diff --git a/src/vmecpp/cpp/vmecpp/vmec/output_quantities/output_quantities.cc b/src/vmecpp/cpp/vmecpp/vmec/output_quantities/output_quantities.cc
index 231ac5eaa..20eb229fd 100644
--- a/src/vmecpp/cpp/vmecpp/vmec/output_quantities/output_quantities.cc
+++ b/src/vmecpp/cpp/vmecpp/vmec/output_quantities/output_quantities.cc
@@ -3187,9 +3187,12 @@ vmecpp::MercierFileContents vmecpp::ComputeMercierStability(
     // S
     mercier.s[jF] = mercier_intermediate.s[jF];
 
+    // mercier.f90 forms this as sqs = 0.5*(vp_real(i) + vp_real(i+1))*sign_jac
+    // and divides SHEAR, ITOR' and PRES' by it, so the Jacobian sign belongs
+    // here rather than only on WELL.
     const double vp_full = (mercier_intermediate.vp_real[jHo] +
                             mercier_intermediate.vp_real[jHi]) /
-                           2.0;
+                           2.0 * vmec_internal_results.sign_of_jacobian;
     if (vp_full == 0.0) {
       // skip this surface
       continue;
diff --git a/src/vmecpp/cpp/vmecpp_large_cpp_tests/vmec/output_quantities/output_quantities_test.cc b/src/vmecpp/cpp/vmecpp_large_cpp_tests/vmec/output_quantities/output_quantities_test.cc
index 6c66552c9..3d64690bf 100644
--- a/src/vmecpp/cpp/vmecpp_large_cpp_tests/vmec/output_quantities/output_quantities_test.cc
+++ b/src/vmecpp/cpp/vmecpp_large_cpp_tests/vmec/output_quantities/output_quantities_test.cc
@@ -671,7 +671,7 @@ TEST_P(MercierStabilityTest, CheckMercierStability) {
 
     const double vp_full = (static_cast<double>(mercier["vp_real"][jHo]) +
                             static_cast<double>(mercier["vp_real"][jHi])) /
-                           2.0;
+                           2.0 * sign_of_jacobian;
 
     // The running sum advances even on surfaces the assembly skips, and is
     // scaled by deltaS only at the end, as the assembly does.