If you have the starting and ending times of the P, Q, R, S, and T waves in your EKG, the definitions are:
- ST — End of the QRS complex (end of the S-wave) to the beginning of the T-wave
- PR — Beginning of the P-wave to the beginning of the QRS complex
- QT — Beginning of the QRS complex to the end of the T-wave
Note that the length of the ST segment is rarely used in clinical electrocardiogrpahy because the end of the QRS complex may not always be easy to define. The morphology of the ST segment (shape, and deviation from the isoelectric baseline that is usually defined as the amplitude of the PR segment) is important. It is nevertheless important to identify it for that reason.
A good online reference is ECG Durations (link). See the latest edition of Braunwald’s Heart Disease for a thorough discussion.