For a given scale length and tuning:
· Increasing string diameter (gauge) will increase the tension, decreasing the diameter will lower tension.
· Tuning up or down one whole step will change the tension by ~.8kg.
NOTE: This is especially important to know, since tuning up all courses 1 whole step (e.g: trying out Turkish tuning on a normally Arabic-tuned oud) could possibly damage your instrument due to the great increase in tension. It's best to lower the overall string gauges when tuning up a whole step. On the other hand, if you are tuning down 1 whole step, the tension will be greatly decreased, resulting in a drastic sound change (not necessarily for the worse), or possibly rendering the strings too loose for good playability.
· For every 1cm change in scale length (at the same tuning) the tension will change by ~.13kg