Firewood Calculator

A cord is 128 cubic feet of stacked wood — conventionally four feet high, eight feet long and four feet deep. It is a legally defined quantity, and in most US states it is the only lawful unit for selling firewood.

A face cord is four feet high, eight feet long and however deep the pieces happen to be. At the common 16-inch length that is 42.7 cubic feet, or exactly one third of a cord. At 24 inches it is half a cord. At 12 inches it is a quarter.

The consequence is that a price quoted per face cord is meaningless without the piece length, and two sellers quoting the same figure can be a factor of two apart. "Rick", "truckload" and "pile" are worse — none of them are quantities at all.

The practical defence is simple: ask for the price per full cord and the piece length, work out the stack dimensions you should receive, and measure the delivery when it arrives. Sixteen-inch wood stacked four feet high needs 24 feet of length to make a cord. If it arrives in a heap, stack it before paying.

One further detail: the 128 cubic feet includes the air between the pieces. Actual solid wood in a well-stacked cord is around 80 to 90 cubic feet, and loosely stacked or crooked wood contains less. Published BTU-per-cord figures already account for this, which is another reason to compare heat rather than volume.

Every published firewood heat table assumes seasoned wood at 20% moisture. Freshly felled hardwood is 45 to 60%, and the difference is substantial.

Part of the loss is straightforward: water in the wood has to be heated to boiling and vaporised before the wood itself can burn properly, and that energy comes out of the fire. But the larger part is combustion quality. Wet wood burns cool, and a cool fire does not burn the volatile gases that make up half the energy in wood. They leave up the chimney unburnt.

That is where creosote comes from. Unburnt volatiles condense on a cool flue as a tarry deposit, and creosote is the fuel in a chimney fire. Burning wet wood is simultaneously the most expensive way to run a stove and the most dangerous, and the resin content of softwoods — the usual scapegoat — has very little to do with it. Dry pine burns cleanly; wet oak does not.

Seasoning is mechanical rather than clever. Split it, because a round dries only from the ends and can sit wet for years. Stack it off the ground in a single row with air moving through, in sun and wind. Cover the top only — a tarp over the sides traps moisture and makes a humidifier. Ash is ready in six months, maple in a year, oak in two.

And measure rather than trust. A moisture meter costs about twenty dollars, and it must be used on a freshly split face with the pins across the grain. The outside of a log reads dry long before the middle is, which is why "seasoned" wood delivered in autumn so often reads 35% inside.

Species varies heat by a factor of two, from about 13 million BTU per cord for white pine to 27 for hickory. The appliance varies it by a factor of seven and a half.

An open masonry fireplace delivers around 10% of the wood’s energy to the room, and that figure flatters it. The fire draws combustion air from the room — air the house has already heated — and sends it up the chimney, so on a cold night an open fireplace can leave a house net colder while feeling warm to anyone sitting in front of it. It is a pleasant thing to own and it is not a heating system.

A modern EPA-certified stove reaches 75% or better, and the mechanism is worth understanding: secondary combustion introduces preheated air above the fire to burn the smoke that an older stove sends up the flue. That is where both the extra heat and the dramatically lower emissions come from — they are the same phenomenon. Catalytic stoves achieve something similar at lower temperatures using a combustor, which allows a longer, lower, cleaner burn but is a consumable needing replacement every few years.

Between those, a fireplace insert converts an existing open fireplace into something like a stove for a fraction of the cost of rebuilding, and it is usually the highest-return change available to anyone with a masonry fireplace and a wood supply.

Firewood is one of the few remaining commodities routinely sold by undefined measure to buyers who cannot easily check, and a few habits remove most of the risk.