A cord of firewood is 128 cubic feet of stacked wood, so a stack 16 ft long, 4 ft high and 16 in deep holds 85.3 cu ft, which is two thirds of a cord or 2 face cords. Enter your own stack above to get cords, face cords, ricks and the real price per cord, or switch to the winter tab to see how many cords your house burns in a season.
How the firewood calculator works out cords
The stack tab multiplies length, height and depth, then divides by the size of a cord. Iowa State and Utah State extension both define the cord the same way: 4 ft by 4 ft by 8 ft, 128 cubic feet of "wood and air space".
Cords = length (ft) × height (ft) × depth (ft) ÷ 128 cu ft
Face cords count the 4 ft by 8 ft faces instead: face cords = length × height ÷ 32 sq ft. A face cord is one third of a cord only when the pieces are 16 in long. That is the whole trick behind "cheap" face cord prices, and the reason the calculator asks for depth.
Cord, face cord and rick: how they compare
| Unit | Face (high × long) | Depth | Volume | Share of a cord |
|---|---|---|---|---|
| Full cord | 4 ft × 8 ft | 4 ft | 128 cu ft | 1 |
| Face cord or rick, 16 in pieces | 4 ft × 8 ft | 16 in | 42.7 cu ft | 1/3 |
| Face cord, 12 in pieces | 4 ft × 8 ft | 12 in | 32 cu ft | 1/4 |
| Face cord, 18 in pieces | 4 ft × 8 ft | 18 in | 48 cu ft | 3/8 |
| Pickup bed, level load | about 64 cu ft | 1/2 |
USU Extension gives "32 to 48 cubic feet" for a face cord and about 64 cu ft for a full-size pickup loaded level. "Rick" is a regional word for a face cord. In metric, one stere (1 m³ stacked) is 0.276 cord, and a cord is 3.62 m³.
Price per cord: is the quote fair?
Enter the price in the stack tab and the calculator divides it by the cords in the stack. A seller asking $95 for a face cord of 16 in pieces is asking $285 per cord. The same $95 for a face cord of 12 in pieces is $380 per cord. Before you pay, measure the stack with your own tape, not with the seller's word for "a cord". Legal cord rules vary by state, and NIST Handbook 130 is the model most states follow.
How many cords of firewood do you need for a winter?
The winter tab estimates the heat your house loses in a season and divides it by the usable heat in a cord.
Heat needed (Btu) = floor area × 0.667 Btu/h per °F per sq ft × 24 h × heating degree days
Cords = heat needed × wood share ÷ (Btu per cord × stove efficiency)
The 0.667 comes from a published worked example: a 1,200 sq ft house loses 800 Btu per hour per degree of temperature difference. The tight and leaky settings move it 25% down or up, which is our adjustment, not a measurement. Heating degree days measure how cold your climate is; the IECC zones run from under 3,000 HDD in zone 2 to 12,600 in zone 7, and the calculator starts from a typical value inside your zone. Type your own if you have it.
Worked example: 1,800 sq ft in zone 5
- Heat lost: 1,800 sq ft × 0.667 × 24 × 6,300 HDD = 181.4 million Btu.
- Red oak carries 3,536 lb per cord at about 7,000 Btu per lb, or 24.8 million Btu. At 60% efficiency one cord gives 14.9 million usable Btu.
- 181.4 ÷ 14.9 = 12.2 cords if wood is the only heat, about 37 face cords. At a 50% share it is 6.1 cords.
The published example (1,200 sq ft, 5,000 HDD, a 20 million Btu cord, 50% stove) comes to 9.6 cords with these formulas.
Firewood species: how much heat is in a cord?
Density decides it. Weights below are Iowa State's, at 7,000 Btu per pound, and the last column repeats the 1,800 sq ft example at 60% efficiency.
| Species | Weight per cord | Million Btu per cord | Cords for the example |
|---|---|---|---|
| Shagbark hickory | 4,072 lb | 28.5 | 10.6 |
| Red oak | 3,536 lb | 24.8 | 12.2 |
| Green ash | 3,296 lb | 23.1 | 13.1 |
| Silver maple | 2,752 lb | 19.3 | 15.7 |
| Cottonwood | 2,272 lb | 15.9 | 19.0 |
| Basswood | 1,984 lb | 13.9 | 21.8 |
Cottonwood needs nearly twice as many cords as hickory, so compare price per million Btu, not price per cord.
What the estimate leaves out
Iowa State reports an average of 3 to 7 cords a year for home heating, mostly supplemental, so a full-wood result of 12 cords is a cold-climate, whole-house figure. EPA points out that overall efficiency is often 10 to 20 points below the advertised combustion figure, and that certified stoves use about a third less wood than older ones. Wet wood wastes heat boiling off water: burn wood at about 20% moisture.
Next questions
Size the stove that burns the wood in the wood stove size calculator, and see what the same house needs in Btu from the BTU calculator or the furnace size calculator. To compare fuels, run the propane usage calculator and the heat pump size calculator for the same floor area, and measure the space with the square footage calculator. All of these sit in the heating and plumbing calculators hub.