If its a transformer with a sinusoidal waveform supply [such as a mains transformer] - you use the most fundamental of all the transformer equations. [Volts RMS] where E=Applied [usually primary] rms voltage f=frequency [Hz] N=turns on the winding [primary] where the voltage E is applied A=Magnetic circuit cross-sectional-area enclosed by the winding [m^2] Bm = the maximum flux density [Tesla] leading to [Tesla]
actually i want to build a transformer on my own. the formula for finding the primary winding of the transformer is Np= (Ep X 10^8) / 4.44*F*A*Bm where Np = number of primary turns. Ep = known primary input voltage in volts. Bm = maximum flux density in gauss. A = effective core area in cm^2 F = input frequency in Hz. so here both N and Bm are unknown. so how can i calculate Bm then so that i can find the number of primary windings.
The maximum flux density Bm depends very much on the material used. Mains frequency transformer steels can be operated at least to 1.5 Tesla [15,000 gauss] - and probably up to 1.7 or 1.8 Tesla depending on the core losses and magnetizing current harmonics one is willing to tolerate. Other materials like ferrites have much lower practical Bm values.
The best resource for limits of the core material is the manufacturer that makes the material, but I'm speaking as an OEM. If you're going to dissolve everything off an existing transformer and wind it yourself, go with what TNK said.