Sweetness is commonly measured by comparison to reference solutions of sucrose. Sucrose is the standard to which all other sweeteners are compared. Humans can recognize sweetness in about 1 or 2% sucrose solution. Coffee is typically sweetened to about the level of 5% sucrose. Soft drinks are usually about as sweet as 10% sucrose. 15% sucrose is really sweet and starts to feel a little syrupy. Taste panelists are often trained to quantitate sweetness on a 15 cm line scale, for convenience, using 2-15% sucrose solutions as references.
Other sweeteners are then tasted at a series of dilutions to determine the concentration that is as sweet as a given percent sucrose reference. For example, if a 1% solution of sweetener X is as sweet as a 10% sucrose solution, then sweetener X is said to be 10 times as potent as sucrose.
For a detailed discussion, see "A Systematic Study of Concentration-Response Relationships of Sweeteners," G.E. DuBois, D.E. Walters, S.S. Schiffman, Z.S. Warwick, B.J. Booth, S.D. Pecore, K. Gibes, B.T. Carr, and L.M. Brands, in Sweeteners: Discovery, Molecular Design and Chemoreception, D.E. Walters, F.T. Orthoefer, and G.E. DuBois, Eds., American Chemical Society, Washington, DC (1991), pp 261-276.