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 be different, depending on the species you are using as fire wood. Density of wood will vary at different moisture contents and unless you know at which moisture content it was determined it is kind of useless. To be able to calculate the density you would need to know the basic specific gravity of the wood species. The specific gravity where the wood is at its lightest weight, as in, oven dried to where it 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
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

If you consider that 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 then this cord could potentially produce:
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 approx 20% of 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 pound weight 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
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