diff --git a/src/sakia/money/relative_zerosum.py b/src/sakia/money/relative_zerosum.py
index c000a30b3f02524becad42c5fdd35869cafeaab9..d320c8c3ee3a388b2fef9ac024629793022afd2a 100644
--- a/src/sakia/money/relative_zerosum.py
+++ b/src/sakia/money/relative_zerosum.py
@@ -10,7 +10,7 @@ class RelativeZSum(BaseReferential):
     _REF_STR_ = QT_TRANSLATE_NOOP('RelativeZSum', "{0} {1}R0{2}")
     _UNITS_STR_ = QT_TRANSLATE_NOOP('RelativeZSum', "R0 {0}")
     _FORMULA_STR_ = QT_TRANSLATE_NOOP('RelativeZSum',
-                                      """R0 = (R / UD(t)) - (( M(t-1) / N(t) ) / UD(t))
+                                      """R0 = (Q / UD(t)) - (( M(t-1) / N(t) ) / UD(t))
                                         <br >
                                         <table>
                                         <tr><td>R0</td><td>Relative value at zero sum</td></tr>