Specific Rotation Calculator
Determine the specific rotation of an optically active compound from a polarimeter reading, path length, and concentration.
What specific rotation measures
Specific rotation, written [α], is a physical constant that describes how strongly a chiral (optically active) compound rotates plane-polarized light, normalized for concentration and path length so results can be compared between labs and instruments.
The formula used
[α] = α ÷ (l × c), where α is the observed rotation in degrees read from a polarimeter, l is the sample path length in decimeters, and c is the concentration of the solution in grams per milliliter. Specific rotation is usually reported alongside the temperature and wavelength of light used (commonly the sodium D-line at 20°C), since both affect the value.
- A positive value indicates a dextrorotatory (+) compound; a negative value indicates levorotatory (-).
- Path length is almost always 1 dm (10 cm) for standard polarimeter cells.
This tool is intended for general chemistry education and laboratory reference purposes.