diff --git a/src/MISC/fix_ttm.cpp b/src/MISC/fix_ttm.cpp
index 1c27e8affe..e759e600c9 100644
--- a/src/MISC/fix_ttm.cpp
+++ b/src/MISC/fix_ttm.cpp
@@ -515,7 +515,7 @@ void FixTTM::end_of_step()
                   total_nnodes,MPI_DOUBLE,MPI_SUM,world);
 
     if (me == 0) {
-      fprintf(fp,BIGINT_FORMAT,update->ntimestep);
+      fmt::print(fp,"{}",update->ntimestep);
 
       double T_a;
       for (int ixnode = 0; ixnode < nxnodes; ixnode++)
diff --git a/src/USER-MISC/fix_ttm_mod.cpp b/src/USER-MISC/fix_ttm_mod.cpp
index 49e9cc02a6..9051414d07 100644
--- a/src/USER-MISC/fix_ttm_mod.cpp
+++ b/src/USER-MISC/fix_ttm_mod.cpp
@@ -33,6 +33,7 @@
 #include "citeme.h"
 #include "math_const.h"
 #include "utils.h"
+#include "fmt/format.h"
 
 using namespace LAMMPS_NS;
 using namespace FixConst;
@@ -763,7 +764,7 @@ void FixTTMMod::end_of_step()
     MPI_Allreduce(&t_surface_l,&surface_l,
                   1,MPI_INT,MPI_MIN,world);
     if (me == 0) {
-      fprintf(fp,BIGINT_FORMAT,update->ntimestep);
+      fmt::print(fp,"{}",update->ntimestep);
       double T_a;
       for (int ixnode = 0; ixnode < nxnodes; ixnode++)
         for (int iynode = 0; iynode < nynodes; iynode++)