Wood density is an important measurement for several reasons. For one, it gives a good indication of how heavy the wood is, which will affect transportation and construction costs. Density also effects how easy many processes, like pressure treatments, will be for a particular type of wood. Density also governs how well a particular type of wood will do as a furniture or floor material. There are many types of oak, but they tend to be denser than many woods and make suitable construction timbers.
There is great disparity in the density of oak wood. Oak species grow around the world, and each species has its own density. Climate and temperature also play an important role. Oak wood that has dried out will tend to be more dense that oak wood filled with water, because water keeps the cells moist, expanded and heavier while dry cells are more compact, shrinking in on themselves, but also making the oak lighter.
Density of trees is typically measured by kilograms per cubic meter. The oak trees tends to average between 600 and 900kg/meter cubed, with 680 being a popular measurement for the most common types, like red oak. At the high end of the density scale, oak averages the same as the denser tropical hardwoods or compact fiberboard, making it extremely durable.
Mahogany has similar density measurements to oak, averaging between 495 and 850kg/cubic meter, depending on dryness and growth. Elm and birch trees also have similar densities, although they rarely reach above 700kg/cubic meter, bringing them closer to the softer types of oak. Ebony and lignum vitae regularly score higher than the densest types of oak, at 1,000kg/cubic meter or above.
Specific gravity is another common measurement used when comparing tree hardness and density. It is a comparison of the density of the wood with the density of water, which is set at 1.0. Numbers lower than this will float in water, while higher and extremely dense materials will sink. Oak tends to have a specific gravity of between 0.80 and 0.95, common numbers for an American hardwood.
Hardness is a scale used to show how durable the oak wood is, closely related to its density. It is measured in pounds per square inch, with the number of pounds necessary to crack or damage a board being the measurement marker for the wood. Oak scores between 1000 and 1400 on this scale, with some species like swamp white oak reaching as high as 1600.