diff --git a/milli/src/heed_codec/mod.rs b/milli/src/heed_codec/mod.rs
index ff8285357..a070c66eb 100644
--- a/milli/src/heed_codec/mod.rs
+++ b/milli/src/heed_codec/mod.rs
@@ -1,10 +1,12 @@
 mod beu32_str_codec;
 mod obkv_codec;
 mod roaring_bitmap;
+mod roaring_bitmap_length;
 mod str_str_u8_codec;
 pub mod facet;
 
 pub use self::beu32_str_codec::BEU32StrCodec;
 pub use self::obkv_codec::ObkvCodec;
 pub use self::roaring_bitmap::{BoRoaringBitmapCodec, CboRoaringBitmapCodec, RoaringBitmapCodec};
+pub use self::roaring_bitmap_length::{BoRoaringBitmapLenCodec, CboRoaringBitmapLenCodec, RoaringBitmapLenCodec};
 pub use self::str_str_u8_codec::StrStrU8Codec;
diff --git a/milli/src/heed_codec/roaring_bitmap/cbo_roaring_bitmap_codec.rs b/milli/src/heed_codec/roaring_bitmap/cbo_roaring_bitmap_codec.rs
index 31eb949b3..8ccf831e3 100644
--- a/milli/src/heed_codec/roaring_bitmap/cbo_roaring_bitmap_codec.rs
+++ b/milli/src/heed_codec/roaring_bitmap/cbo_roaring_bitmap_codec.rs
@@ -8,7 +8,7 @@ use roaring::RoaringBitmap;
 /// This is the limit where using a byteorder became less size efficient
 /// than using a direct roaring encoding, it is also the point where we are able
 /// to determine the encoding used only by using the array of bytes length.
-const THRESHOLD: usize = 7;
+pub const THRESHOLD: usize = 7;
 
 /// A conditionnal codec that either use the RoaringBitmap
 /// or a lighter ByteOrder en/decoding method.
diff --git a/milli/src/heed_codec/roaring_bitmap/mod.rs b/milli/src/heed_codec/roaring_bitmap/mod.rs
index 866567637..6f8045c92 100644
--- a/milli/src/heed_codec/roaring_bitmap/mod.rs
+++ b/milli/src/heed_codec/roaring_bitmap/mod.rs
@@ -1,5 +1,5 @@
 mod bo_roaring_bitmap_codec;
-mod cbo_roaring_bitmap_codec;
+pub mod cbo_roaring_bitmap_codec;
 mod roaring_bitmap_codec;
 
 pub use self::bo_roaring_bitmap_codec::BoRoaringBitmapCodec;
diff --git a/milli/src/heed_codec/roaring_bitmap_length/bo_roaring_bitmap_len_codec.rs b/milli/src/heed_codec/roaring_bitmap_length/bo_roaring_bitmap_len_codec.rs
new file mode 100644
index 000000000..e749680a0
--- /dev/null
+++ b/milli/src/heed_codec/roaring_bitmap_length/bo_roaring_bitmap_len_codec.rs
@@ -0,0 +1,11 @@
+use std::mem;
+
+pub struct BoRoaringBitmapLenCodec;
+
+impl heed::BytesDecode<'_> for BoRoaringBitmapLenCodec {
+    type DItem = u64;
+
+    fn bytes_decode(bytes: &[u8]) -> Option<Self::DItem> {
+        Some((bytes.len() / mem::size_of::<u32>()) as u64)
+    }
+}
diff --git a/milli/src/heed_codec/roaring_bitmap_length/cbo_roaring_bitmap_len_codec.rs b/milli/src/heed_codec/roaring_bitmap_length/cbo_roaring_bitmap_len_codec.rs
new file mode 100644
index 000000000..4f728f1cd
--- /dev/null
+++ b/milli/src/heed_codec/roaring_bitmap_length/cbo_roaring_bitmap_len_codec.rs
@@ -0,0 +1,22 @@
+use std::mem;
+
+use super::{BoRoaringBitmapLenCodec, RoaringBitmapLenCodec};
+use crate::heed_codec::roaring_bitmap::cbo_roaring_bitmap_codec::THRESHOLD;
+
+pub struct CboRoaringBitmapLenCodec;
+
+impl heed::BytesDecode<'_> for CboRoaringBitmapLenCodec {
+    type DItem = u64;
+
+    fn bytes_decode(bytes: &[u8]) -> Option<Self::DItem> {
+        if bytes.len() <= THRESHOLD * mem::size_of::<u32>() {
+            // If there is threshold or less than threshold integers that can fit into this array
+            // of bytes it means that we used the ByteOrder codec serializer.
+            BoRoaringBitmapLenCodec::bytes_decode(bytes)
+        } else {
+            // Otherwise, it means we used the classic RoaringBitmapCodec and
+            // that the header takes threshold integers.
+            RoaringBitmapLenCodec::bytes_decode(bytes)
+        }
+    }
+}
diff --git a/milli/src/heed_codec/roaring_bitmap_length/mod.rs b/milli/src/heed_codec/roaring_bitmap_length/mod.rs
new file mode 100644
index 000000000..e503c5c7a
--- /dev/null
+++ b/milli/src/heed_codec/roaring_bitmap_length/mod.rs
@@ -0,0 +1,7 @@
+mod bo_roaring_bitmap_len_codec;
+mod cbo_roaring_bitmap_len_codec;
+mod roaring_bitmap_len_codec;
+
+pub use self::bo_roaring_bitmap_len_codec::BoRoaringBitmapLenCodec;
+pub use self::cbo_roaring_bitmap_len_codec::CboRoaringBitmapLenCodec;
+pub use self::roaring_bitmap_len_codec::RoaringBitmapLenCodec;
diff --git a/milli/src/heed_codec/roaring_bitmap_length/roaring_bitmap_len_codec.rs b/milli/src/heed_codec/roaring_bitmap_length/roaring_bitmap_len_codec.rs
new file mode 100644
index 000000000..042b5cf6b
--- /dev/null
+++ b/milli/src/heed_codec/roaring_bitmap_length/roaring_bitmap_len_codec.rs
@@ -0,0 +1,83 @@
+use std::io::{self, Read, BufRead};
+use std::mem;
+
+use byteorder::{ReadBytesExt, LittleEndian};
+
+const SERIAL_COOKIE_NO_RUNCONTAINER: u32 = 12346;
+const SERIAL_COOKIE: u16 = 12347;
+
+pub struct RoaringBitmapLenCodec;
+
+impl RoaringBitmapLenCodec {
+    // FIXME should be exported in the RoaringBitmap crate
+    fn deserialize_from_slice(mut bytes: &[u8]) -> io::Result<u64> {
+        let (size, has_offsets) = {
+            let cookie = bytes.read_u32::<LittleEndian>()?;
+            if cookie == SERIAL_COOKIE_NO_RUNCONTAINER {
+                (bytes.read_u32::<LittleEndian>()? as usize, true)
+            } else if (cookie as u16) == SERIAL_COOKIE {
+                return Err(io::Error::new(
+                    io::ErrorKind::Other,
+                    "run containers are unsupported",
+                ));
+            } else {
+                return Err(io::Error::new(io::ErrorKind::Other, "unknown cookie value"));
+            }
+        };
+
+        if size > u16::max_value() as usize + 1 {
+            return Err(io::Error::new(
+                io::ErrorKind::Other,
+                "size is greater than supported",
+            ));
+        }
+
+        let mut description_bytes = vec![0u8; size * 4];
+        bytes.read_exact(&mut description_bytes)?;
+        let description_bytes = &mut &description_bytes[..];
+
+        if has_offsets {
+            bytes.consume(size * 4);
+        }
+
+        let mut length = 0;
+        for _ in 0..size {
+            let _key = description_bytes.read_u16::<LittleEndian>()?;
+            let len = u64::from(description_bytes.read_u16::<LittleEndian>()?) + 1;
+            length += len;
+
+            if len <= 4096 {
+                bytes.consume(len as usize * mem::size_of::<u16>());
+            } else {
+                bytes.consume(1024 * mem::size_of::<u64>())
+            }
+        }
+
+        Ok(length)
+    }
+}
+
+impl heed::BytesDecode<'_> for RoaringBitmapLenCodec {
+    type DItem = u64;
+
+    fn bytes_decode(bytes: &[u8]) -> Option<Self::DItem> {
+        RoaringBitmapLenCodec::deserialize_from_slice(bytes).ok()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use crate::heed_codec::RoaringBitmapCodec;
+    use heed::BytesEncode;
+    use roaring::RoaringBitmap;
+
+    #[test]
+    fn deserialize_roaring_bitmap_length() {
+        let bitmap: RoaringBitmap = (0..500).chain(800..800_000).chain(920_056..930_032).collect();
+        let bytes = RoaringBitmapCodec::bytes_encode(&bitmap).unwrap();
+        let len = RoaringBitmapLenCodec::deserialize_from_slice(&bytes).unwrap();
+        assert_eq!(bitmap.len(), len);
+    }
+}
diff --git a/milli/src/index.rs b/milli/src/index.rs
index 12ad86b22..f8efa324e 100644
--- a/milli/src/index.rs
+++ b/milli/src/index.rs
@@ -12,8 +12,8 @@ use crate::fields_ids_map::FieldsIdsMap;
 use crate::{default_criteria, Criterion, Search, FacetDistribution};
 use crate::{BEU32, DocumentId, FieldId, ExternalDocumentsIds};
 use crate::{
-    RoaringBitmapCodec, BEU32StrCodec, StrStrU8Codec, ObkvCodec,
-    BoRoaringBitmapCodec, CboRoaringBitmapCodec,
+    RoaringBitmapCodec, RoaringBitmapLenCodec, BEU32StrCodec,
+    StrStrU8Codec, ObkvCodec, BoRoaringBitmapCodec, CboRoaringBitmapCodec,
 };
 
 pub const CRITERIA_KEY: &str = "criteria";
diff --git a/milli/src/lib.rs b/milli/src/lib.rs
index 66d134f4e..0fa966ee8 100644
--- a/milli/src/lib.rs
+++ b/milli/src/lib.rs
@@ -26,6 +26,7 @@ pub use self::external_documents_ids::ExternalDocumentsIds;
 pub use self::fields_ids_map::FieldsIdsMap;
 pub use self::heed_codec::{BEU32StrCodec, StrStrU8Codec, ObkvCodec};
 pub use self::heed_codec::{RoaringBitmapCodec, BoRoaringBitmapCodec, CboRoaringBitmapCodec};
+pub use self::heed_codec::{RoaringBitmapLenCodec, BoRoaringBitmapLenCodec, CboRoaringBitmapLenCodec};
 pub use self::index::Index;
 pub use self::search::{Search, FacetDistribution, FacetCondition, SearchResult};
 pub use self::update_store::UpdateStore;