diff --git a/phf/src/lib.rs b/phf/src/lib.rs
index 01830a4c..35a146cb 100644
--- a/phf/src/lib.rs
+++ b/phf/src/lib.rs
@@ -138,6 +138,18 @@ pub enum Slice<T: 'static> {
     Dynamic(Vec<T>),
 }
 
+// NOTE: we need this because `Deref::deref` is not a `const-fn` so we can't use
+// `len` as written in `Map`/`Set`/`OrderedMap`/`OrderedSet` since it involves
+// an implicit indirection
+#[cfg(not(feature = "std"))]
+impl<T> Slice<T> {
+    pub(crate) const fn len(&self) -> usize {
+        match self {
+            Self::Static(slice) => slice.len(),
+        }
+    }
+}
+
 impl<T> Deref for Slice<T> {
     type Target = [T];
 
diff --git a/phf/src/map.rs b/phf/src/map.rs
index 3df11b3c..1140f3d9 100644
--- a/phf/src/map.rs
+++ b/phf/src/map.rs
@@ -46,15 +46,29 @@ where
 
 impl<K, V> Map<K, V> {
     /// Returns true if the `Map` is empty.
+    #[cfg(feature = "std")]
     pub fn is_empty(&self) -> bool {
         self.len() == 0
     }
 
+    /// Returns true if the `Map` is empty.
+    #[cfg(not(feature = "std"))]
+    pub const fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
     /// Returns the number of entries in the `Map`.
+    #[cfg(feature = "std")]
     pub fn len(&self) -> usize {
         self.entries.len()
     }
 
+    /// Returns the number of entries in the `Map`.
+    #[cfg(not(feature = "std"))]
+    pub const fn len(&self) -> usize {
+        self.entries.len()
+    }
+
     /// Determines if `key` is in the `Map`.
     pub fn contains_key<T: ?Sized>(&self, key: &T) -> bool
     where
diff --git a/phf/src/ordered_map.rs b/phf/src/ordered_map.rs
index de888aef..c2066d3b 100644
--- a/phf/src/ordered_map.rs
+++ b/phf/src/ordered_map.rs
@@ -52,15 +52,29 @@ where
 
 impl<K, V> OrderedMap<K, V> {
     /// Returns the number of entries in the `Map`.
+    #[cfg(feature = "std")]
     pub fn len(&self) -> usize {
         self.entries.len()
     }
 
+    /// Returns the number of entries in the `Map`.
+    #[cfg(not(feature = "std"))]
+    pub const fn len(&self) -> usize {
+        self.entries.len()
+    }
+
     /// Returns true if the `Map` is empty.
+    #[cfg(feature = "std")]
     pub fn is_empty(&self) -> bool {
         self.len() == 0
     }
 
+    /// Returns true if the `Map` is empty.
+    #[cfg(not(feature = "std"))]
+    pub const fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
     /// Returns a reference to the value that `key` maps to.
     pub fn get<T: ?Sized>(&self, key: &T) -> Option<&V>
     where
diff --git a/phf/src/ordered_set.rs b/phf/src/ordered_set.rs
index d1e93293..35163da0 100644
--- a/phf/src/ordered_set.rs
+++ b/phf/src/ordered_set.rs
@@ -30,15 +30,29 @@ where
 
 impl<T> OrderedSet<T> {
     /// Returns the number of elements in the `OrderedSet`.
+    #[cfg(feature = "std")]
     pub fn len(&self) -> usize {
         self.map.len()
     }
 
+    /// Returns the number of elements in the `OrderedSet`.
+    #[cfg(not(feature = "std"))]
+    pub const fn len(&self) -> usize {
+        self.map.len()
+    }
+
     /// Returns true if the `OrderedSet` contains no elements.
+    #[cfg(feature = "std")]
     pub fn is_empty(&self) -> bool {
         self.len() == 0
     }
 
+    /// Returns true if the `OrderedSet` contains no elements.
+    #[cfg(not(feature = "std"))]
+    pub const fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
     /// Returns a reference to the set's internal static instance of the given
     /// key.
     ///
diff --git a/phf/src/set.rs b/phf/src/set.rs
index 1a3f1acc..64394d59 100644
--- a/phf/src/set.rs
+++ b/phf/src/set.rs
@@ -29,15 +29,29 @@ where
 
 impl<T> Set<T> {
     /// Returns the number of elements in the `Set`.
+    #[cfg(feature = "std")]
     pub fn len(&self) -> usize {
         self.map.len()
     }
 
+    /// Returns the number of elements in the `Set`.
+    #[cfg(not(feature = "std"))]
+    pub const fn len(&self) -> usize {
+        self.map.len()
+    }
+
     /// Returns true if the `Set` contains no elements.
+    #[cfg(feature = "std")]
     pub fn is_empty(&self) -> bool {
         self.len() == 0
     }
 
+    /// Returns true if the `Set` contains no elements.
+    #[cfg(not(feature = "std"))]
+    pub const fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
     /// Returns a reference to the set's internal static instance of the given
     /// key.
     ///