diff --git a/src/cache.rs b/src/cache.rs
index 2ce63dd..1b1f359 100644
--- a/src/cache.rs
+++ b/src/cache.rs
@@ -2,18 +2,18 @@ use super::FontRef;
 
 /// Uniquely generated value for identifying and caching fonts.
 #[derive(Copy, Clone, PartialOrd, Ord, PartialEq, Eq, Hash, Debug)]
-pub struct CacheKey(pub(crate) u64);
+pub struct CacheKey(pub(crate) usize);
 
 impl CacheKey {
     /// Generates a new cache key.
     pub fn new() -> Self {
-        use core::sync::atomic::{AtomicU64, Ordering};
-        static KEY: AtomicU64 = AtomicU64::new(1);
+        use core::sync::atomic::{AtomicUsize, Ordering};
+        static KEY: AtomicUsize = AtomicUsize::new(1);
         Self(KEY.fetch_add(1, Ordering::Relaxed))
     }
 
     /// Returns the underlying value of the key.
-    pub fn value(self) -> u64 {
+    pub fn value(self) -> usize {
         self.0
     }
 }
@@ -42,10 +42,10 @@ impl<T> FontCache<T> {
     pub fn get<'a>(
         &'a mut self,
         font: &FontRef,
-        id_override: Option<[u64; 2]>,
+        id_override: Option<[usize; 2]>,
         mut f: impl FnMut(&FontRef) -> T,
-    ) -> ([u64; 2], &'a T) {
-        let id = id_override.unwrap_or([font.key.value(), u64::MAX]);
+    ) -> ([usize; 2], &'a T) {
+        let id = id_override.unwrap_or([font.key.value(), usize::MAX]);
         let (found, index) = self.find(id);
         if found {
             let entry = &mut self.entries[index];
@@ -72,7 +72,7 @@ impl<T> FontCache<T> {
         }
     }
 
-    fn find(&self, id: [u64; 2]) -> (bool, usize) {
+    fn find(&self, id: [usize; 2]) -> (bool, usize) {
         let mut lowest = 0;
         let mut lowest_epoch = self.epoch;
         for (i, entry) in self.entries.iter().enumerate() {
@@ -94,6 +94,6 @@ impl<T> FontCache<T> {
 
 struct Entry<T> {
     epoch: u64,
-    id: [u64; 2],
+    id: [usize; 2],
     data: T,
 }
diff --git a/src/scale/hinting_cache.rs b/src/scale/hinting_cache.rs
index 83e0461..9432641 100644
--- a/src/scale/hinting_cache.rs
+++ b/src/scale/hinting_cache.rs
@@ -10,7 +10,7 @@ use skrifa::{
 const MAX_CACHED_HINT_INSTANCES: usize = 8;
 
 pub struct HintingKey<'a> {
-    pub id: [u64; 2],
+    pub id: [usize; 2],
     pub outlines: &'a OutlineGlyphCollection<'a>,
     pub size: Size,
     pub coords: &'a [NormalizedCoord],
@@ -58,7 +58,7 @@ impl HintingCache {
 }
 
 struct HintingEntry {
-    id: [u64; 2],
+    id: [usize; 2],
     instance: HintingInstance,
     serial: u64,
 }
diff --git a/src/scale/mod.rs b/src/scale/mod.rs
index ffb6ce7..5d37fdf 100644
--- a/src/scale/mod.rs
+++ b/src/scale/mod.rs
@@ -340,7 +340,7 @@ pub struct ScalerBuilder<'a> {
     font: FontRef<'a>,
     outlines: Option<OutlineGlyphCollection<'a>>,
     proxy: &'a ScalerProxy,
-    id: [u64; 2],
+    id: [usize; 2],
     coords: &'a mut Vec<SkrifaNormalizedCoord>,
     size: f32,
     hint: bool,
diff --git a/src/shape/cache.rs b/src/shape/cache.rs
index fa6b31a..6284007 100644
--- a/src/shape/cache.rs
+++ b/src/shape/cache.rs
@@ -27,7 +27,7 @@ impl FontEntry {
 
 pub struct FeatureEntry {
     pub epoch: Epoch,
-    pub id: [u64; 2],
+    pub id: [usize; 2],
     pub coords: Vec<i16>,
     pub tags: [u32; 4],
     pub store: FeatureStore,
@@ -55,7 +55,7 @@ impl FeatureCache {
 
     pub fn entry<'a>(
         &'a mut self,
-        id: [u64; 2],
+        id: [usize; 2],
         coords: &[i16],
         has_feature_vars: bool,
         tags: &[u32; 4],
@@ -77,7 +77,7 @@ impl FeatureCache {
 
     fn find_entry(
         &mut self,
-        id: [u64; 2],
+        id: [usize; 2],
         coords: &[i16],
         has_feature_vars: bool,
         tags: &[u32; 4],
diff --git a/src/shape/mod.rs b/src/shape/mod.rs
index f0a0fa3..d4f0d26 100644
--- a/src/shape/mod.rs
+++ b/src/shape/mod.rs
@@ -318,7 +318,7 @@ impl ShapeContext {
     pub fn builder_with_id<'a>(
         &'a mut self,
         font: impl Into<FontRef<'a>>,
-        id: [u64; 2],
+        id: [usize; 2],
     ) -> ShaperBuilder<'a> {
         ShaperBuilder::new_with_id(self, font, id)
     }
@@ -367,7 +367,7 @@ pub struct ShaperBuilder<'a> {
     state: &'a mut State,
     feature_cache: &'a mut FeatureCache,
     font: FontRef<'a>,
-    font_id: [u64; 2],
+    font_id: [usize; 2],
     font_entry: &'a FontEntry,
     coords: &'a mut Vec<i16>,
     charmap: Charmap<'a>,
@@ -384,7 +384,7 @@ impl<'a> ShaperBuilder<'a> {
     /// context and font.
     fn new(context: &'a mut ShapeContext, font: impl Into<FontRef<'a>>) -> Self {
         let font = font.into();
-        let id = [font.key.value(), u64::MAX];
+        let id = [font.key.value(), usize::MAX];
         Self::new_with_id(context, font, id)
     }
 
@@ -393,7 +393,7 @@ impl<'a> ShaperBuilder<'a> {
     fn new_with_id(
         context: &'a mut ShapeContext,
         font: impl Into<FontRef<'a>>,
-        id: [u64; 2],
+        id: [usize; 2],
     ) -> Self {
         let font = font.into();
         let (font_id, font_entry) = context.font_cache.get(&font, Some(id), FontEntry::new);
diff --git a/src/shape/partition.rs b/src/shape/partition.rs
index 1bedf48..32f4f3e 100644
--- a/src/shape/partition.rs
+++ b/src/shape/partition.rs
@@ -24,7 +24,7 @@ pub trait SelectedFont: PartialEq {
     /// Returns a reference to the underlying font.
     fn font(&self) -> FontRef;
 
-    fn id_override(&self) -> Option<[u64; 2]> {
+    fn id_override(&self) -> Option<[usize; 2]> {
         None
     }
 
@@ -215,7 +215,7 @@ where
     let font_ref = font.font();
     let id = font
         .id_override()
-        .unwrap_or([font_ref.key.value(), u64::MAX]);
+        .unwrap_or([font_ref.key.value(), usize::MAX]);
     let mut shaper = context
         .builder_with_id(font.font(), id)
         .script(options.script())