How Much Firewood Do I Need? Calculator Based on Your Winter Fuel Use
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The most reliable way to estimate how much firewood you need is to start from the fuel you already burned last winter. Your propane, heating oil, gas or electric bill tells you how much heat your home used. This page converts that into cords of firewood for the wood you plan to buy.
How the calculator works
Every fuel has a known amount of heat per unit. The U.S. Energy Information Administration (EIA) lists these values:
| Fuel | Heat per unit |
|---|---|
| Propane | 91,452 BTU per gallon |
| Heating oil | 138,500 BTU per gallon |
| Natural gas | 100,000 BTU per therm (1,036 BTU per cubic foot) |
| Electricity | 3,412 BTU per kWh |
The calculation has three steps:
- Heat in last winter's fuel = amount used × BTU per unit.
- Heat that actually reached your home = step 1 × the efficiency of your old equipment. Efficiency is the share of the fuel's heat that heats your home instead of going up the flue. It is usually on the equipment label or in the manual.
- Cords needed = step 2 ÷ (million BTU per cord of your wood × wood stove efficiency).
Million BTU per cord comes from the BTU chart, which lists 44 species from Utah State University Extension. Pick the species you will actually burn. A cord of osage-orange holds more than twice the heat of a cord of eastern redcedar, so the species matters a lot.
Worked examples
These use made-up usage amounts to show the math. They are not typical figures for any home. All three assume red oak (24.6 million BTU per cord), a 70% efficient wood stove, and old equipment that was 80% efficient.
| Last winter's fuel | Heat in the fuel | Useful heat (× 80%) | Cords of red oak |
|---|---|---|---|
| 800 gallons of propane | 73.2 million BTU | 58.5 million BTU | 3.4 cords |
| 600 gallons of heating oil | 83.1 million BTU | 66.5 million BTU | 3.9 cords |
| 900 therms of natural gas | 90.0 million BTU | 72.0 million BTU | 4.2 cords |
Take the propane row. 800 × 91,452 = 73.2 million BTU. At 80% efficiency, 58.5 million BTU reached the house. One cord of red oak in a 70% stove gives 24.6 × 0.70 = 17.2 million BTU, so 58.5 ÷ 17.2 = 3.4 cords.
Adjusting for your own situation
- Wood heating only part of the house? Lower the "share" box. If the stove will supply half your heat, enter 50.
- A different wood changes the answer. Switch the species in the calculator. With the same propane example, a cord of white pine (15.9 million BTU) gives 15.9 × 0.70 = 11.1 million BTU, so you need 58.5 ÷ 11.1 = about 5.3 cords instead of 3.4. See the BTU chart for every species.
- Damp wood burns worse. The calculation assumes wood at 20% moisture or less, which the DOE gives as the standard for good firewood. Wet wood wastes heat boiling off water, so you would need more of it. See how long to season firewood.
- Round up. The result is an estimate. A colder winter, a stove that burns less efficiently than its rating, or wood that is not fully dry all push the number up. Buying a little extra is cheaper than running short.
Converting cords into what you actually buy
Sellers often sell face cords, not full cords. A face cord of 16-inch pieces is one third of a full cord, so 3.4 cords is about 10 face cords. The cord calculator and the face cord vs full cord page show how to convert, and how to check what a seller actually delivered.
Sources
- U.S. Energy Information Administration, British thermal units (Btu): heat content of propane, heating oil, natural gas and electricity.
- U.S. Department of Energy, Wood and Pellet Heating: wood stove efficiency ranges and the 20% moisture standard.
- Utah State University Extension, Wood Heating, Table 1: million BTU per cord by species.