We provide new quantitative versions of Helly's theorem. For example, we show that for every family {Pi:i∈I} of closed half‐spaces in Rn such that P=⋂i∈IPi has positive volume, there exist s⩽αn and i1,…,is∈I such that
right center left3ptthickmathspacetrueitalicvolMJX-TeXAtom-ORDn(PMJX-TeXAtom-ORDiMJX-TeXAtom-ORD1∩⋯∩PMJX-TeXAtom-ORDiMJX-TeXAtom-ORDs)⩽(Cn) MJX-TeXAtom-ORDnitalicvolMJX-TeXAtom-ORDn(P), where α,C>0 are absolute constants. These results complement and improve previous work of Bárány et al and Naszódi. Our method combines the work of Srivastava on approximate John's decompositions with few vectors, a new estimate on the corresponding constant in the Brascamp–Lieb inequality and an appropriate variant of Ball's proof of the reverse isoperimetric inequality.