Firewood Heat and Cord Value Calculator
Measure a neatly stacked load, choose the closest wood species, and account for moisture and appliance efficiency. The results compare heat and cost on the same delivered-heat basis and show how many of that stack you need for your target.
Useful heat delivered (million BTU)5.73
- Stacked volume (cubic feet)
- 42.56
- Full-cord equivalent
- 0.33
- Estimated wood and water weight
- 1,173 lb dry wood; 235 lb water
- Available heat (million BTU)
- 8.18
- Cost per delivered million BTU
- $30.56
- Exact stacks needed for target
- 5.24
- Whole stacks to buy
- 6
- Full cords needed for target
- 1.74
- Exact cost to reach target
- $916.93
- Cost using whole stacks
- $1,050.00
Heat uses Utah State University Extension dry-cord values and a 20% dry-basis moisture reference.
Volume assumes a neatly stacked pile; a loose or irregular pile contains less wood than its measured space suggests.
How to use this calculator
- Select the species that most closely matches the wood in the stack.
- Measure the stack’s average length, height, and depth in feet.
- Enter a dry-basis moisture reading from a freshly split face and a realistic appliance efficiency.
- Enter the stack price and the useful heat you want, then compare the exact and whole-stack quantities.
How firewood heat and cord value are calculated
A full cord is 128 cubic feet of neatly stacked firewood and air space. Start by measuring the offered stack: V = length × height × depth. Its full-cord equivalent is C = V ÷ 128. This matters because a face cord has no fixed volume: an 8-foot-long, 4-foot-high stack is deeper when its logs are longer.
The calculator uses the selected species’ published dry weight per cord and heat per cord from Utah State University Extension. Dry wood weight is W = C × Wd, and water weight is Wwater = W × M, where M is moisture content as a decimal on a dry basis.
Species heat values are referenced to 20% moisture. To estimate the effect of a different reading, heat per cord is adjusted by Qcord = max(0, E20 − Wd × 1,200 × (M − 0.20) ÷ 1,000,000). The 1,200 BTU term represents heat used to evaporate each additional pound of water. Available stack heat is Q = C × Qcord; useful heat is Qu = Q × η, where η is appliance efficiency.
What changes the result most
Stack depth and moisture can move a comparison more than the label “face cord” suggests. A standard 8-by-4 face stack is 32 cubic feet with 12-inch pieces, about one quarter cord, but 48 cubic feet with 18-inch pieces, about three eighths cord. Dense woods generally yield more heat per cord, while a wetter stack spends more of its combustion energy heating water rather than the room.
Efficiency is equally important for value. The calculator divides the stack price by delivered heat, so a less efficient appliance increases the effective cost even when the wood price does not change. It also calculates the exact stack quantity for a heat target and the number of whole stacks you would actually buy.
What this estimate does not measure
The table values are regional averages, not a test of the particular tree or load. Bark, decay, species mix, piece size, air gaps, and measurement error can change the amount of solid wood in a stack. The moisture adjustment is an energy estimate; it does not predict ignition, draft, incomplete combustion, creosote, or whether a fuel is safe for a particular appliance.
Worked example
An 8-foot-long stack, 4 feet high and 1.33 feet deep, has a volume of 42.56 cubic feet, or 0.33 full cords. Using red oak at 20% moisture, the Utah State table value of 24.6 million BTU per cord gives 8.18 million BTU available from the stack. At 70% appliance efficiency, it delivers 5.73 million BTU.
If that stack costs $175, its delivered-heat cost is $30.56 per million BTU. A 30 million BTU target needs 5.24 stacks exactly, so buying whole stacks means six stacks and $1,050. The same target represents 1.74 moisture-adjusted full cords at that efficiency.
Common questions
How many face cords equal one full cord?
A face cord is not a fixed fraction of a full cord because its depth is the log length. An 8-foot by 4-foot stack with 16-inch logs is about 42.7 cubic feet, or one third of a 128-cubic-foot full cord. Measure depth rather than relying on the name used by the seller.
How does log length change the size of a face cord?
For the same 8-foot-long and 4-foot-high face stack, 12-inch logs make a 32-cubic-foot stack and 18-inch logs make a 48-cubic-foot stack. The longer-log stack therefore contains 50% more stacked volume. It is not a fair price comparison until volume is included.
Which firewood species produces the most heat per cord?
Among the species in this calculator, white oak has the highest published heat value at 29.1 million BTU per dry cord in the cited Utah State table. Black locust, beech, and apple are also high. Availability, seasoning, piece size, and the actual stack volume still affect the heat you buy.
How much heat is lost when firewood is wetter than 20%?
This estimate subtracts 1,200 BTU for every pound of water above the 20% dry-basis reference. The loss depends on the species’ dry cord weight, so a heavier cord has a larger calculated water penalty at the same moisture reading. Real wet wood can perform worse because it is also harder to light and burn completely.
What appliance efficiency should I enter for a stove or fireplace?
Use the efficiency stated for your specific appliance if it is available, and treat it as an estimate in everyday use. A fireplace, older stove, or poorly operated appliance can deliver far less heat to the room than a modern tested stove. Installation, draft, maintenance, and firing rate all matter.
Is wood moisture content measured on a wet or dry basis?
This calculator follows the supplied dry-basis formula: water weight divided by oven-dry wood weight. Many consumer moisture meters and firewood recommendations use wet-basis readings instead, so verify how your meter reports its percentage before entering it. A reading must be taken on a freshly split interior face for a useful estimate.