I'm assuming what is not clear to you is the difference between RMS voltage and peak-to-peak voltage.
Let's dig in a bit of theory, then!

When you probe in "AC mode" a sine wave (such as the output of a transformer) on your multimeter, what you are reading is the RMS voltage; in other words, this is the voltage that is comparable to a DC equivalent, in terms of power delivered to a resistive load (such as light bulbs or heating appliances). For example, 230V (or 115V, depending on where you live) is the
RMS voltage of the power grid.
On the other hand, the peak-to-peak voltage is the voltage you literally would see between peaks on an oscilloscope, were you to probe the signal (don't do it on the power grid directly, though...). It's the voltage measured between peaks of the sinusoidal voltage wave, and it's also closely related to the voltage you get when you rectify it. The peak-to-peak voltage is twice as high as the same rectified voltage (assuming capacitance on the output and no load).
RMS and peak-to-peak voltages are related by this equivalence (assuming a perfect sine wave): Vptp = 2 * sqrt(2) * Vrms = 2 * 1.414 * Vrms
In summary:
Vptp = 2 * sqrt(2) * Vrms = 2 * 1.414 * Vrms
Vrectif = 0.5 * Vptp
Hence:
Vrectif = 1.414 * Vrms
I'll leave to you the calculations as an exercise, though you should expect about 20 turns for a DC output of 30V.
Finally, if the voltages don't quite match between theory and circuit, and are a couple of volts off, remember that bridge rectifiers have a forward voltage drop...