The　reference　intensity　ratio　(RIR)　is　the　intensity　ratio　of　a　specified　peak　of　the　target　crystalline　phase　to　a　certain　peak　of　the　reference　phase　in　the　powder　X-ray　diffraction　pattern　of　a　mixture　of　the　target　and　reference　phases　in　a　weight　ratio　of　1:1.　The　RIR　represents　the　diffraction　power　of　the　target　phase　compared　with　that　of　the　reference　phase.　A　quantitative　phase　analysis　(QPA)　can　be　readily　made　on　the　basis　of　only　a　powder　pattern　when　the　RIR　values　of　all　component　phases　in　the　mixture　are　available.　The　RIR　of　a　target　phase　can　be　measured　experimentally　or　calculated　in　accordance　with　its　crystal　structure.　RIR　values　are　given　in　most　powder　diffraction　files　(PDFs)　published　by　the　International　Centre　for　Diffraction　Data,　but　there　are　still　more　than　110　000　PDFs　for　which　the　RIR　is　absent.　Newly　discovered　phases　may　be　present　in　a　mixture　as　components　of　unknown　weight　fractions,　for　which　the　RIR　values　can　be　neither　measured　nor　calculated　with　the　currently　available　method.　Herein　is　presented　a　new　method　of　calculating　the　RIR　of　a　crystalline　phase　from　its　lattice　parameters,　the　chemical　content　of　the　unit　cell　and　a　list　of　normalized　intensities　of　reflections　over　a　sufficiently　large　diffraction　angle　range.　The　new　method　can　be　used　to　calculate　not　only　the　absent　RIRs　in　the　PDFs　but　also　the　RIR　of　a　component　phase　of　a　mixture　of　unknown　weight　fraction　when　the　lattice　parameters　and　chemical　content　of　the　unit　cell　of　the　corresponding　phase　are　known.