Spike's Calculators

# Amount of Heat(Btu's) from A Cord of Firewood

One pound of wood is one pound of wood. The density of wood, pounds per cubic foot, will differ depending on the species you use as firewood. Wood density will vary at different moisture contents; unless you know at which moisture content it was determined, it is not helpful. To calculate the density, you need to know the "basic" specific gravity of the wood species. This is the specific gravity number where the wood is at its lightest weight, oven-dried and contains 0% moisture.

##### Calculating Weight @ 0% MC
The formula to calculate the weight at 0% moisture:
`volume(cubic feet) x Specific Gravity(Basic) x weight of one cubic foot of water = weight in lbs`
It is easy to Google what the basic specific gravity of a certain species is.
One source lists Eastern Hemlock as having a basic specific gravity of 0.41
On average, one cord of wood (128 Cubic Ft ) contains a volume of 85 cubic feet of solid wood.
```The weight of one cubic foot of water = 62.4 lbs
What one pound of dry wood (0%MC) can produce = 8600 Btu's```

Then the weight of one cord of Eastern Hemlock @ 0% MC would be:
`85 * 0.41 * 62.4 = 2174.64 lbs`
Since this is all dry wood, this cord could potentially produce the following:
`2174.64 * 8600Btu/lb = 18696400 Btu's 18.7 million Btu's`

##### Calculating Weight @ Various % MC
To calculate the weight of this cord @ various MC% you will have to add to the first formula:
(1 + MC/100)
and it becomes:
`volume(c f) x Specific Gravity(B) x 62.4 *(1 + MC/100)`
Since, most likely, the firewood you are burning will contain approximately 20% moisture after being air dried:
One cord of Eastern Hemlock @ 20% MC:
```85 * 0.41 * 62.4*(1 + 20/100) =
85 * 0.41 * 62.4 * 1.2 = 2609.568 lbs ```
So now you are thinking more pounds per cord = more Btu's!! Let's make it heavier by adding more water!!
If you read how to calculate Btu's per pound of dried wood, then you would know that the dried wood is the part that produces Btu's and not the additional moisture in the wood.
Proportional Weight of Wood and Water in One Pound

At 20% MC one pound of wood will contain 0.83 lb of dry wood and 0.17 lb of water
```8600 * 0.83 = 7138 Btu's @ 20% MC
2609.85 * 7138 = 18629109.3 Btu's = 18.6 MBtu's.```
That is close to the same as the previous result @ 0 MC of 18696400 Btu's = 18.7 MBtu's