diff --git a/src/default/spiffs_config.h b/src/default/spiffs_config.h
index 65667fc..31c0e27 100644
--- a/src/default/spiffs_config.h
+++ b/src/default/spiffs_config.h
@@ -119,6 +119,17 @@
 #define SPIFFS_USE_MAGIC                (0)
 #endif
 
+#if SPIFFS_USE_MAGIC
+// Only valid when SPIFFS_USE_MAGIC is enabled. If SPIFFS_USE_MAGIC_LENGTH is
+// enabled, the magic will also be dependent on the length of the filesystem.
+// For example, a filesystem configured and formatted for 4 megabytes will not
+// be accepted for mounting with a configuration defining the filesystem as 2
+// megabytes.
+#ifndef SPIFFS_USE_MAGIC_LENGTH
+#define SPIFFS_USE_MAGIC_LENGTH         (0)
+#endif
+#endif
+
 // SPIFFS_LOCK and SPIFFS_UNLOCK protects spiffs from reentrancy on api level
 // These should be defined on a multithreaded system
 
@@ -131,7 +142,6 @@
 #define SPIFFS_UNLOCK(fs)
 #endif
 
-
 // Enable if only one spiffs instance with constant configuration will exist
 // on the target. This will reduce calculations, flash and memory accesses.
 // Parts of configuration must be defined below instead of at time of mount.
diff --git a/src/spiffs.h b/src/spiffs.h
index 4b3e768..9a25449 100644
--- a/src/spiffs.h
+++ b/src/spiffs.h
@@ -282,6 +282,33 @@ typedef struct {
 
 // functions
 
+#if SPIFFS_USE_MAGIC && SPIFFS_USE_MAGIC_LENGTH
+/**
+ * Special function. This takes a spiffs config struct and returns
+ * the number of blocks this file system was formatted with.
+ * This function relies on following in the passed config struct:
+ * hal_read_f, phys_addr, and log_page_size.
+ *
+ * Do note, that caution should be taken with the value returned here.
+ *
+ * There is no check whatsoever that the flash actually contains a file
+ * system - meaning that just any number could be returned.
+ *
+ * There is neither any check that the configuration is set correctly.
+ * The block size may for instance differ.
+ *
+ * One must be sure of the page size and the physical address when
+ * calling this function.
+ *
+ * The resulting value can then be used to set the config structs
+ * phys_size by multiplying it with the known block size.
+ *
+ * @param config        essential parts of the physical and logical
+ *                      configuration of the file system
+ */
+s32_t SPIFFS_probe_nbr_of_blocks(spiffs_config *config);
+#endif // SPIFFS_USE_MAGIC && SPIFFS_USE_MAGIC_LENGTH
+
 /**
  * Initializes the file system dynamic parameters and mounts the filesystem.
  * If SPIFFS_USE_MAGIC is enabled the mounting may fail with SPIFFS_ERR_NOT_A_FS
diff --git a/src/spiffs_hydrogen.c b/src/spiffs_hydrogen.c
index 25aae24..d625d7b 100644
--- a/src/spiffs_hydrogen.c
+++ b/src/spiffs_hydrogen.c
@@ -61,6 +61,17 @@ s32_t SPIFFS_format(spiffs *fs) {
   return 0;
 }
 
+#if SPIFFS_USE_MAGIC && SPIFFS_USE_MAGIC_LENGTH
+
+s32_t SPIFFS_probe_nbr_of_blocks(spiffs_config *config) {
+  SPIFFS_LOCK(fs);
+  s32_t block_count = spiffs_probe(config);
+  SPIFFS_UNLOCK(fs);
+  return block_count;
+}
+
+#endif // SPIFFS_USE_MAGIC && SPIFFS_USE_MAGIC_LENGTH
+
 s32_t SPIFFS_mount(spiffs *fs, spiffs_config *config, u8_t *work,
     u8_t *fd_space, u32_t fd_space_size,
     void *cache, u32_t cache_size,
diff --git a/src/spiffs_nucleus.c b/src/spiffs_nucleus.c
index ba55722..a9d7005 100644
--- a/src/spiffs_nucleus.c
+++ b/src/spiffs_nucleus.c
@@ -243,7 +243,7 @@ s32_t spiffs_erase_block(
 
 #if SPIFFS_USE_MAGIC
   // finally, write magic
-  spiffs_obj_id magic = SPIFFS_MAGIC(fs);
+  spiffs_obj_id magic = SPIFFS_MAGIC(fs, bix);
   res = _spiffs_wr(fs, SPIFFS_OP_C_WRTHRU | SPIFFS_OP_T_OBJ_LU2, 0,
       SPIFFS_MAGIC_PADDR(fs, bix),
       sizeof(spiffs_obj_id), (u8_t *)&magic);
@@ -259,6 +259,30 @@ s32_t spiffs_erase_block(
 }
 
 
+#if SPIFFS_USE_MAGIC && SPIFFS_USE_MAGIC_LENGTH
+s32_t spiffs_probe(
+    spiffs_config *cfg) {
+  s32_t res;
+  spiffs dummy_fs; // create a dummy fs struct just to be able to use macros
+  memcpy(&dummy_fs.cfg, cfg, sizeof(spiffs_config));
+  spiffs_obj_id magic;
+  u32_t paddr = SPIFFS_MAGIC_PADDR(&dummy_fs, (spiffs_block_ix)0);
+#if SPIFFS_HAL_CALLBACK_EXTRA
+  // not any proper fs to report here, so callback with null
+  // (cross fingers that no-one gets angry)
+  res = cfg->hal_read_f((void *)0, paddr, sizeof(spiffs_obj_id), (u8_t *)&magic);
+#else
+  res = cfg->hal_read_f(paddr, sizeof(spiffs_obj_id), (u8_t *)&magic);
+#endif
+  SPIFFS_CHECK_RES(res);
+
+  // unwind the magic crap to get nbr of blocks
+  dummy_fs.block_count = 0;
+  return SPIFFS_MAGIC(&dummy_fs, 0) ^ magic;
+}
+#endif // SPIFFS_USE_MAGIC && SPIFFS_USE_MAGIC_LENGTH
+
+
 static s32_t spiffs_obj_lu_scan_v(
     spiffs *fs,
     spiffs_obj_id obj_id,
@@ -311,7 +335,7 @@ s32_t spiffs_obj_lu_scan(
         sizeof(spiffs_obj_id), (u8_t *)&magic);
 
     SPIFFS_CHECK_RES(res);
-    if (magic != SPIFFS_MAGIC(fs)) {
+    if (magic != SPIFFS_MAGIC(fs, bix)) {
       if (unerased_bix == (spiffs_block_ix)-1) {
         // allow one unerased block as it might be powered down during an erase
         unerased_bix = bix;
diff --git a/src/spiffs_nucleus.h b/src/spiffs_nucleus.h
index b64d10c..6e4e8ac 100644
--- a/src/spiffs_nucleus.h
+++ b/src/spiffs_nucleus.h
@@ -131,7 +131,15 @@
 #define SPIFFS_OBJ_ID_DELETED           ((spiffs_obj_id)0)
 #define SPIFFS_OBJ_ID_FREE              ((spiffs_obj_id)-1)
 
-#define SPIFFS_MAGIC(fs)                ((spiffs_obj_id)(0x20140529 ^ SPIFFS_CFG_LOG_PAGE_SZ(fs)))
+#if SPIFFS_USE_MAGIC
+#if !SPIFFS_USE_MAGIC_LENGTH
+#define SPIFFS_MAGIC(fs, bix)           \
+  ((spiffs_obj_id)(0x20140529 ^ SPIFFS_CFG_LOG_PAGE_SZ(fs)))
+#else // SPIFFS_USE_MAGIC_LENGTH
+#define SPIFFS_MAGIC(fs, bix)           \
+  ((spiffs_obj_id)(0x20140529 ^ SPIFFS_CFG_LOG_PAGE_SZ(fs) ^ ((fs)->block_count - (bix))))
+#endif // SPIFFS_USE_MAGIC_LENGTH
+#endif // SPIFFS_USE_MAGIC
 
 #define SPIFFS_CONFIG_MAGIC             (0x20090315)
 
@@ -530,6 +538,11 @@ s32_t spiffs_erase_block(
     spiffs *fs,
     spiffs_block_ix bix);
 
+#if SPIFFS_USE_MAGIC && SPIFFS_USE_MAGIC_LENGTH
+s32_t spiffs_probe(
+    spiffs_config *cfg);
+#endif // SPIFFS_USE_MAGIC && SPIFFS_USE_MAGIC_LENGTH
+
 // ---------------
 
 s32_t spiffs_obj_lu_scan(
diff --git a/src/test/params_test.h b/src/test/params_test.h
index bb5f52d..37fb473 100644
--- a/src/test/params_test.h
+++ b/src/test/params_test.h
@@ -16,7 +16,9 @@
 #define SPIFFS_PHYS_ADDR      (4*1024*1024)
 
 // test using filesystem magic
-//#define SPIFFS_USE_MAGIC    1
+#define SPIFFS_USE_MAGIC    1
+// test using filesystem magic length
+#define SPIFFS_USE_MAGIC_LENGTH   1
 
 #define SECTOR_SIZE         65536
 #define LOG_BLOCK           (SECTOR_SIZE*2)
diff --git a/src/test/test_hydrogen.c b/src/test/test_hydrogen.c
index 2a83e30..1dfd66f 100644
--- a/src/test/test_hydrogen.c
+++ b/src/test/test_hydrogen.c
@@ -35,6 +35,76 @@ TEST(info)
 }
 TEST_END(info)
 
+#if SPIFFS_USE_MAGIC
+TEST(magic)
+{
+  fs_reset_specific(0, 0, 65536*16, 65536, 65536, 256);
+  SPIFFS_unmount(FS);
+
+  TEST_CHECK_EQ(fs_mount_specific(0, 65536*16, 65536, 65536, 256), SPIFFS_OK);
+  SPIFFS_unmount(FS);
+
+  TEST_CHECK_NEQ(fs_mount_specific(0, 65536*16, 65536, 65536, 128), SPIFFS_OK);
+  TEST_CHECK_EQ(SPIFFS_errno(FS), SPIFFS_ERR_NOT_A_FS);
+
+  TEST_CHECK_NEQ(fs_mount_specific(4, 65536*16, 65536, 65536, 256), SPIFFS_OK);
+  TEST_CHECK_EQ(SPIFFS_errno(FS), SPIFFS_ERR_NOT_A_FS);
+
+  return TEST_RES_OK;
+}
+TEST_END(magic)
+
+
+#if SPIFFS_USE_MAGIC_LENGTH
+TEST(magic_length)
+{
+  fs_reset_specific(0, 0, 65536*16, 65536, 65536, 256);
+  SPIFFS_unmount(FS);
+
+  TEST_CHECK_EQ(fs_mount_specific(0, 65536*16, 65536, 65536, 256), SPIFFS_OK);
+  SPIFFS_unmount(FS);
+
+  TEST_CHECK_NEQ(fs_mount_specific(0, 65536*8, 65536, 65536, 256), SPIFFS_OK);
+  TEST_CHECK_EQ(SPIFFS_errno(FS), SPIFFS_ERR_NOT_A_FS);
+
+  TEST_CHECK_NEQ(fs_mount_specific(0, 65536*15, 65536, 65536, 256), SPIFFS_OK);
+  TEST_CHECK_EQ(SPIFFS_errno(FS), SPIFFS_ERR_NOT_A_FS);
+
+  TEST_CHECK_NEQ(fs_mount_specific(0, 65536*17, 65536, 65536, 256), SPIFFS_OK);
+  TEST_CHECK_EQ(SPIFFS_errno(FS), SPIFFS_ERR_NOT_A_FS);
+
+  TEST_CHECK_NEQ(fs_mount_specific(0, 65536*256, 65536, 65536, 256), SPIFFS_OK);
+  TEST_CHECK_EQ(SPIFFS_errno(FS), SPIFFS_ERR_NOT_A_FS);
+
+  return TEST_RES_OK;
+}
+TEST_END(magic_length)
+
+
+TEST(magic_length_probe)
+{
+  fs_reset_specific(0, 0, 65536*16, 65536, 65536, 256);
+  TEST_CHECK_EQ(SPIFFS_probe_nbr_of_blocks(&__fs.cfg), 16);
+
+  fs_reset_specific(0, 0, 65536*24, 65536, 65536, 256);
+  TEST_CHECK_EQ(SPIFFS_probe_nbr_of_blocks(&__fs.cfg), 24);
+
+  fs_reset_specific(0, 0, 32768*16, 32768, 32768, 128);
+  TEST_CHECK_EQ(SPIFFS_probe_nbr_of_blocks(&__fs.cfg), 16);
+
+  fs_reset_specific(0, 0, 16384*37, 16384, 16384, 128);
+  TEST_CHECK_EQ(SPIFFS_probe_nbr_of_blocks(&__fs.cfg), 37);
+
+  fs_reset_specific(0, 0, 4096*11, 4096, 4096, 256);
+  TEST_CHECK_EQ(SPIFFS_probe_nbr_of_blocks(&__fs.cfg), 11);
+
+  return TEST_RES_OK;
+}
+TEST_END(magic_length_probe)
+
+#endif // SPIFFS_USE_MAGIC_LENGTH
+
+#endif // SPIFFS_USE_MAGIC
 
 TEST(missing_file)
 {