diff --git a/rslib/src/search/parser.rs b/rslib/src/search/parser.rs
index 99185fb26..55ab6b5fc 100644
--- a/rslib/src/search/parser.rs
+++ b/rslib/src/search/parser.rs
@@ -6,11 +6,13 @@ use nom::branch::alt;
 use nom::bytes::complete::escaped;
 use nom::bytes::complete::is_not;
 use nom::bytes::complete::tag;
+use nom::character::complete::alphanumeric1;
 use nom::character::complete::anychar;
 use nom::character::complete::char;
 use nom::character::complete::none_of;
 use nom::character::complete::one_of;
 use nom::combinator::map;
+use nom::combinator::recognize;
 use nom::combinator::verify;
 use nom::error::ErrorKind as NomErrorKind;
 use nom::multi::many0;
@@ -101,6 +103,7 @@ pub enum PropertyKind {
     Ease(f32),
     Position(u32),
     Rated(i32, RatingKind),
+    CustomDataNumber { key: String, value: f32 },
 }
 
 #[derive(Debug, PartialEq, Eq, Clone)]
@@ -403,6 +406,7 @@ fn parse_prop(prop_clause: &str) -> ParseResult<SearchNode> {
         tag("pos"),
         tag("rated"),
         tag("resched"),
+        recognize(preceded(tag("cdn:"), alphanumeric1)),
     ))(prop_clause)
     .map_err(|_| {
         parse_failure(
@@ -442,7 +446,13 @@ fn parse_prop(prop_clause: &str) -> ParseResult<SearchNode> {
         "reps" => PropertyKind::Reps(parse_u32(num, prop_clause)?),
         "lapses" => PropertyKind::Lapses(parse_u32(num, prop_clause)?),
         "pos" => PropertyKind::Position(parse_u32(num, prop_clause)?),
-        _ => unreachable!(),
+        other => {
+            let Some(prop) = other.strip_prefix("cdn:") else { unreachable!() };
+            PropertyKind::CustomDataNumber {
+                key: prop.into(),
+                value: parse_f32(num, prop_clause)?,
+            }
+        }
     };
 
     Ok(SearchNode::Property {
@@ -903,6 +913,16 @@ mod test {
                 kind: PropertyKind::Ease(3.3)
             })]
         );
+        assert_eq!(
+            parse("prop:cdn:abc<=1")?,
+            vec![Search(Property {
+                operator: "<=".into(),
+                kind: PropertyKind::CustomDataNumber {
+                    key: "abc".into(),
+                    value: 1.0
+                }
+            })]
+        );
 
         Ok(())
     }
@@ -1112,6 +1132,18 @@ mod test {
                 provided: "DUE<5".into(),
             },
         );
+        assert_err_kind(
+            "prop:cdn=5",
+            InvalidPropProperty {
+                provided: "cdn=5".to_string(),
+            },
+        );
+        assert_err_kind(
+            "prop:cdn:=5",
+            InvalidPropProperty {
+                provided: "cdn:=5".to_string(),
+            },
+        );
 
         assert_err_kind(
             "prop:lapses",
diff --git a/rslib/src/search/sqlwriter.rs b/rslib/src/search/sqlwriter.rs
index ffbb8113e..9381bea24 100644
--- a/rslib/src/search/sqlwriter.rs
+++ b/rslib/src/search/sqlwriter.rs
@@ -354,6 +354,13 @@ impl SqlWriter<'_> {
                 write!(self.sql, "factor {} {}", op, (ease * 1000.0) as u32).unwrap()
             }
             PropertyKind::Rated(days, ease) => self.write_rated(op, i64::from(*days), ease)?,
+            PropertyKind::CustomDataNumber { key, value } => {
+                write!(
+                    self.sql,
+                    "extract_custom_data_number(c.data, '{key}') {op} {value}"
+                )
+                .unwrap();
+            }
         }
 
         Ok(())
@@ -1166,6 +1173,10 @@ mod test {
             )
         );
         assert_eq!(s(ctx, "prop:rated>-5:3").0, s(ctx, "rated:5:3").0);
+        assert_eq!(
+            &s(ctx, "prop:cdn:r=1").0,
+            "(extract_custom_data_number(c.data, 'r') = 1)"
+        );
 
         // note types by name
         assert_eq!(
diff --git a/rslib/src/search/writer.rs b/rslib/src/search/writer.rs
index 520ebd288..b43522cfa 100644
--- a/rslib/src/search/writer.rs
+++ b/rslib/src/search/writer.rs
@@ -171,6 +171,7 @@ fn write_property(operator: &str, kind: &PropertyKind) -> String {
             RatingKind::AnyAnswerButton => format!("prop:rated{}{}", operator, u),
             RatingKind::ManualReschedule => format!("prop:resched{}{}", operator, u),
         },
+        CustomDataNumber { key, value } => format!("prop:cdn:{key}{operator}{value}"),
     }
 }
 
diff --git a/rslib/src/storage/sqlite.rs b/rslib/src/storage/sqlite.rs
index b07f17942..e6fcd69b1 100644
--- a/rslib/src/storage/sqlite.rs
+++ b/rslib/src/storage/sqlite.rs
@@ -13,6 +13,7 @@ use regex::Regex;
 use rusqlite::functions::FunctionFlags;
 use rusqlite::params;
 use rusqlite::Connection;
+use serde_json::Value;
 use unicase::UniCase;
 
 use super::upgrades::SCHEMA_MAX_VERSION;
@@ -24,6 +25,7 @@ use crate::error::DbErrorKind;
 use crate::prelude::*;
 use crate::scheduler::timing::local_minutes_west_for_stamp;
 use crate::scheduler::timing::v1_creation_date;
+use crate::storage::card::data::CardData;
 use crate::text::without_combining;
 
 fn unicase_compare(s1: &str, s2: &str) -> Ordering {
@@ -67,6 +69,7 @@ fn open_or_create_collection_db(path: &Path) -> Result<Connection> {
     add_regexp_tags_function(&db)?;
     add_without_combining_function(&db)?;
     add_fnvhash_function(&db)?;
+    add_extract_custom_data_number_function(&db)?;
 
     db.create_collation("unicase", unicase_compare)?;
 
@@ -190,6 +193,28 @@ fn add_regexp_tags_function(db: &Connection) -> rusqlite::Result<()> {
     )
 }
 
+/// eg. extract_custom_data_number(card.data, 'r') -> float | null
+fn add_extract_custom_data_number_function(db: &Connection) -> rusqlite::Result<()> {
+    db.create_scalar_function(
+        "extract_custom_data_number",
+        2,
+        FunctionFlags::SQLITE_DETERMINISTIC,
+        move |ctx| {
+            assert_eq!(ctx.len(), 2, "called with unexpected number of arguments");
+
+            let Ok(card_data) = ctx.get_raw(0).as_str() else { return Ok(None) };
+            if card_data.is_empty() {
+                return Ok(None);
+            }
+            let Ok(key) = ctx.get_raw(1).as_str() else { return Ok(None) };
+            let custom_data = &CardData::from_str(card_data).custom_data;
+            let Ok(value) = serde_json::from_str::<Value>(custom_data) else { return Ok(None) };
+            let num = value.get(key).and_then(|v| v.as_f64());
+            Ok(num)
+        },
+    )
+}
+
 /// Fetch schema version from database.
 /// Return (must_create, version)
 fn schema_version(db: &Connection) -> Result<(bool, u8)> {