diff --git a/cpp/src/arrow/util/decimal.cc b/cpp/src/arrow/util/decimal.cc
index 5999beae68079..1d84d2ce634f7 100644
--- a/cpp/src/arrow/util/decimal.cc
+++ b/cpp/src/arrow/util/decimal.cc
@@ -116,6 +116,9 @@ struct DecimalRealConversion : public BaseDecimalRealConversion {
       return OverflowError(real, precision, scale);
     }
 
+    // The algorithm below requires the destination decimal type
+    // to be strictly more precise than the source float type
+    // (see `kSafeMulByTenTo` calculation).
     if constexpr (kMaxPrecision <= kMantissaDigits) {
       return Derived::FromPositiveRealApprox(real, precision, scale);
     }