diff --git a/CMakeLists.txt b/CMakeLists.txt
index 70152bb6b..6530016fe 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -347,6 +347,7 @@ list(APPEND libopenmc_SOURCES
   src/endf.cpp
   src/error.cpp
   src/event.cpp
+  src/file_utils.cpp
   src/finalize.cpp
   src/geometry.cpp
   src/geometry_aux.cpp
diff --git a/include/openmc/file_utils.h b/include/openmc/file_utils.h
index 786d35955..11b20b4aa 100644
--- a/include/openmc/file_utils.h
+++ b/include/openmc/file_utils.h
@@ -1,11 +1,7 @@
 #ifndef OPENMC_FILE_UTILS_H
 #define OPENMC_FILE_UTILS_H
 
-#include <algorithm> // any_of
-#include <cctype>    // for isalpha
-#include <fstream> // for ifstream
 #include <string>
-#include <sys/stat.h>
 
 namespace openmc {
 
@@ -13,48 +9,18 @@ namespace openmc {
 //! Determine if a path is a directory
 //! \param[in] path Path to check
 //! \return Whether the path is a directory
-inline bool dir_exists(const std::string& path)
-{
-  struct stat s;
-  if (stat(path.c_str(), &s) != 0)
-    return false;
-
-  return s.st_mode & S_IFDIR;
-}
+bool dir_exists(const std::string& path);
 
 //! Determine if a file exists
 //! \param[in] filename Path to file
 //! \return Whether file exists
-inline bool file_exists(const std::string& filename)
-{
-  // rule out file being a path to a directory
-  if (dir_exists(filename))
-    return false;
-
-  std::ifstream s {filename};
-  return s.good();
-}
+bool file_exists(const std::string& filename);
 
 // Gets the file extension of whatever string is passed in. This is defined as
 // a sequence of strictly alphanumeric characters which follow the last period,
 // i.e. at least one alphabet character is present, and zero or more numbers.
 // If such a sequence of characters is not found, an empty string is returned.
-inline std::string get_file_extension(const std::string& filename)
-{
-  // check that at least one letter is present
-  const auto last_period_pos = filename.find_last_of('.');
-
-  // no file extension
-  if (last_period_pos == std::string::npos)
-    return "";
-
-  const std::string ending = filename.substr(last_period_pos + 1);
-
-  // check that at least one character is present.
-  const bool has_alpha = std::any_of(ending.begin(), ending.end(),
-    [](char x) { return static_cast<bool>(std::isalpha(x)); });
-  return has_alpha ? ending : "";
-}
+std::string get_file_extension(const std::string& filename);
 
 } // namespace openmc
 
diff --git a/src/file_utils.cpp b/src/file_utils.cpp
new file mode 100644
index 000000000..15b30e3b6
--- /dev/null
+++ b/src/file_utils.cpp
@@ -0,0 +1,46 @@
+#include "openmc/file_utils.h"
+
+#include <algorithm> // any_of
+#include <cctype>    // for isalpha
+#include <fstream>   // for ifstream
+#include <sys/stat.h>
+
+namespace openmc {
+
+bool dir_exists(const std::string& path)
+{
+  struct stat s;
+  if (stat(path.c_str(), &s) != 0)
+    return false;
+
+  return s.st_mode & S_IFDIR;
+}
+
+bool file_exists(const std::string& filename)
+{
+  // rule out file being a path to a directory
+  if (dir_exists(filename))
+    return false;
+
+  std::ifstream s {filename};
+  return s.good();
+}
+
+std::string get_file_extension(const std::string& filename)
+{
+  // check that at least one letter is present
+  const auto last_period_pos = filename.find_last_of('.');
+
+  // no file extension
+  if (last_period_pos == std::string::npos)
+    return "";
+
+  const std::string ending = filename.substr(last_period_pos + 1);
+
+  // check that at least one character is present.
+  const bool has_alpha = std::any_of(ending.begin(), ending.end(),
+    [](char x) { return static_cast<bool>(std::isalpha(x)); });
+  return has_alpha ? ending : "";
+}
+
+} // namespace openmc