the total usable heat in a given stack actually goes up as it seasons even though the stack gets lighter. The water figure is worth a look too. A 400 kg load coming down from 28% to 20% has to shed 40 litres, and that water has to leave through the ends of the logs and out of the store, which is why airflow and a rain cover beat a sealed shed every time.

Weighing a sample when you have no meter

The scales method is the reference method, and it is the one the certification labs use under ISO 18134. Split a thin slice from the middle of a representative log, weigh it straight away, then dry it and weigh it again once the weight stops falling between weighings. A domestic oven on its lowest setting will get you close if you re-weigh every hour and stop when two weighings match. Use the middle of the log, never an end grain offcut that has been sitting in the wind: the outside of a stacked log dries first and will flatter your wood by several points.

The same warning applies to a meter. Pushing the pins into the weathered outside of a log tells you about the last fortnight of weather, not the log. Split it first, press the pins into the fresh face across the grain, and take two or three readings from different logs in the stack. Our guides on using a moisture meter properly and what to do when the reading comes back too high go through the technique and the usual false alarms, and our meter picks cover what is worth buying.

What to do with the number

Under 20%: burn it. This is the standard England sets for wood sold in units under 2m³, and it is where a stove runs clean. 20% to 25%: it will light and burn, but you are losing heat and sooting the glass; keep it for shoulder-season evenings and move drier wood to the front of the store. Above 25%: stack it and wait. Our seasoning time calculator estimates how many more months that will take for your species and storage, and seasoning firewood to under 20% covers the stacking that speeds it up. If you have just taken delivery of a load that reads wet, the Ready to Burn rules set out what the seller was required to supply.