Chance to Jam = ( Your ECM Strength / Target's Sensor Strength ) * 100%

And if you use multiple jammers:
Chance to Jam = (1 -(1 - Your ECM Strength / Target's Sensor Strength ) ^ The number of jammers of this strength)*100%

Note that when calculating the chance to jam a single target using a rainbow fit, the above formula must be modified, considering that a rainbow fit will have different jam strengths, per racial jammer, dependent on the target race type. As an example, jamming a ladar sensor ship:

Chance to Jam = (1 -(1 - Ladar Jammer Strength / Target's Sensor Strength ) x (1 - Ladar Jam Strength on non Ladar Jammers / Target's Sensor Strength )^3 ) x 100%

(High) A641 B274 B449 B520 D792 D845 N110 S047
(Low) A239 C140 C391 J244 N290 N944 R051 U210
(Null) C248 E545 K329 K346 S199 V283 Z060 Z142
Class 1 H121 P060 Q317 V301 Y790 Z647 Z971
Class 2 C125 D364 D382 G024 I182 N766 R943
Class 3 C247 L4777 M267 N968 O477 O883 X702
Class 4 E175 M609 O128 T405 X877 Y683