A cord is a stack; the heat is in the wood inside it
The energy in firewood tracks the mass of dry wood substance, and almost nothing else. Species matters because dense species pack more wood into the same stacked volume — a cord of hickory carries roughly twice the oven-dry weight of a cord of aspen, and delivers roughly twice the heat. Beyond that, a pound of oven-dry wood of any species releases close to the same energy, somewhere around 8,600 BTU on a gross basis. The species tables people quote are density tables in disguise.
How much solid wood a cord contains is its own variable. A cord is 128 cubic feet of stack including the air between pieces, and around 80 cubic feet of that is typically wood. Straight, uniformly split pieces stacked carefully do better; round, crooked or knotty wood does worse. Change that figure and every energy number on this page moves proportionally, which is a reminder that a cord is a purchasing unit rather than a physical constant. If the question is whether what was delivered is a cord at all, the firewood cord calculator handles the measurement.
The moisture penalty is two penalties, and the small one gets all the attention
The familiar story is that wet wood has to boil off its water before it can heat anything, and the story is true. It is also, on its own, a smaller effect than most people expect. Water takes roughly 1,200 BTU per pound to warm, vaporise and leave up the flue. A cord of red oak at 50 percent moisture carries about 1,760 pounds of water, which is around 2.1 million BTU spent on evaporation. Against a firebox release of about 26 million, that is a penalty in the region of five percent compared with the same wood at 20 percent.
Five percent does not match anyone's experience of burning green wood, and the reason is that the latent heat is not where the loss happens. The real damage is to combustion itself. Water vapour and cool gases hold the firebox temperature down, wood gases that should burn instead go up the flue unburned, the fire needs constant attention and produces little usable output, and the appliance never reaches the conditions its efficiency rating assumes. That is the effect this page models as a separate derate, and it is deliberately labelled as an estimate rather than physics, because unlike the latent heat it depends on the appliance, the draught and how the fire is run.
| Moisture content, dry basis | Water in a cord of red oak | Latent penalty | Total shortfall as modelled |
|---|---|---|---|
| 15% | about 530 lb | slightly better than 20% | none |
| 20% | about 700 lb | reference point | reference point |
| 30% | about 1,060 lb | around 2% | around 13% |
| 50% | about 1,760 lb | around 5% | around 39% |
| 75% | about 2,640 lb | around 8% | around 48% |
The right-hand column is why a cord of green wood is not a bargain at any discount that gets offered for it. You are buying water by weight, paying to haul it and stack it, and then paying again in output.
Reading a moisture meter without fooling yourself
A pin meter reads moisture content on the dry basis: pounds of water per hundred pounds of oven-dry wood, which is why numbers above 100 percent are possible and common in freshly felled timber. Some sources quote a wet basis instead, in which the same wood reads a lower figure, and quietly mixing the two is a frequent source of confusion when comparing published tables.
The measurement technique matters as much as the meter. Take a piece from the middle of the stack, split it, and press the pins into the freshly exposed face, across the grain, near the centre of the piece. A reading taken off the outside of a split, off end grain, or off wood that has been rained on tells you about that surface, which dries and rewets within days and is not the wood that will be burning. Take several readings from several pieces; a stack is not uniform.
What seasoning time depends on
Species is the smaller variable. The larger ones are entirely under your control: how small it is split, how it is stacked, and how much sun and moving air it gets. Water leaves a log mainly through the end grain and the split faces, so a round left unsplit can take years while the same wood split small can be ready in a single good summer. The multipliers in the calculator reflect that, and the round-wood figure is not an exaggeration.
Stacking well means a single row rather than a heap, off the ground so the bottom course is not wicking, with the top covered and the sides wide open. The mistake that undoes an entire summer is wrapping a stack in a tarp all the way down, which stops airflow and traps moisture the wood has given off, so the pile arrives at winter no drier than it started. A woodshed with open sides does the same job as a tarp on top without the trap.
Drying also slows as it goes. Wood loses its free water quickly and then approaches equilibrium with the surrounding air asymptotically, so the last stretch from 25 percent to 20 takes disproportionately longer than the first drop from 60 to 40. Any months figure, including the one on this page, is a rough planning number, and the meter is the only thing that actually answers the question.
The part that is not about efficiency
Burning wet wood produces creosote, and creosote in a flue is the fuel for a chimney fire. A firebox that never gets hot enough sends unburned volatiles up a relatively cool chimney, where they condense and accumulate. That is a safety matter rather than an economic one, and it is outside what a calculator can assess: how often a specific flue needs inspecting and sweeping is a question for a qualified chimney sweep who can look at it. What the arithmetic on this page can tell you is that the wet wood was never cheaper, so there was nothing gained to set against it. Once you know how much heat a cord actually delivers, the heating fuel reserve calculator turns it into days of heat and a refill trigger, and the heating degree day calculator tells you how much of the winter that is likely to cover.
Questions people ask
How many BTU are in a cord of firewood?
Roughly 10 to 30 million, and the spread is almost entirely density and moisture. A cord of a dense hardwood like hickory or white oak, seasoned to 20 percent, carries around 4,000 pounds of oven-dry wood and releases somewhere near 30 million BTU in the firebox before appliance efficiency. A cord of aspen or pine carries half the dry weight and releases roughly half the heat. Published tables differ from each other by noticeable margins because they assume different solid-wood fractions per cord and different moisture, which is why this calculator makes both an input rather than burying them.
How long does firewood take to season?
Anywhere from a single summer to more than two years, and how it is handled matters more than what it is. Split to stove size, stacked in a single row off the ground with the top covered and the sides open, in a temperate climate, softwoods and light hardwoods are commonly ready in six to ten months and dense oaks want closer to two years. Leave the same wood in the round and every one of those figures roughly doubles or worse, because water leaves mainly through end grain and split faces. Piling it or wrapping it in a tarp can mean it never gets there at all. Treat any months figure as planning only and settle the question with a meter.
How much heat do I lose burning wet wood?
Less than folklore says on the pure physics, and much more than folklore says once combustion is included. Boiling off the extra water in a cord at 50 percent moisture rather than 20 costs around five percent of the energy released. But a firebox running on wet wood stays cool, burns incompletely and sends unburned gases up the flue, and that is the effect that dominates. Modelled together the shortfall lands nearer 40 percent, which is closer to what people actually experience. The second figure is an estimate rather than a physical constant, because it depends on the appliance, the draught and how the fire is run.
Does species matter as much as people say?
It matters, and it matters for one reason: density. A pound of oven-dry wood releases close to the same energy whatever tree it came from, so the species tables are really density tables. Hickory beats pine per cord because a cord of hickory holds about twice the dry weight, not because the wood is better fuel per pound. That reframing has a practical consequence: if you are buying by weight, or cutting your own and comparing effort, dense species look far less dominant than they do per cord. Moisture is the larger lever than species in almost every real comparison.
Is it worth buying green wood cheaper and seasoning it myself?
It can be, provided two conditions hold, and both are about time rather than price. The first is that you have somewhere to stack it properly with sun and moving air, since green wood stored badly stays green. The second is that you are buying it far enough ahead — often a full year for dense hardwood, sometimes two — that it will be dry before you need it, which means buying next winter fuel this spring rather than buying this winter fuel in November. Where the arithmetic goes wrong is buying green wood in autumn at a discount and burning it in January, because then you have paid to haul water and you get the full shortfall on top.