First-principles density functional theory (DFT) is used to investigate the electronic and magnetic properties of Sr
4Rh
3O
10, a member of the Ruddlesden-Popper series. Based on the DFT calculations taking into account the co-operative effect of Coulomb interaction (
U) and spin-orbit couplings (SOC), Sr
4Rh
3O
10 is found to be a half metallic ferromagnet (HMF) with total angular moment
μtot=12
μB per unit cell. The material has almost 100
% spin-polarization at the Fermi level despite of sizable SOC. Replacement of Rh atom by the isovalent Co atom is considered. Upon full-replacement of Co, a low-spin to intermediate spin transition happens resulting in a HMF state with the total angular moment three-time larger (i.e.
μtot=36
μB per unit cell), compared to Sr
4Rh
3O
10. We propose Sr
4Rh
3O
10 and Sr
4Co
3O
10 as candidates of half metals.