We present a detailed photometric study of the dwarf irregular galaxy NGC 6822 aimed at investigating the properties of its stellar populations and, in particular, the presence of stellar radial gradients. Our goal is to analyse the stellar populations in six fields, which cover the whole bar of this dwarf galaxy. We derived the quantitative star formation history (SFH) of the six fields using the IAC method, involving IAC-pop/MinnIAC codes. The solutions we derived show an enhanced star formation rate (SFR) in Fields 1 and 3 during the past 500 Myr. The SFRs of the other fields are almost extinguished at very recent epochs and. We study the radial gradients of the SFR and consider the total mass converted into stars in two time intervals (between 0 and 0.5 Gyr ago and between 0.5 and 13.5 Gyr ago). We find that the scale lengths of the young and intermediate-to-old populations are perfectly compatible, with the exception of the young populations in Fields 1 and 3. The recent SF in these two fields is greater than in the other ones. This might be an indication that in these two fields we are sampling incipient spiral arms. Further evidence and new observations are required to prove this hypothesis. In addition, we derived the age-metallicity relations. As expected, the metallicity increases with time for all of the fields. We do not observe any radial gradient in the metallicity. Based on observations collected with the ACS on board the NASA/ESA HST.The photometric catalogue is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/572/A26