section_path: Physical properties/Specific gravity
section_name: Specific gravity
paragraph_text: Specific gravity
  
  The single most revealing property of wood as an indicator of wood quality is specific gravity (Timell 1986), as both pulp yield and lumber strength are determined by it. Specific gravity is the ratio of the mass of a substance to the mass of an equal volume of water; density is the ratio of a mass of a quantity of a substance to the volume of that quantity and is expressed in mass per unit substance, e.g., grams per milliliter (g/cm or g/ml). The terms are essentially equivalent as long as the metric system is used. Upon drying, wood shrinks and its density increases. Minimum values are associated with green (water-saturated) wood and are referred to as basic specific gravity (Timell 1986).
  
  The U.S. Forest Products Laboratory lists a variety of ways to define specific gravity (G) and density (ρ) for wood:
  
  The FPL has adopted G and G for specific gravity, in accordance with the ASTM D2555 standard. These are scientifically useful, but don't represent any condition that could physically occur. The FPL Wood Handbook also provides formulas for approximately converting any of these measurements to any other.