What a cord is, and why every other firewood word is vague
A cord is 128 cubic feet of wood, stacked so the pieces lie parallel and touching. The classic picture is a pile 4 ft high, 4 ft deep and 8 ft long: 4 × 4 × 8 = 128. That number includes the air between the pieces, which is why a cord is a volume of stacked wood, not of wood substance.
The cord is the only firewood unit with a legal definition behind it. In the United States, the NIST Handbook 130 Uniform Regulation for the Method of Sale of Commodities requires fireplace and stove wood to be advertised and sold by the cord or a decimal fraction of a cord, and it specifically prohibits the terms face cord, rack, pile, truckload and rick as units of measure unless the actual dimensions are stated. Most states have adopted Handbook 130 by reference into their weights-and-measures law.
Sellers use those words anyway. A face cord is a stack 4 ft high and 8 ft long — the same 32 square feet of face as a cord — but only as deep as the pieces are long. That is the whole trick: the face looks identical, and the volume you get depends entirely on the cut length. Sixteen-inch pieces give you one third of a cord. Twelve-inch pieces give you one quarter. Twenty-four inch pieces give you half.
So the question to ask a seller is never "is that a cord?" It is "what are the three dimensions of the stack, and how long are the pieces?" Those four numbers are what this calculator wants, and from them the answer is arithmetic rather than argument.
The formula, and where the heat number comes from
The volume side is simple. Multiply the length of the face by its height to get square feet of face, multiply by the piece length converted to feet to get stacked cubic feet, and divide by 128 for cords. Divide the face area by 32 instead and you get face cords — the count of 4 × 8 faces, whatever their depth.
The heat side takes four inputs and one physical constant. Wood substance burns at roughly the same energy per pound no matter what tree it came from: about 8,500 BTU per oven-dry pound for hardwoods, a little more for resinous softwoods. What differs enormously is how many pounds of wood substance a cubic foot contains. That is specific gravity, and it ranges from about 0.35 for eastern white pine to 0.72 for shagbark hickory — a factor of two. Multiply specific gravity by 62.4 lb/ft³ (the density of water) and you get pounds of oven-dry wood per cubic foot of solid wood.
A 128 ft³ cord is not 128 ft³ of wood. Split, stacked hardwood typically contains 70 to 90 cubic feet of solid wood per cord, with 80 ft³ the standard working assumption; round, uniform, tightly stacked pieces pack toward the high end and crooked, knotty, loosely thrown wood toward the low end. That is why one seller's cord can genuinely out-heat another's.
Two deductions then apply. Water in the wood must be boiled off and carried out the flue as steam before the wood substance can burn; at about 1,200 BTU per pound of water that is a real loss, proportional to the moisture content on a dry-weight basis. Finally, your appliance sends part of what remains up the chimney. An EPA-certified stove is typically listed in the 60–80% range; an open masonry fireplace can be near zero or even negative on a cold night, because it pulls more heated household air out of the room than it returns.
Worked example: a delivered "cord" of 16-inch red oak for $400
A seller drops a load and you stack it into two rows, each 12 ft long and 4 ft high, cut to 16 in. You paid $400 delivered.
- Face area. 12 ft × 4 ft × 2 rows = 96 ft² of face.
- Piece length in feet. 16 ÷ 12 = 1.3333 ft.
- Stacked volume. 96 × 1.3333 = 128.0 ft³.
- Full cords. 128.0 ÷ 128 = 1.000 cord. The seller was honest.
- Face cords. 96 ÷ 32 = 3.0 faces, each worth 16 ÷ 48 = one third of a cord.
- Price per cord. $400 ÷ 1.000 = $400 per cord.
Now the heat. Red oak has a specific gravity of 0.63, so at 80 ft³ of solid wood per cord the pile holds 80 × 0.63 × 62.4 = 3,144.96 lb of oven-dry wood. At 8,500 BTU/lb that is 26.73 million BTU of gross heat. At 20% moisture you spend 3,144.96 × 0.20 × 1,200 = 754,790 BTU boiling water off, leaving 25.98 MMBtu. Through a 70%-efficient stove you actually get 18.18 MMBtu into the house, at a cost of $400 ÷ 18.18 = $22.00 per million BTU.
Compare that to a rival load: 16-inch eastern white pine, three 4 × 8 faces for $250. That is also 1.000 cord, so $250 per cord looks like the better buy. But pine's specific gravity is 0.35, giving 1,747.2 lb dry, 14.85 MMBtu gross, 10.10 MMBtu usable — and $24.75 per million BTU. The cheaper cord is the more expensive heat, and you handle the same number of logs for less of it.
How to read the result before you hand over money
Start with price per full cord. This is the only figure on which two offers are comparable, and it is where most overpaying happens: a $150 "cord" of 16-inch wood delivered as one 4 × 8 face is $450 per cord, three times what the buyer thinks they agreed. Firewood prices vary far too much by region, species and season for any national figure to be meaningful — call three local sellers, convert each quote to a full-cord basis with this calculator, and compare those.
Then look at cost per million BTU delivered. It folds species, moisture and your stove into one number and lets you compare wood against the fuel you would otherwise burn. The table below converts your pile's usable heat into gallons of propane, gallons of heating oil, therms of gas and kilowatt-hours, so you can multiply by your own utility rate.
Finally, sanity-check the stacked volume against the delivery vehicle before you accept a load. A standard 8 ft pickup bed, filled level to the top of the sides, holds roughly 65 ft³ — half a cord at best, and much less if it is heaped loosely rather than stacked. A load that arrives thrown loose into a dump trailer will shrink noticeably when you stack it, which is precisely why the legal unit is defined on stacked wood.
What a "face cord" is worth, by piece length
| Piece length | Stacked volume of one face | Fraction of a full cord | Faces needed for a full cord |
|---|---|---|---|
| 12 in | 32.0 ft³ | 0.250 | 4.00 |
| 14 in | 37.3 ft³ | 0.292 | 3.43 |
| 16 in | 42.7 ft³ | 0.333 | 3.00 |
| 18 in | 48.0 ft³ | 0.375 | 2.67 |
| 20 in | 53.3 ft³ | 0.417 | 2.40 |
| 24 in | 64.0 ft³ | 0.500 | 2.00 |
| 48 in | 128.0 ft³ | 1.000 | 1.00 |
Fraction of a cord = piece length in inches ÷ 48. Multiply a face-cord price by the last column to get the price per full cord.
Wood density and heat by species
| Species | Specific gravity | Oven-dry lb per cord | Gross MMBtu per cord |
|---|---|---|---|
| Shagbark hickory | 0.72 | 3,594 | 30.5 |
| Black locust | 0.69 | 3,445 | 29.3 |
| White oak | 0.68 | 3,395 | 28.9 |
| American beech | 0.64 | 3,195 | 27.2 |
| Red oak | 0.63 | 3,145 | 26.7 |
| Sugar maple | 0.63 | 3,145 | 26.7 |
| White ash | 0.60 | 2,995 | 25.5 |
| Red maple | 0.54 | 2,696 | 22.9 |
| American elm | 0.50 | 2,496 | 21.2 |
| Douglas-fir | 0.48 | 2,396 | 20.4 |
| Ponderosa pine | 0.42 | 2,097 | 17.8 |
| Quaking aspen | 0.38 | 1,897 | 16.1 |
| Eastern white pine | 0.35 | 1,747 | 14.9 |
Gross heat before the moisture and appliance deductions. Multiply the pounds column by 8,500 to reproduce the last column.
Unseasoned wood is a chimney fire waiting to happen
Wood that has not dried below about 20% moisture burns cool. Cool combustion leaves unburned tars and volatiles in the smoke, and those condense on the flue wall as creosote — the fuel for a chimney fire. The heat penalty is the smaller half of the problem.
Split, stacked off the ground, top-covered and open to the wind, most hardwoods need a full summer; oak commonly needs two. Check with a pin moisture meter on a freshly split face, not on the end grain of a piece that has been in the pile for a year. Have the flue swept and inspected annually regardless of how dry you think your wood is.
Mistakes that cost buyers money
- Accepting a "cord" without dimensions. Ask for length, height and piece length before delivery, and stack it before you pay if you can.
- Measuring the rack instead of the wood. Height means the average height of piled wood, so measure a low spot and a high spot and split the difference.
- Comparing $/cord across species. A cord of white pine holds about half the wood substance of a cord of hickory. Compare cost per million BTU instead.
- Buying by weight without a moisture figure. A ton of green wood can be more than a third water, and you pay for the water twice — once by the pound and once by the BTU you spend boiling it off.
- Assuming a pickup bed holds a cord. A level-full 8 ft bed is roughly 65 ft³, about half a cord, and a half-ton truck is over its payload with a full cord of green oak aboard.
- Stacking loosely and calling it a cord. Cross-hatched, criss-crossed or thrown piles contain far less wood than a parallel-stacked cord of the same outside dimensions.
What this calculator assumes, and what it cannot know
The volume side is exact: it is a rectangular box divided by 128, and if your measurements are right, the cord figure is right. The heat side is a model, and it depends on three things it has to assume.
The first is solid content per cord, set at 80 ft³ by default. The true figure for your pile depends on how it was cut and stacked and can plausibly sit anywhere from about 65 to 95 ft³; change the input if you have reason to. The second is the 8,500 BTU per oven-dry pound heat of combustion. That is a good average for hardwoods; resinous softwoods such as pine and fir run several hundred BTU per pound higher, which slightly narrows the gap the table above shows. The third is your appliance's real-world efficiency, which is lower than its laboratory rating whenever you damp it down.
The calculator does not model bark (which burns, but at lower density), rot or insect damage in the pile, the extra heat a catalytic stove extracts from smoke, or the standing-loss behaviour of an outdoor wood boiler. It also cannot value your labour: a cord of hickory and a cord of aspen take about the same time to buck, split and stack, and the hickory gives you twice the heat for it.
If you are working from standing timber rather than a delivered stack, size the log first with the board feet log scale calculator, and estimate the tree with the tree height calculator.
Firewood terms worth knowing
- Full cord (standard cord)
- 128 cubic feet of stacked wood — the 4 × 4 × 8 pile. The only unit defined in NIST Handbook 130.
- Face cord, rick, run
- A stack 4 ft high and 8 ft long, one piece-length deep. Its fraction of a cord is the piece length in inches divided by 48.
- Sheldon cord
- A seller's term for any stack deeper than 4 ft. It has no fixed size and must be quoted with dimensions.
- Specific gravity
- Oven-dry mass of wood divided by the mass of an equal volume of water. Multiply by 62.4 to get pounds of dry wood per cubic foot of solid wood.
- Moisture content (dry basis)
- Pounds of water per pound of oven-dry wood, as a percent. What a wood moisture meter reads. Green wood is often 60–100%; seasoned firewood is 15–20%.
- MMBtu
- One million British thermal units. The standard unit for comparing heating fuels; a gallon of propane holds 0.0915 MMBtu.