diff --git a/libraries/AP_Logger/AP_Logger.cpp b/libraries/AP_Logger/AP_Logger.cpp
index 77d05e358d..25638c1e6f 100644
--- a/libraries/AP_Logger/AP_Logger.cpp
+++ b/libraries/AP_Logger/AP_Logger.cpp
@@ -92,7 +92,15 @@ const AP_Param::GroupInfo AP_Logger::var_info[] = {
     // @User: Standard
     // @Units: s
     AP_GROUPINFO("_FILE_TIMEOUT",  6, AP_Logger, _params.file_timeout,     HAL_LOGGING_FILE_TIMEOUT),
-    
+
+    // @Param: _FILE_MB_FREE
+    // @DisplayName: Old logs on the SD card will be deleted to maintain this amount of free space
+    // @Description: Set this such that the free space is larger than your largest typical flight log
+    // @Units: MB
+    // @Range: 10 1000
+    // @User: Standard
+    AP_GROUPINFO("_FILE_MB_FREE",  7, AP_Logger, _params.min_MB_free, 500),
+
     AP_GROUPEND
 };
 
diff --git a/libraries/AP_Logger/AP_Logger.h b/libraries/AP_Logger/AP_Logger.h
index 1c7eadb4dc..f5ba692e51 100644
--- a/libraries/AP_Logger/AP_Logger.h
+++ b/libraries/AP_Logger/AP_Logger.h
@@ -343,6 +343,7 @@ public:
         AP_Int8 log_replay;
         AP_Int8 mav_bufsize; // in kilobytes
         AP_Int16 file_timeout; // in seconds
+        AP_Int16 min_MB_free;
     } _params;
 
     const struct LogStructure *structure(uint16_t num) const;
diff --git a/libraries/AP_Logger/AP_Logger_File.cpp b/libraries/AP_Logger/AP_Logger_File.cpp
index f3c32f89dd..5d18c389e6 100644
--- a/libraries/AP_Logger/AP_Logger_File.cpp
+++ b/libraries/AP_Logger/AP_Logger_File.cpp
@@ -34,6 +34,9 @@ extern const AP_HAL::HAL& hal;
 #define HAL_LOGGER_WRITE_CHUNK_SIZE 4096
 #endif
 
+#define MB_to_B 1000000
+#define B_to_MB 0.000001
+
 /*
   constructor
  */
@@ -190,22 +193,6 @@ int64_t AP_Logger_File::disk_space()
     return AP::FS().disk_space(_log_directory);
 }
 
-// returns the available space in _log_directory as a percentage
-// returns -1.0f on error
-float AP_Logger_File::avail_space_percent()
-{
-    int64_t avail = disk_space_avail();
-    if (avail == -1) {
-        return -1.0f;
-    }
-    int64_t space = disk_space();
-    if (space == -1) {
-        return -1.0f;
-    }
-
-    return (avail/(float)space) * 100;
-}
-
 // find_oldest_log - find oldest log in _log_directory
 // returns 0 if no log was found
 uint16_t AP_Logger_File::find_oldest_log()
@@ -286,17 +273,19 @@ void AP_Logger_File::Prep_MinSpace()
         return;
     }
 
+    const int64_t target_free = (int64_t)_front._params.min_MB_free * MB_to_B;
+
     _cached_oldest_log = 0;
 
     uint16_t log_to_remove = first_log_to_remove;
 
     uint16_t count = 0;
     do {
-        float avail = avail_space_percent();
-        if (is_equal(avail, -1.0f)) {
+        int64_t avail = disk_space_avail();
+        if (avail == -1) {
             break;
         }
-        if (avail >= min_avail_space_percent) {
+        if (avail >= target_free) {
             break;
         }
         if (count++ > MAX_LOG_FILES+10) {
@@ -310,8 +299,8 @@ void AP_Logger_File::Prep_MinSpace()
             break;
         }
         if (file_exists(filename_to_remove)) {
-            hal.console->printf("Removing (%s) for minimum-space requirements (%.2f%% < %.0f%%)\n",
-                                filename_to_remove, (double)avail, (double)min_avail_space_percent);
+            hal.console->printf("Removing (%s) for minimum-space requirements (%.0fMB < %.0fMB)\n",
+                                filename_to_remove, (double)avail*B_to_MB, (double)target_free*B_to_MB);
             EXPECT_DELAY_MS(2000);
             if (AP::FS().unlink(filename_to_remove) == -1) {
                 hal.console->printf("Failed to remove %s: %s\n", filename_to_remove, strerror(errno));
@@ -352,7 +341,12 @@ bool AP_Logger_File::NeedPrep()
         return false;
     }
 
-    if (avail_space_percent() < min_avail_space_percent) {
+    const int64_t actual = disk_space_avail();
+    if (actual == -1) {
+        return false;
+    }
+
+    if (actual < (int64_t)_front._params.min_MB_free * MB_to_B) {
         return true;
     }
 
diff --git a/libraries/AP_Logger/AP_Logger_File.h b/libraries/AP_Logger/AP_Logger_File.h
index d32d4e7a95..afcc53f6fd 100644
--- a/libraries/AP_Logger/AP_Logger_File.h
+++ b/libraries/AP_Logger/AP_Logger_File.h
@@ -97,17 +97,10 @@ private:
     uint16_t find_oldest_log();
     int64_t disk_space_avail();
     int64_t disk_space();
-    float avail_space_percent();
 
     bool file_exists(const char *filename) const;
     bool log_exists(const uint16_t lognum) const;
 
-#if CONFIG_HAL_BOARD == HAL_BOARD_SITL || CONFIG_HAL_BOARD == HAL_BOARD_LINUX
-    // I always seem to have less than 10% free space on my laptop:
-    const float min_avail_space_percent = 0.1f;
-#else
-    const float min_avail_space_percent = 10.0f;
-#endif
     // write buffer
     ByteBuffer _writebuf;
     const uint16_t _writebuf_chunk;