P_mW = 10^(dBm/10).
Power in milliwatts P_mW = 10^(P_dBm/10), then watts as mW/1000. This is the standard log definition for RF/engineering dBm referenced to 1 mW; very large values are rejected as non-finite overflow.
This calculator uses plain decimal numbers. If a result looks wrong, check whether you used a comma instead of a dot (or vice versa) where the field expects one.