The chart below shows the air dried density of common wood species – this is the number given in the first column after the wood name.

This indicates the approx weight of one cubic foot of the wood when air-dried (about 20% moisture).

For the purposes of the Burn Time Calculator on this site, we have used the following densities for our calculations:
25 – Soft
30 – Medium
38 – Hard
44 – Extra Hard

Chart above is from the Engineering Toolbox at:
http://www.engineeringtoolbox.com/wood-combustion-heat-d_372.html