diff --git a/features/filesystem/fat/FATFileSystem.cpp b/features/filesystem/fat/FATFileSystem.cpp
index a936a03419..f39029686b 100644
--- a/features/filesystem/fat/FATFileSystem.cpp
+++ b/features/filesystem/fat/FATFileSystem.cpp
@@ -308,7 +308,7 @@ int FATFileSystem::unmount()
 
 /* See http://elm-chan.org/fsw/ff/en/mkfs.html for details of f_mkfs() and
  * associated arguments. */
-int FATFileSystem::format(BlockDevice *bd, int allocation_unit) {
+int FATFileSystem::format(BlockDevice *bd, bd_size_t cluster_size) {
     FATFileSystem fs;
     int err = fs.mount(bd, false);
     if (err) {
@@ -317,7 +317,7 @@ int FATFileSystem::format(BlockDevice *bd, int allocation_unit) {
 
     // Logical drive number, Partitioning rule, Allocation unit size (bytes per cluster)
     fs.lock();
-    FRESULT res = f_mkfs(fs._fsid, 1, allocation_unit);
+    FRESULT res = f_mkfs(fs._fsid, 1, cluster_size);
     fs.unlock();
     if (res != FR_OK) {
         return fat_error_remap(res);
diff --git a/features/filesystem/fat/FATFileSystem.h b/features/filesystem/fat/FATFileSystem.h
index d46535b9c4..6324093e07 100644
--- a/features/filesystem/fat/FATFileSystem.h
+++ b/features/filesystem/fat/FATFileSystem.h
@@ -49,18 +49,18 @@ public:
      *  The block device to format should be mounted when this function is called.
      *
      *  @param bd
-     *    This is the block device that will be formated.
+     *    This is the block device that will be formatted.
      *
-     *  @param allocation_unit
-     *    This is the number of bytes per cluster size. The valid value is N
-     *    times the sector size. N is a power of 2 from 1 to 128 for FAT
-     *    volume and upto 16MiB for exFAT volume. If zero is given,
-     *    the default allocation unit size is selected by the underlying
-     *    filesystem, which depends on the volume size.
+     *  @param cluster_size
+     *    This is the number of bytes per cluster. A larger cluster size will decrease
+     *    the overhead of the FAT table, but also increase the minimum file size. The
+     *    cluster_size must be a multiple of the underlying device's allocation unit
+     *    and is currently limited to a max of 32,768 bytes. If zero, a cluster size
+     *    will be determined from the device's allocation unit. Defaults to zero.
      *   
      *  @return         0 on success, negative error code on failure
      */
-    static int format(BlockDevice *bd, int allocation_unit = 0);
+    static int format(BlockDevice *bd, bd_size_t cluster_size = 0);
 
     /** Mounts a filesystem to a block device
      *