Firewood Calculator

How much firewood you need in one number

Most firewood advice is vague. People say it depends on your house and leave it there. This calculator turns that into a number by starting with the heat your home loses in a winter and dividing by the heat one cord of your wood delivers through your appliance.

Three inputs do most of the work. The first is how many square feet you heat, because heat loss scales with the area that touches cold outside air. The second is your climate, because a house in northern Minnesota loses about four times the heat of one on the Gulf Coast. The third is your appliance, and this is where people get surprised. An open fireplace throws most of its heat up the chimney, while a modern EPA-certified wood stove keeps most of it in the room.

The result is the number of cords for one heating season, plus face cords and an estimated cost. Treat it as the amount to buy and stack, then adjust next year once you know how fast your own pile goes down. A drafty house with old windows will burn more than a tight one, and a mild winter will burn less.

The math behind the cord estimate

The calculator works in BTUs, the same unit you see on a furnace label. It multiplies your heated square footage by a climate factor to get the heat your home needs for the season, then divides by the usable heat in one cord.

Seasoned hardwood holds about 24 million BTU per cord, a mixed load about 20 million, and softwood like pine about 15 million. Your appliance only delivers a share of that. An EPA-certified wood stove runs about 70 percent, a fireplace insert about 60 percent, an outdoor boiler about 50 percent, and an open fireplace a dismal 10 percent. The EPA's Burn Wise program makes the same point about matching the right wood to the right appliance.

Usable heat per cord is the wood's heat value times the appliance efficiency. A cord of hardwood in a certified stove gives about 24 times 0.70, which is 16.8 million BTU of heat into the room. Divide the season's heat load by that figure, then add 15 percent to cover moisture and the cold snaps when you burn harder than usual.

The 15 percent buffer is not padding. Wood that is not fully seasoned carries water that has to boil off before it heats anything, and a late cold snap always eats more than the average week suggests.

Worked example: a 2,000 square foot house in a cold climate

Say you heat 2,000 square feet in a cold climate, where the season runs about 40,000 BTU per square foot. The heat load is 2,000 times 40,000, which is 80,000,000 BTU, or 80 million BTU.

You burn seasoned hardwood at 24 million BTU per cord in an EPA-certified stove that delivers 70 percent. Usable heat per cord is 24 times 0.70, which is 16.8 million BTU. Divide 80 by 16.8 and you get 4.76 cords. Add the 15 percent buffer and the total is about 5.5 cords for the winter.

At $300 a cord that is roughly $1,643 of wood, and about 5.5 times 3, or 16.5 face cords. Most people stack in face cords and buy in full cords, so it helps to know both numbers before you call a seller.

A smaller house scales straight down. A 1,200 square foot home in the same cold climate needs 1,200 times 40,000, which is 48 million BTU. Divided by 16.8 that is 2.86 cords, or about 3.3 cords with the buffer. Heating a modest home with wood is a few cords, not a few dozen, when the stove is efficient.

Worked example: a fireplace burns eight times the wood

Run the same 2,000 square foot house through an open fireplace. At 10 percent efficiency the usable heat per cord is only 24 times 0.10, which is 2.4 million BTU. The season needs 80 divided by 2.4, which is 33.3 cords, or about 38 cords with the buffer.

That is roughly eight times the wood of the certified stove, for the same warm rooms. At $300 a cord the fireplace winter costs about $11,500 in wood against $1,643 for the stove. Even a used stove pays for itself in a single season at those numbers.

A fireplace insert splits the difference. At 60 percent efficiency the usable heat per cord is 14.4 million BTU, so the season needs 80 divided by 14.4, which is 5.6 cords, or about 6.4 cords with the buffer. An insert turns an open fireplace into a real heater without a full remodel.

If you heat only part of the house with wood and use a furnace or heat pump for the rest, set the square footage to just the rooms the stove warms. Guessing high is the fastest way to end up with a pile of wet wood in April.

Cords, face cords and ricks: what you are buying

A full cord is a stack 8 feet long, 4 feet high and 4 feet deep, which is 128 cubic feet of stacked wood. It is a legal unit in most states, and a seller who advertises a cord is supposed to mean the full 128 cubic feet.

A face cord, also called a rick or a stove cord, is a stack 8 feet long and 4 feet high, but only as deep as the pieces are long. If the pieces are 16 inches, a face cord is about 42.7 cubic feet, or one third of a full cord. If a seller stacks 24 inch pieces, a face cord is half a cord, and the price should reflect that. The University of Arkansas Extension publishes a firewood species and buying guide that is useful when you are comparing offers.

Always ask two questions before you buy: how many cubic feet, and how long are the pieces. A face cord at a cheap price can be half the wood of the same stack cut longer. The other check is moisture, which is where a seller either has real seasoned wood or a pile of green logs.

Seasoning, moisture and storing your wood

Wet wood is the most common reason a stove underperforms. Water in the wood has to boil away before the wood can heat your house, so every pound of water steals heat you paid for. The EPA says wood burns best at a moisture content under 20 percent and should be seasoned at least six months for softwood and a year for hardwood.

Split your wood before you stack it. Split pieces dry several times faster than whole rounds because the water escapes through the exposed faces. Stack it off the ground on rails or pallets, with the top covered and the sides open to the wind. Covering the sides traps moisture and slows drying. The USDA Forest Service Forest Products Laboratory publishes the wood energy data behind these numbers, including the finding that a full cord of air-dried oak holds about 25 million BTU.

A $20 moisture meter takes the guesswork out. Split a piece, press the pins into the freshly split face, and read the number. Under 20 percent is ready. If it reads 25 or higher, give it more time. Seasoned wood is also lighter and sounds hollow when you knock two pieces together.

Buy ahead. Wood you buy this fall is often not ready until next fall, so the cheapest way to heat with wood is to stay a year ahead of yourself. That way you are always burning wood that has had a full year to dry, and your stove runs at the efficiency you paid for.