diff --git a/libafl/src/stages/calibrate.rs b/libafl/src/stages/calibrate.rs
index 1d46d72a9a..6b19043532 100644
--- a/libafl/src/stages/calibrate.rs
+++ b/libafl/src/stages/calibrate.rs
@@ -23,8 +23,7 @@ use crate::{
 };
 
 /// The metadata to keep unstable entries
-/// In libafl, the stability is the number of the unstable entries divided by the size of the map
-/// This is different from AFL++, which shows the number of the unstable entries divided by the number of filled entries.
+/// Formula is same as AFL++: number of unstable entries divided by the number of filled entries.
 #[cfg_attr(
     any(not(feature = "serdeany_autoreg"), miri),
     allow(clippy::unsafe_derive_deserialize)
@@ -32,7 +31,7 @@ use crate::{
 #[derive(Serialize, Deserialize, Clone, Debug)]
 pub struct UnstableEntriesMetadata {
     unstable_entries: HashSet<usize>,
-    map_len: usize,
+    filled_entries_count: usize,
 }
 impl_serdeany!(UnstableEntriesMetadata);
 
@@ -42,7 +41,7 @@ impl UnstableEntriesMetadata {
     pub fn new() -> Self {
         Self {
             unstable_entries: HashSet::new(),
-            map_len: 0,
+            filled_entries_count: 0,
         }
     }
 
@@ -54,8 +53,8 @@ impl UnstableEntriesMetadata {
 
     /// Getter
     #[must_use]
-    pub fn map_len(&self) -> usize {
-        self.map_len
+    pub fn filled_entries_count(&self) -> usize {
+        self.filled_entries_count
     }
 }
 
@@ -152,12 +151,27 @@ where
             .observers_mut()
             .post_exec_all(state, &input, &exit_kind)?;
 
-        let map_first = &executor.observers()[&self.map_observer_handle]
-            .as_ref()
-            .to_vec();
-
+        let observers = &executor.observers();
+        let map_first = observers[&self.map_observer_handle].as_ref();
+        let map_first_filled_count = match state
+            .named_metadata_map()
+            .get::<MapFeedbackMetadata<O::Entry>>(&self.map_name)
+        {
+            Some(metadata) => metadata.num_covered_map_indexes,
+            None => map_first.count_bytes().try_into().map_err(|len| {
+                Error::illegal_state(
+                    format!(
+                        "map's filled entry count ({}) is greater than usize::MAX ({})",
+                        len,
+                        usize::MAX,
+                    )
+                    .as_str(),
+                )
+            })?,
+        };
+        let map_first_entries = map_first.to_vec();
+        let map_first_len = map_first.to_vec().len();
         let mut unstable_entries: Vec<usize> = vec![];
-        let map_len: usize = map_first.len();
         // Run CAL_STAGE_START - 1 times, increase by 2 for every time a new
         // run is found to be unstable or to crash with CAL_STAGE_MAX total runs.
         let mut i = 1;
@@ -203,11 +217,11 @@ where
                     .unwrap()
                     .history_map;
 
-                if history_map.len() < map_len {
-                    history_map.resize(map_len, O::Entry::default());
+                if history_map.len() < map_first_len {
+                    history_map.resize(map_first_len, O::Entry::default());
                 }
 
-                for (idx, (first, (cur, history))) in map_first
+                for (idx, (first, (cur, history))) in map_first_entries
                     .iter()
                     .zip(map.iter().zip(history_map.iter_mut()))
                     .enumerate()
@@ -230,11 +244,11 @@ where
         if unstable_found {
             let metadata = state.metadata_or_insert_with(UnstableEntriesMetadata::new);
 
-            // If we see new stable entries executing this new corpus entries, then merge with the existing one
+            // If we see new unstable entries executing this new corpus entries, then merge with the existing one
             for item in unstable_entries {
                 metadata.unstable_entries.insert(item); // Insert newly found items
             }
-            metadata.map_len = map_len;
+            metadata.filled_entries_count = map_first_filled_count;
         } else if !state.has_metadata::<UnstableEntriesMetadata>() {
             send_default_stability = true;
             state.add_metadata(UnstableEntriesMetadata::new());
@@ -299,16 +313,18 @@ where
         if unstable_found {
             if let Some(meta) = state.metadata_map().get::<UnstableEntriesMetadata>() {
                 let unstable_entries = meta.unstable_entries().len();
-                let map_len = meta.map_len();
-                debug_assert_ne!(map_len, 0, "The map_len must never be 0");
+                debug_assert_ne!(
+                    map_first_filled_count, 0,
+                    "The map's filled count must never be 0"
+                );
                 mgr.fire(
                     state,
                     Event::UpdateUserStats {
                         name: Cow::from("stability"),
                         value: UserStats::new(
                             UserStatsValue::Ratio(
-                                (map_len - unstable_entries) as u64,
-                                map_len as u64,
+                                (map_first_filled_count - unstable_entries) as u64,
+                                map_first_filled_count as u64,
                             ),
                             AggregatorOps::Avg,
                         ),
@@ -322,7 +338,10 @@ where
                 Event::UpdateUserStats {
                     name: Cow::from("stability"),
                     value: UserStats::new(
-                        UserStatsValue::Ratio(map_len as u64, map_len as u64),
+                        UserStatsValue::Ratio(
+                            map_first_filled_count as u64,
+                            map_first_filled_count as u64,
+                        ),
                         AggregatorOps::Avg,
                     ),
                     phantom: PhantomData,