Besides classical functions and dB conversions, RadioCalc includes a converter of Power to field strength commonly used for EMC/EMS test-set-up. Also impedance can be represented as a function of reflection coefficient (somehow like an electronic Smith-chart). This and many other functions are obtained by clicking into the arrow of the "Conversion off" window.
Radiocalc - Convertisseur Radio Universel
Thanks to RadioCalc the different dB charts are obsolete by computing the ratios of Power (W) or Voltages (V) according to dB = 10*log(P2/P1) ou dB = 20*log(V2/V1).
Just for a trial : type 100/10 (or just 10) and push the dB button as the 'W' mode is chosen, the result will be 10 dB, then choose the 'V' mode by clicking in the V button (in the Quick dB window) push again dB, the result is now 20 dB.
From - Temcom - Consulting for test & measurement and electronics in France
(This was Earlier Radiocalc, i have now updated the link to the new site in France)