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Geranyl diphosphate is a ten-carbon prenyl diphosphate, chemically the diphosphate ester of geraniol. The neutral acid has molecular formula ${\mathrm{C}}_{10}{\mathrm{H}}_{20}{\mathrm{O}}_7{\mathrm{P}}_2$; this formula names a protonation state rather than every ionic form encountered in an enzyme assay.^[ref]

A prenyltransferase can form geranyl diphosphate by joining five-carbon dimethylallyl diphosphate to five-carbon isopentenyl diphosphate, releasing diphosphate. The reaction constructs a longer carbon skeleton while retaining one activated diphosphate group.^[ref]

Consider an ideal isolated reaction supplied with $2\mu \mathrm{mol}$ of dimethylallyl diphosphate and $3\mu \mathrm{mol}$ of isopentenyl diphosphate. The one-to-one substrate ratio permits at most $2\mu \mathrm{mol}$ of geranyl diphosphate, leaving $1\mu \mathrm{mol}$ of isopentenyl diphosphate if conversion is complete. Carbon accounting gives $2(5+5)=2(10)$ for the material incorporated into product.

This upper bound is a useful assay check, not a measured yield. A signal reporting diphosphate release measures a coproduct; chemical identification of the organic product is needed to establish that the intended geranyl diphosphate was made. Product identity includes bond geometry as well as atom counts.