Firewood BTU Calculator

Understanding BTUs and Firewood Energy

A British Thermal Unit (BTU) is the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. When applied to firewood, BTU ratings describe the total heat energy released when a specific quantity of wood is burned completely.

In this context, we talk about BTUs per cord. A cord is a volume measure, so different species with different densities hold different amounts of energy in the same volume. Dense hardwoods such as oak and hickory contain more wood fiber (and therefore more stored energy) per cord than light softwoods such as pine or spruce.

The calculator uses typical BTU-per-cord values for each species, assuming properly seasoned wood. It then adjusts those values based on moisture content to reflect how much of that energy is effectively available for heating your home rather than evaporating water from the wood.

Key Formula Used in the Calculator

The core idea is that total potential BTUs depend on species and volume, while usable BTUs are reduced by moisture. Conceptually:

Total BTUs = BTU per cord (dry) × number of cords

To account for moisture, we apply a simple adjustment factor that lowers usable BTUs as moisture increases:

Where:

• BTUusable is the approximate usable heat output after moisture losses.
• BTUdry is the total BTUs if the wood were ideally dry (based on species and cords).
• M is the moisture content percentage (wet basis).
• k is a simplifying factor representing average efficiency loss per unit of moisture. The calculator uses a reasonable constant so that higher moisture gives noticeably lower usable BTUs.

This model is intentionally simplified; real-world combustion is more complex and depends on stove design, draft, and how you operate your system.

Typical BTU Values by Wood Species

Different species vary widely in the amount of heat they provide per cord. The table below lists approximate values for properly seasoned wood (around 20% moisture content). Values are rounded and may differ from the specific values used internally by the calculator, but they are close enough for planning and comparison.

Use the table as a quick reference if you are deciding which logs to buy or which piles to reserve for the coldest weather. High-BTU hardwoods make sense for overnight burns and the coldest days, while lower-BTU softwoods are useful for shoulder seasons and starting fires.

Interpreting Your Firewood BTU Results

The calculator typically provides two key values:

• Total potential BTUs: The theoretical heat content of your wood based on species and cords, assuming ideal combustion and well-seasoned fuel.
• Moisture-adjusted BTUs: An estimate of how much of that energy is realistically available for heating after accounting for moisture losses.

To put these numbers into context, consider that a small, well-insulated home might need roughly 30–50 million BTUs over a heating season, while a larger, less efficient house in a cold climate could require 80–120 million BTUs or more. These are rough ranges and can vary significantly with climate, insulation, air leakage, and thermostat habits.

When you see your result:

• If your moisture-adjusted BTUs are far below your estimated seasonal needs, you may need to buy more wood, improve insulation, or supplement with another heat source.
• If your total BTUs are close to your expected needs but moisture-adjusted BTUs are much lower, your main opportunity is to improve seasoning and storage so more of the wood’s energy actually heats your home.
• If your usable BTUs significantly exceed seasonal needs, you might be able to carry some wood over into the next season, as long as it stays dry.

Remember that your stove or fireplace is not 100% efficient. A high-efficiency wood stove might convert 70–80% of the wood’s usable BTUs into space heat, while an open fireplace may deliver far less. The calculator focuses on the fuel side (what is in the wood), not on appliance efficiency.

Worked Example: Planning a Winter with Oak Firewood

Imagine you live in a moderately cold climate and estimate that your home needs around 60 million BTUs of delivered heat to get through the winter. You have access to seasoned oak and want to see if two cords are enough.

1. Select species: Choose Oak from the dropdown.
2. Enter amount: Type 2 in the Amount (cords) field.
3. Moisture content: Your moisture meter shows about 20%, so leave the default at 20.
4. Run the calculation: Click the calculate button.

Assume the calculator uses a base value of about 25 million BTU per cord of seasoned oak. Then:

• Total potential BTUs (dry basis): 25 million BTU/cord × 2 cords = 50 million BTUs.
• Moisture-adjusted BTUs: The tool applies its moisture factor. You might see something like 42–45 million usable BTUs reported.

Comparing to your 60 million BTU target, you can see that two cords of oak are probably not enough on their own, especially after factoring in stove efficiency. You might plan for:

• Adding another cord of oak, or
• Supplementing with another heat source (e.g., electric, gas, or heat pump) on the coldest days, or
• Improving air sealing and insulation so your total seasonal BTU requirement drops.

This simple example shows how the calculator helps turn abstract BTU numbers into practical heating decisions.

How Moisture Content Affects Heat Output

Moisture content is one of the most important factors in how much heat you actually feel from your firewood. High moisture content means:

• More of the fire’s energy is used to boil water out of the logs instead of heating your home.
• Lower flue gas temperatures, which can increase creosote buildup and require more frequent chimney cleaning.
• More smoke, harder starts, and higher risk of smoldering, inefficient burns.

As a rule of thumb:

• 15–20% moisture: Ideal range for most stoves; good balance of energy content and clean burning.
• 25–30% moisture: Marginal; you will notice more smoke and less heat per cord.
• 35–50%+ moisture: Considered green wood; best left to dry for at least one more season before regular use.

The calculator’s moisture slider or input lets you visualize how much heat you are leaving on the table if you burn wood that is not fully seasoned. Reducing moisture content from, say, 35% to 20% can increase usable BTUs dramatically, even though the underlying species and cords stay the same.

Tips for Storing and Seasoning Firewood

To maximize the BTUs you get from each cord, proper storage and seasoning are essential.

• Split early: Split logs expose more surface area, allowing moisture to escape faster. Split wood can season in 6–12 months, depending on species and climate.
• Keep it off the ground: Stack wood on pallets, runners, or a wood rack to minimize ground moisture and rot.
• Allow airflow: Cover the top of your stack to shed rain and snow, but leave the sides open. Solid tarps wrapped around the entire pile can trap moisture.
• Face sun and wind: If possible, orient stacks so prevailing winds and sunlight help drive off moisture.
• Rotate your stock: Burn the oldest, driest wood first, and keep greener or newly delivered wood for future seasons.

Well-seasoned wood not only delivers more BTUs, it also burns cleaner, reduces creosote buildup, and is easier on your stove and chimney system.

Assumptions and Limitations of This Calculator

While the tool provides useful estimates, it does not replace a professional heating load calculation or detailed fuel analysis. Understanding the assumptions behind the numbers will help you use the results appropriately:

• Typical BTU values: BTU-per-cord figures for each species are based on commonly cited reference values for properly seasoned wood. Real-world values can vary by tree age, growing conditions, and exact species (e.g., red vs. white oak).
• Standard cord definition: The calculations assume a full cord is about 128 cubic feet of tightly stacked wood (4 ft × 4 ft × 8 ft). Face cords or loose piles contain less wood volume and thus fewer BTUs.
• Simplified moisture model: The impact of moisture on usable BTUs is approximated with a single factor. In reality, the relationship depends on combustion temperature, airflow, and stove design.
• No appliance efficiency: The calculator estimates energy in the wood itself. It does not account for stove or fireplace efficiency, which can significantly reduce the heat that actually stays in your home.
• Climate and house differences: Seasonal BTU needs vary widely with climate zone, insulation, airtightness, window quality, and occupant behavior. Any seasonal comparisons here are broad generalizations.

Because of these limitations, treat the outputs as planning tools rather than exact predictions. When in doubt, consult with a local chimney professional or heating contractor who understands your specific house and climate.

Safety note: Always operate wood-burning appliances according to the manufacturer’s instructions, burn only appropriately seasoned wood, and have your chimney inspected and cleaned regularly to reduce the risk of chimney fires from creosote buildup.

Frequently Asked Questions

How accurate are BTU-per-cord numbers for firewood?

BTU-per-cord numbers are averages. Individual trees and loads of wood can be higher or lower depending on growth conditions, exact species mix, and how the wood was processed and stored. The calculator’s values are suitable for planning but should not be treated as laboratory-grade measurements.

Do I need a moisture meter to use this calculator?

You can use the tool with an estimated moisture value, but a moisture meter improves accuracy. Affordable pin-type meters are widely available and can quickly tell you whether your wood is in the ideal 15–20% range.

Can I mix different species in one calculation?

The calculator assumes a single dominant species. If your pile is mixed, you can either pick the species that makes up most of the volume or run separate calculations for each species and add the BTU results together.

How does this compare to other heating fuels?

To compare firewood with propane, natural gas, or electricity, you can convert everything to BTUs. For example, one gallon of propane contains about 91,500 BTUs. By comparing cost per million BTUs, you can see which fuel is most economical in your area, assuming similar appliance efficiencies.

What if my results do not match my real-world experience?

Differences are usually due to one or more of the following: wood not being as dry as assumed, lower appliance efficiency than expected, draft or chimney issues, or an inaccurate estimate of seasonal BTU needs. Use the calculator as a guide and adjust your expectations based on experience over one or two heating seasons.

Related Heating and Energy Planning

Estimating your firewood BTUs is only one piece of planning for winter comfort. Consider pairing this calculator with other tools that estimate overall heating demand, compare fuel costs, or help you size heating equipment. Combining several simple estimates usually gives a clearer picture than relying on a single number.