The output power of a transformer is the same as its input power. So the voltage ratio is the root of its impedance ratio.
Example:
1000 ohms primary to 8 ohms secondary.
The impedance ratio is 1000/8= 125.
If the input voltage is 10V then the power is 1/10W.
Then the output power is also 1/10W and the output voltage is 0.894V.
The voltage ratio is 10/0.894= 11.19 which is the root of 125.