So dB is a measure of power relative to 1 Watt and dBm is measure of power relative to 1 Milliwatt.
Covert input (linear) level, typically an rms Volt value to Power levels.
X_dB = 20 * log10(x_in);
x_in = 10^(X_dB / 20);
X_dBm = X_dB + 30;
x_in = 10^((X_dBm - 30) / 20);
x_in_dBm = x_in * 10^(30 / 20);
To recap, it is easier to add and subtract in dB space than divide or multiply in linear. dB is a unit to measure power where input is the amplitude units (rms Volts in my example). Going from dB to dBm is an 30dB difference in power. This is a factor of 1000 in linear Power. To convert to linear amplitude units, this becomes a scaling of sqrt(1000).