Hi all! I apolagize if this has done before, but I could not find this in the excellent amp faq that Joris compiled. There has been an abundance of ohm related questions here, so here I give you the quick answer to calculating ohms. R = ohm R1 = ohm of cabinet 1 R2 = ohm of cabinet 2 etc. R1 * R2 * R3 (etc.) ----------------------- = R R1 + R2 + R3 (etc.) For example, if you have two cabs, a 8ohm cab and a 4ohm cab and you want to know how many ohms that will make, you simply use the formula here above. 8 * 4 -------- = R 8 + 4 So, 8 * 4 = 32 and 8 + 4 = 12 Finally 32 / 12 = 2.66666....ohms. If the total ohms number is lower than your amplifier specifications recommend, it is not safe to use that particiliuar combination of cabinets as it would most likely result in a fried amp. On the other hand, if the number is higher than your amplifier specifications recommend it is safe, but the amp won´t be pumping out as many watts as it could have. As a sidenote, if your amp is capable of 2ohms, and you have a 8ohm cab and a 4ohm cab, which is safe in this instance. The 4ohm cab will be much louder then the 8ohm one. Hope this helps!