diff --git a/doc/includes/references.h b/doc/includes/references.h
index b7a32e40..546299d7 100644
--- a/doc/includes/references.h
+++ b/doc/includes/references.h
@@ -169,7 +169,7 @@ struct mjData_ {
   int     nf;                // number of friction constraints
   int     nl;                // number of limit constraints
   int     nefc;              // number of constraints
-  int     nnzJ;              // number of non-zeros in constraint Jacobian
+  int     nJ;                // number of non-zeros in constraint Jacobian
   int     nisland;           // number of detected constraint islands
 
   // global properties
@@ -361,13 +361,13 @@ struct mjData_ {
   int*    efc_J_rownnz;      // number of non-zeros in constraint Jacobian row   (nefc x 1)
   int*    efc_J_rowadr;      // row start address in colind array                (nefc x 1)
   int*    efc_J_rowsuper;    // number of subsequent rows in supernode           (nefc x 1)
-  int*    efc_J_colind;      // column indices in constraint Jacobian            (nnzJ x 1)
+  int*    efc_J_colind;      // column indices in constraint Jacobian            (nJ x 1)
   int*    efc_JT_rownnz;     // number of non-zeros in constraint Jacobian row T (nv x 1)
   int*    efc_JT_rowadr;     // row start address in colind array              T (nv x 1)
   int*    efc_JT_rowsuper;   // number of subsequent rows in supernode         T (nv x 1)
-  int*    efc_JT_colind;     // column indices in constraint Jacobian          T (nnzJ x 1)
-  mjtNum* efc_J;             // constraint Jacobian                              (nnzJ x 1)
-  mjtNum* efc_JT;            // constraint Jacobian transposed                   (nnzJ x 1)
+  int*    efc_JT_colind;     // column indices in constraint Jacobian          T (nJ x 1)
+  mjtNum* efc_J;             // constraint Jacobian                              (nJ x 1)
+  mjtNum* efc_JT;            // constraint Jacobian transposed                   (nJ x 1)
   mjtNum* efc_pos;           // constraint position (equality, contact)          (nefc x 1)
   mjtNum* efc_margin;        // inclusion margin (contact)                       (nefc x 1)
   mjtNum* efc_frictionloss;  // frictionloss (friction)                          (nefc x 1)
diff --git a/include/mujoco/mjdata.h b/include/mujoco/mjdata.h
index 44342c90..96b37acc 100644
--- a/include/mujoco/mjdata.h
+++ b/include/mujoco/mjdata.h
@@ -197,7 +197,7 @@ struct mjData_ {
   int     nf;                // number of friction constraints
   int     nl;                // number of limit constraints
   int     nefc;              // number of constraints
-  int     nnzJ;              // number of non-zeros in constraint Jacobian
+  int     nJ;                // number of non-zeros in constraint Jacobian
   int     nisland;           // number of detected constraint islands
 
   // global properties
@@ -389,13 +389,13 @@ struct mjData_ {
   int*    efc_J_rownnz;      // number of non-zeros in constraint Jacobian row   (nefc x 1)
   int*    efc_J_rowadr;      // row start address in colind array                (nefc x 1)
   int*    efc_J_rowsuper;    // number of subsequent rows in supernode           (nefc x 1)
-  int*    efc_J_colind;      // column indices in constraint Jacobian            (nnzJ x 1)
+  int*    efc_J_colind;      // column indices in constraint Jacobian            (nJ x 1)
   int*    efc_JT_rownnz;     // number of non-zeros in constraint Jacobian row T (nv x 1)
   int*    efc_JT_rowadr;     // row start address in colind array              T (nv x 1)
   int*    efc_JT_rowsuper;   // number of subsequent rows in supernode         T (nv x 1)
-  int*    efc_JT_colind;     // column indices in constraint Jacobian          T (nnzJ x 1)
-  mjtNum* efc_J;             // constraint Jacobian                              (nnzJ x 1)
-  mjtNum* efc_JT;            // constraint Jacobian transposed                   (nnzJ x 1)
+  int*    efc_JT_colind;     // column indices in constraint Jacobian          T (nJ x 1)
+  mjtNum* efc_J;             // constraint Jacobian                              (nJ x 1)
+  mjtNum* efc_JT;            // constraint Jacobian transposed                   (nJ x 1)
   mjtNum* efc_pos;           // constraint position (equality, contact)          (nefc x 1)
   mjtNum* efc_margin;        // inclusion margin (contact)                       (nefc x 1)
   mjtNum* efc_frictionloss;  // frictionloss (friction)                          (nefc x 1)
diff --git a/include/mujoco/mjxmacro.h b/include/mujoco/mjxmacro.h
index a47a0a32..796b6c5a 100644
--- a/include/mujoco/mjxmacro.h
+++ b/include/mujoco/mjxmacro.h
@@ -689,13 +689,13 @@
     X( int,      efc_J_rownnz,      MJ_D(nefc),    1 ) \
     X( int,      efc_J_rowadr,      MJ_D(nefc),    1 ) \
     X( int,      efc_J_rowsuper,    MJ_D(nefc),    1 ) \
-    X( int,      efc_J_colind,      MJ_D(nnzJ),    1 ) \
+    X( int,      efc_J_colind,      MJ_D(nJ),      1 ) \
     X( int,      efc_JT_rownnz,     MJ_M(nv),      1 ) \
     X( int,      efc_JT_rowadr,     MJ_M(nv),      1 ) \
     X( int,      efc_JT_rowsuper,   MJ_M(nv),      1 ) \
-    X( int,      efc_JT_colind,     MJ_D(nnzJ),    1 ) \
-    X( mjtNum,   efc_J,             MJ_D(nnzJ),    1 ) \
-    X( mjtNum,   efc_JT,            MJ_D(nnzJ),    1 ) \
+    X( int,      efc_JT_colind,     MJ_D(nJ),      1 ) \
+    X( mjtNum,   efc_J,             MJ_D(nJ),      1 ) \
+    X( mjtNum,   efc_JT,            MJ_D(nJ),      1 ) \
     X( mjtNum,   efc_pos,           MJ_D(nefc),    1 ) \
     X( mjtNum,   efc_margin,        MJ_D(nefc),    1 ) \
     X( mjtNum,   efc_frictionloss,  MJ_D(nefc),    1 ) \
@@ -755,7 +755,7 @@
     X( int,       nf                 ) \
     X( int,       nl                 ) \
     X( int,       nefc               ) \
-    X( int,       nnzJ               ) \
+    X( int,       nJ                 ) \
     X( int,       nisland            ) \
     X( mjtNum,    time               ) \
     X( uintptr_t, threadpool         )
diff --git a/introspect/structs.py b/introspect/structs.py
index c8993c31..0d5107fd 100644
--- a/introspect/structs.py
+++ b/introspect/structs.py
@@ -4692,7 +4692,7 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  doc='number of constraints',
              ),
              StructFieldDecl(
-                 name='nnzJ',
+                 name='nJ',
                  type=ValueType(name='int'),
                  doc='number of non-zeros in constraint Jacobian',
              ),
@@ -5588,7 +5588,7 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                      inner_type=ValueType(name='int'),
                  ),
                  doc='column indices in constraint Jacobian',
-                 array_extent=('nnzJ',),
+                 array_extent=('nJ',),
              ),
              StructFieldDecl(
                  name='efc_JT_rownnz',
@@ -5616,7 +5616,7 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  type=PointerType(
                      inner_type=ValueType(name='int'),
                  ),
-                 doc='column indices in constraint Jacobian          T (nnzJ x 1)',  # pylint: disable=line-too-long
+                 doc='column indices in constraint Jacobian          T (nJ x 1)',  # pylint: disable=line-too-long
              ),
              StructFieldDecl(
                  name='efc_J',
@@ -5624,7 +5624,7 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                      inner_type=ValueType(name='mjtNum'),
                  ),
                  doc='constraint Jacobian',
-                 array_extent=('nnzJ',),
+                 array_extent=('nJ',),
              ),
              StructFieldDecl(
                  name='efc_JT',
@@ -5632,7 +5632,7 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                      inner_type=ValueType(name='mjtNum'),
                  ),
                  doc='constraint Jacobian transposed',
-                 array_extent=('nnzJ',),
+                 array_extent=('nJ',),
              ),
              StructFieldDecl(
                  name='efc_pos',
diff --git a/mjx/mujoco/mjx/_src/io.py b/mjx/mujoco/mjx/_src/io.py
index 893e7cb1..00ba75e5 100644
--- a/mjx/mujoco/mjx/_src/io.py
+++ b/mjx/mujoco/mjx/_src/io.py
@@ -408,7 +408,7 @@ def get_data_into(
     ncon = (d_i.contact.dist <= 0).sum()
     efc_active = (d_i.efc_J != 0).any(axis=1)
     nefc = int(efc_active.sum())
-    result_i.nnzJ = nefc * m.nv
+    result_i.nJ = nefc * m.nv
     if ncon != result_i.ncon or nefc != result_i.nefc:
       mujoco._functions._realloc_con_efc(result_i, ncon=ncon, nefc=nefc)  # pylint: disable=protected-access
     result_i.efc_J_rownnz[:] = np.repeat(m.nv, nefc)
diff --git a/python/mujoco/structs.cc b/python/mujoco/structs.cc
index 0c97081e..56a6c8ee 100644
--- a/python/mujoco/structs.cc
+++ b/python/mujoco/structs.cc
@@ -747,7 +747,7 @@ void MjDataWrapper::Serialize(std::ostream& output) const {
   X(ncon);
   X(ne);
   X(nf);
-  X(nnzJ);
+  X(nJ);
   X(nefc);
   X(nisland);
   X(time);
@@ -824,7 +824,7 @@ MjDataWrapper MjDataWrapper::Deserialize(std::istream& input) {
   X(ncon);
   X(ne);
   X(nf);
-  X(nnzJ);
+  X(nJ);
   X(nefc);
   X(nisland);
   X(time);
diff --git a/src/engine/engine_core_constraint.c b/src/engine/engine_core_constraint.c
index dc2a7658..2c62248a 100644
--- a/src/engine/engine_core_constraint.c
+++ b/src/engine/engine_core_constraint.c
@@ -1943,7 +1943,7 @@ static int mj_nc(const mjModel* m, mjData* d, int* nnz) {
 // driver: call all functions above
 void mj_makeConstraint(const mjModel* m, mjData* d) {
   // clear sizes
-  d->ne = d->nf = d->nl = d->nefc = d->nnzJ = 0;
+  d->ne = d->nf = d->nl = d->nefc = d->nJ = 0;
 
   // disabled or Jacobian not allocated: return
   if (mjDISABLED(mjDSBL_CONSTRAINT)) {
@@ -1951,13 +1951,13 @@ void mj_makeConstraint(const mjModel* m, mjData* d) {
   }
 
   // precount sizes for constraint Jacobian matrices
-  int *nnz = mj_isSparse(m) ? &(d->nnzJ) : NULL;
+  int *nnz = mj_isSparse(m) ? &(d->nJ) : NULL;
   int ne_allocated = mj_ne(m, d, nnz);
   int nf_allocated = mj_nf(m, d, nnz);
   int nl_allocated = mj_nl(m, d, nnz);
   int nefc_allocated = ne_allocated + nf_allocated + nl_allocated + mj_nc(m, d, nnz);
   if (!mj_isSparse(m)) {
-    d->nnzJ = nefc_allocated * m->nv;
+    d->nJ = nefc_allocated * m->nv;
   }
   d->nefc = nefc_allocated;
 
@@ -1998,12 +1998,11 @@ void mj_makeConstraint(const mjModel* m, mjData* d) {
       mjERROR("nefc mis-allocation: found nefc=%d but allocated %d", d->nefc, nefc_allocated);
     }
 
-    // check that nnzJ was computed correctly
+    // check that nJ was computed correctly
     if (d->nefc > 0) {
-      int nnzJ = d->efc_J_rownnz[d->nefc - 1] + d->efc_J_rowadr[d->nefc - 1];
-      if (d->nnzJ != nnzJ) {
-        mjERROR("constraint Jacobian mis-allocation: found nnzJ=%d but allocated %d",
-                nnzJ, d->nnzJ);
+      int nJ = d->efc_J_rownnz[d->nefc - 1] + d->efc_J_rowadr[d->nefc - 1];
+      if (d->nJ != nJ) {
+        mjERROR("constraint Jacobian mis-allocation: found nJ=%d but allocated %d", nJ, d->nJ);
       }
     }
   } else if (d->nefc > nefc_allocated) {
diff --git a/src/engine/engine_io.c b/src/engine/engine_io.c
index 88ca9bd2..6891da68 100644
--- a/src/engine/engine_io.c
+++ b/src/engine/engine_io.c
@@ -1815,7 +1815,7 @@ static void _resetData(const mjModel* m, mjData* d, unsigned char debug_value) {
   d->nf = 0;
   d->nl = 0;
   d->nefc = 0;
-  d->nnzJ = 0;
+  d->nJ = 0;
   d->nisland = 0;
 
   // clear global properties
diff --git a/test/engine/engine_io_test.cc b/test/engine/engine_io_test.cc
index fc38adc7..7b65de4a 100644
--- a/test/engine/engine_io_test.cc
+++ b/test/engine/engine_io_test.cc
@@ -156,7 +156,7 @@ TEST_F(EngineIoTest, ResetVariableSizes) {
   EXPECT_EQ(data->ne, 0);
   EXPECT_EQ(data->nf, 0);
   EXPECT_EQ(data->nefc, 0);
-  EXPECT_EQ(data->nnzJ, 0);
+  EXPECT_EQ(data->nJ, 0);
   EXPECT_EQ(data->ncon, 0);
 
   mj_deleteData(data);
diff --git a/unity/Runtime/Bindings/MjBindings.cs b/unity/Runtime/Bindings/MjBindings.cs
index 262be737..906c2c7f 100644
--- a/unity/Runtime/Bindings/MjBindings.cs
+++ b/unity/Runtime/Bindings/MjBindings.cs
@@ -4851,7 +4851,7 @@ public unsafe struct mjData_ {
   public int nf;
   public int nl;
   public int nefc;
-  public int nnzJ;
+  public int nJ;
   public int nisland;
   public double time;
   public fixed double energy[2];