diff --git a/src/libmain/shared.cc b/src/libmain/shared.cc
index 22ae51e47..2247aeca4 100644
--- a/src/libmain/shared.cc
+++ b/src/libmain/shared.cc
@@ -386,18 +386,12 @@ RunPager::~RunPager()
 }
 
 
-string showBytes(uint64_t bytes)
-{
-    return (format("%.2f MiB") % (bytes / (1024.0 * 1024.0))).str();
-}
-
-
 PrintFreed::~PrintFreed()
 {
     if (show)
-        std::cout << format("%1% store paths deleted, %2% freed\n")
-            % results.paths.size()
-            % showBytes(results.bytesFreed);
+        std::cout << fmt("%d store paths deleted, %s freed\n",
+            results.paths.size(),
+            showBytes(results.bytesFreed));
 }
 
 Exit::~Exit() { }
diff --git a/src/libstore/optimise-store.cc b/src/libstore/optimise-store.cc
index c032a5e22..a0d482ddf 100644
--- a/src/libstore/optimise-store.cc
+++ b/src/libstore/optimise-store.cc
@@ -276,21 +276,15 @@ void LocalStore::optimiseStore(OptimiseStats & stats)
     }
 }
 
-static string showBytes(uint64_t bytes)
-{
-    return (format("%.2f MiB") % (bytes / (1024.0 * 1024.0))).str();
-}
-
 void LocalStore::optimiseStore()
 {
     OptimiseStats stats;
 
     optimiseStore(stats);
 
-    printInfo(
-        format("%1% freed by hard-linking %2% files")
-        % showBytes(stats.bytesFreed)
-        % stats.filesLinked);
+    printInfo("%s freed by hard-linking %d files",
+        showBytes(stats.bytesFreed),
+        stats.filesLinked);
 }
 
 void LocalStore::optimisePath(const Path & path)
diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index 9e7142e01..f07e99885 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -1641,4 +1641,11 @@ AutoCloseFD createUnixDomainSocket(const Path & path, mode_t mode)
     return fdSocket;
 }
 
+
+string showBytes(uint64_t bytes)
+{
+    return fmt("%.2f MiB", bytes / (1024.0 * 1024.0));
+}
+
+
 }
diff --git a/src/libutil/util.hh b/src/libutil/util.hh
index b8e201203..129d59a97 100644
--- a/src/libutil/util.hh
+++ b/src/libutil/util.hh
@@ -573,4 +573,7 @@ template<class... Ts> struct overloaded : Ts... { using Ts::operator()...; };
 template<class... Ts> overloaded(Ts...) -> overloaded<Ts...>;
 
 
+std::string showBytes(uint64_t bytes);
+
+
 }