If i convert ac to dc with a bridge (4 diodes) and my input voltage is 30 volts, what's the output voltage? I've heard that it increases but i don't by how much.
It doesn't increase - it decreases by the voltage dropped across two of the bridge diodes.
The reason you get the apparent increase is because if you fit a reservoir capacitor after the bridge it charges to the peak of the rectified half cycles.
If you are using a capacitor, the voltage across it will rise to the RMS voltage multiplied by sqrt(2) then with 2 times the bridge diode Vf subtracted from the result. (RMS * 1.414) - (Vf * 2).