diff --git a/Cargo.lock b/Cargo.lock
index 293bcff..d21210e 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -230,6 +230,12 @@ dependencies = [
  "winapi",
 ]
 
+[[package]]
+name = "clru"
+version = "0.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "218d6bd3dde8e442a975fa1cd233c0e5fded7596bccfe39f58eca98d22421e0a"
+
 [[package]]
 name = "color_quant"
 version = "1.1.0"
@@ -242,6 +248,7 @@ version = "0.2.0"
 dependencies = [
  "base64 0.13.0",
  "bytes",
+ "clru",
  "crossbeam",
  "directories",
  "heed",
diff --git a/Cargo.toml b/Cargo.toml
index 13a7af4..1301179 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -79,6 +79,7 @@ num_cpus = "1.13.0"
 threadpool = "1.8.1"
 heed = { git = "https://github.com/timokoesters/heed.git", rev = "f6f825da7fb2c758867e05ad973ef800a6fe1d5d", optional = true }
 thread_local = "1.1.3"
+clru = "0.5.0"
 
 [features]
 default = ["conduit_bin", "backend_sqlite"]
diff --git a/src/database.rs b/src/database.rs
index 8cf4f64..fb2874b 100644
--- a/src/database.rs
+++ b/src/database.rs
@@ -48,7 +48,7 @@ pub struct Config {
     #[serde(default = "default_db_cache_capacity_mb")]
     db_cache_capacity_mb: f64,
     #[serde(default = "default_pdu_cache_capacity")]
-    pdu_cache_capacity: u32,
+    pdu_cache_capacity: usize,
     #[serde(default = "default_sqlite_wal_clean_second_interval")]
     sqlite_wal_clean_second_interval: u32,
     #[serde(default = "default_max_request_size")]
@@ -111,7 +111,7 @@ fn default_db_cache_capacity_mb() -> f64 {
     200.0
 }
 
-fn default_pdu_cache_capacity() -> u32 {
+fn default_pdu_cache_capacity() -> usize {
     100_000
 }
 
@@ -294,11 +294,12 @@ impl Database {
                 softfailedeventids: builder.open_tree("softfailedeventids")?,
 
                 referencedevents: builder.open_tree("referencedevents")?,
-                pdu_cache: Mutex::new(LruCache::new(
+                pdu_cache: Mutex::new(clru::CLruCache::with_scale(
                     config
                         .pdu_cache_capacity
                         .try_into()
                         .expect("pdu cache capacity fits into usize"),
+                    utils::scale::ConduitScale,
                 )),
                 auth_chain_cache: Mutex::new(LruCache::new(1_000_000)),
                 shorteventid_cache: Mutex::new(LruCache::new(1_000_000)),
diff --git a/src/database/rooms.rs b/src/database/rooms.rs
index c5b795b..53f5dd0 100644
--- a/src/database/rooms.rs
+++ b/src/database/rooms.rs
@@ -107,7 +107,14 @@ pub struct Rooms {
     /// RoomId + EventId -> Parent PDU EventId.
     pub(super) referencedevents: Arc<dyn Tree>,
 
-    pub(super) pdu_cache: Mutex<LruCache<EventId, Arc<PduEvent>>>,
+    pub(super) pdu_cache: Mutex<
+        clru::CLruCache<
+            EventId,
+            Arc<PduEvent>,
+            std::collections::hash_map::RandomState,
+            utils::scale::ConduitScale,
+        >,
+    >,
     pub(super) shorteventid_cache: Mutex<LruCache<u64, Arc<EventId>>>,
     pub(super) auth_chain_cache: Mutex<LruCache<Vec<u64>, Arc<HashSet<u64>>>>,
     pub(super) eventidshort_cache: Mutex<LruCache<EventId, u64>>,
@@ -1087,7 +1094,7 @@ impl Rooms {
     /// Checks the `eventid_outlierpdu` Tree if not found in the timeline.
     #[tracing::instrument(skip(self))]
     pub fn get_pdu(&self, event_id: &EventId) -> Result<Option<Arc<PduEvent>>> {
-        if let Some(p) = self.pdu_cache.lock().unwrap().get_mut(event_id) {
+        if let Some(p) = self.pdu_cache.lock().unwrap().get(event_id) {
             return Ok(Some(Arc::clone(p)));
         }
 
@@ -1109,10 +1116,14 @@ impl Rooms {
             })
             .transpose()?
         {
-            self.pdu_cache
+            if let Err(_) = self
+                .pdu_cache
                 .lock()
                 .unwrap()
-                .insert(event_id.clone(), Arc::clone(&pdu));
+                .put_with_weight(event_id.clone(), Arc::clone(&pdu))
+            {
+                tracing::warn!("PDU too big to fit in cache ({}), not cached, leaked.", &event_id)
+            };
             Ok(Some(pdu))
         } else {
             Ok(None)
diff --git a/src/utils.rs b/src/utils.rs
index 26d71a8..20a8e90 100644
--- a/src/utils.rs
+++ b/src/utils.rs
@@ -141,3 +141,277 @@ pub fn deserialize_from_str<
     }
     deserializer.deserialize_str(Visitor(std::marker::PhantomData))
 }
+
+pub mod scale {
+    use std::{collections::BTreeMap, mem::size_of, sync::Arc};
+
+    use clru::WeightScale;
+    use ruma::{EventId, RoomId, UserId};
+
+    use crate::PduEvent;
+
+    // This trait captures a plain size_of of a particular value,
+    // but also grabs all "hidden owned memory" behind it dynamically
+    pub trait Weighted {
+        fn size_of() -> usize;
+
+        // inner values that aren't taken into account with size_of
+        //
+        // total memory use = size_of() + inner()
+        fn inner(&self) -> usize;
+    }
+
+    pub trait WeightedExt {
+        fn weight(&self) -> usize;
+    }
+
+    impl<T> WeightedExt for T
+    where
+        T: Weighted + ?Sized,
+    {
+        fn weight(&self) -> usize {
+            Self::size_of() + self.inner()
+        }
+    }
+
+    impl<T> Weighted for Arc<T>
+    where
+        T: Weighted,
+    {
+        fn size_of() -> usize {
+            size_of::<Arc<T>>()
+        }
+
+        fn inner(&self) -> usize {
+            let i = self.as_ref();
+
+            i.weight()
+        }
+    }
+
+    impl<T> Weighted for Option<T>
+    where
+        T: Weighted,
+    {
+        fn size_of() -> usize {
+            size_of::<Option<T>>()
+        }
+
+        fn inner(&self) -> usize {
+            if let Some(i) = self {
+                i.inner()
+            } else {
+                0
+            }
+        }
+    }
+
+    impl<T: ?Sized> Weighted for Box<T>
+    where
+        T: Weighted,
+    {
+        fn size_of() -> usize {
+            size_of::<Box<T>>()
+        }
+
+        fn inner(&self) -> usize {
+            (self as &T).weight()
+        }
+    }
+
+    // impl Weighted for Box<ruma::ServerName> {
+    //     fn size_of() -> usize {
+    //         size_of::<Box<ruma::ServerName>>()
+    //     }
+
+    //     fn inner(&self) -> usize {
+    //         (self as &ruma::ServerName).weight()
+    //     }
+    // }
+
+    impl<T> Weighted for Vec<T>
+    where
+        T: Weighted,
+    {
+        fn size_of() -> usize {
+            size_of::<Vec<T>>()
+        }
+
+        fn inner(&self) -> usize {
+            (T::size_of() * self.len()) + self.iter().fold(0, |i, t| i + t.inner())
+        }
+    }
+
+    impl<A, B> Weighted for BTreeMap<A, B>
+    where
+        A: Weighted,
+        B: Weighted,
+    {
+        fn size_of() -> usize {
+            size_of::<BTreeMap<A, B>>()
+        }
+
+        // FIXME: technically we have all values of key/value pairs,
+        // but btree has a lot of internal nodeleafs andsoforth, so this isn't everything
+        fn inner(&self) -> usize {
+            self.iter().fold(0, |a, (k, v)| a + k.weight() + v.weight())
+        }
+    }
+
+    impl Weighted for u64 {
+        fn size_of() -> usize {
+            size_of::<u64>()
+        }
+
+        fn inner(&self) -> usize {
+            0
+        }
+    }
+
+    impl Weighted for str {
+        fn size_of() -> usize {
+            0 // str is a transmutation of &[u8]
+        }
+
+        fn inner(&self) -> usize {
+            self.as_bytes().len()
+        }
+    }
+
+    impl Weighted for String {
+        fn size_of() -> usize {
+            size_of::<String>()
+        }
+
+        fn inner(&self) -> usize {
+            self.as_str().inner()
+        }
+    }
+
+    impl Weighted for ruma::ServerName {
+        fn size_of() -> usize {
+            0 // ServerName is effectively str
+        }
+
+        fn inner(&self) -> usize {
+            self.as_str().inner()
+        }
+    }
+
+    impl Weighted for EventId {
+        fn size_of() -> usize {
+            size_of::<EventId>()
+        }
+
+        // reconstructing the inner Box<str>
+        fn inner(&self) -> usize {
+            1 // stripped $ symbol
+            + self.localpart().len() // everything up until :, or the end
+            + if let Some(server) = self.server_name() {
+                1 // stripped : symbol
+                + server.inner() // everything from after : to the end
+            } else {
+                0
+            }
+        }
+    }
+
+    impl Weighted for RoomId {
+        fn size_of() -> usize {
+            size_of::<RoomId>()
+        }
+
+        // reconstructing the inner Box<str>
+        fn inner(&self) -> usize {
+            1 // stripped ! symbol
+            + self.localpart().len() // everything up until :
+            + 1 // stripped : symbol
+            + self.server_name().inner() // everything from after : to the end
+        }
+    }
+
+    impl Weighted for UserId {
+        fn size_of() -> usize {
+            size_of::<UserId>()
+        }
+
+        // reconstructing the inner Box<str>
+        fn inner(&self) -> usize {
+            1 // stripped @ symbol
+            + self.localpart().len() // everything up until :
+            + 1 // stripped : symbol
+            + self.server_name().inner() // everything from after : to the end
+        }
+    }
+
+    impl Weighted for ruma::events::pdu::EventHash {
+        fn size_of() -> usize {
+            size_of::<ruma::events::pdu::EventHash>()
+        }
+
+        // reconstructing the inner Box<str>
+        fn inner(&self) -> usize {
+            self.sha256.inner()
+        }
+    }
+
+    impl<A, B: ?Sized> Weighted for ruma::KeyId<A, B> {
+        fn size_of() -> usize {
+            size_of::<ruma::KeyId<A, B>>()
+        }
+
+        fn inner(&self) -> usize {
+            self.as_str().inner()
+        }
+    }
+
+    impl Weighted for serde_json::Value {
+        fn size_of() -> usize {
+            size_of::<serde_json::Value>()
+        }
+
+        fn inner(&self) -> usize {
+            use serde_json::Value;
+
+            match self {
+                Value::String(s) => s.inner(),
+                Value::Array(a) => a.inner(),
+                Value::Object(o) => o
+                    .into_iter()
+                    .fold(0, |a, (s, v)| a + s.weight() + v.weight()),
+                _ => 0,
+            }
+        }
+    }
+
+    impl Weighted for PduEvent {
+        fn size_of() -> usize {
+            size_of::<PduEvent>()
+        }
+
+        fn inner(&self) -> usize {
+            self.event_id.inner()
+                + self.room_id.inner()
+                + self.sender.inner()
+                + self.content.inner()
+                + self.prev_events.inner()
+                + self.auth_events.inner()
+                + self.redacts.inner()
+                + self.unsigned.inner()
+                + self.hashes.inner()
+                + self.signatures.inner()
+        }
+    }
+
+    pub(crate) struct ConduitScale;
+
+    impl<A, B> WeightScale<A, B> for ConduitScale
+    where
+        A: Weighted,
+        B: Weighted,
+    {
+        fn weight(&self, key: &A, value: &B) -> usize {
+            key.weight() + value.weight()
+        }
+    }
+}