diff --git a/RELEASE-NOTES.md b/RELEASE-NOTES.md
index 6fa3e90..dcd7d30 100644
--- a/RELEASE-NOTES.md
+++ b/RELEASE-NOTES.md
@@ -1,3 +1,7 @@
+# 0.21.4
+
+- Make `encoded_len` `const`, allowing the creation of arrays sized to encode compile-time-known data lengths
+
 # 0.21.3
 
 - Implement `source` instead of `cause` on Error types
@@ -12,7 +16,8 @@
 # 0.21.1
 
 - Remove the possibility of panicking during decoded length calculations
-- `DecoderReader` no longer sometimes erroneously ignores padding  [#226](https://github.com/marshallpierce/rust-base64/issues/226)
+- `DecoderReader` no longer sometimes erroneously ignores
+  padding  [#226](https://github.com/marshallpierce/rust-base64/issues/226)
 
 ## Breaking changes
 
@@ -64,7 +69,8 @@ precisely, see the following table.
 
 ## Breaking changes
 
-- Re-exports of preconfigured engines in `engine` are removed in favor of `base64::prelude::...` that are better suited to those who wish to `use` the entire path to a name.
+- Re-exports of preconfigured engines in `engine` are removed in favor of `base64::prelude::...` that are better suited
+  to those who wish to `use` the entire path to a name.
 
 # 0.21.0-beta.1
 
diff --git a/src/encode.rs b/src/encode.rs
index 00ade74..2162d9e 100644
--- a/src/encode.rs
+++ b/src/encode.rs
@@ -94,25 +94,32 @@ pub(crate) fn encode_with_padding<E: Engine + ?Sized>(
 ///
 /// Returns `None` if the encoded length can't be represented in `usize`. This will happen for
 /// input lengths in approximately the top quarter of the range of `usize`.
-pub fn encoded_len(bytes_len: usize, padding: bool) -> Option<usize> {
+pub const fn encoded_len(bytes_len: usize, padding: bool) -> Option<usize> {
     let rem = bytes_len % 3;
 
     let complete_input_chunks = bytes_len / 3;
-    let complete_chunk_output = complete_input_chunks.checked_mul(4);
+    // `let Some(_) = _ else` requires 1.65.0, whereas this messier one works on 1.48
+    let complete_chunk_output =
+        if let Some(complete_chunk_output) = complete_input_chunks.checked_mul(4) {
+            complete_chunk_output
+        } else {
+            return None;
+        };
 
     if rem > 0 {
         if padding {
-            complete_chunk_output.and_then(|c| c.checked_add(4))
+            complete_chunk_output.checked_add(4)
         } else {
             let encoded_rem = match rem {
                 1 => 2,
-                2 => 3,
-                _ => unreachable!("Impossible remainder"),
+                // only other possible remainder is 2
+                // can't use a separate _ => unreachable!() in const fns in ancient rust versions
+                _ => 3,
             };
-            complete_chunk_output.and_then(|c| c.checked_add(encoded_rem))
+            complete_chunk_output.checked_add(encoded_rem)
         }
     } else {
-        complete_chunk_output
+        Some(complete_chunk_output)
     }
 }
 
diff --git a/tests/encode.rs b/tests/encode.rs
index cf61e9a..9d69447 100644
--- a/tests/encode.rs
+++ b/tests/encode.rs
@@ -44,3 +44,34 @@ fn encode_all_bytes_url() {
         &engine::GeneralPurpose::new(&URL_SAFE, PAD).encode(bytes)
     );
 }
+
+#[test]
+fn encoded_len_unpadded() {
+    assert_eq!(0, encoded_len(0, false).unwrap());
+    assert_eq!(2, encoded_len(1, false).unwrap());
+    assert_eq!(3, encoded_len(2, false).unwrap());
+    assert_eq!(4, encoded_len(3, false).unwrap());
+    assert_eq!(6, encoded_len(4, false).unwrap());
+    assert_eq!(7, encoded_len(5, false).unwrap());
+    assert_eq!(8, encoded_len(6, false).unwrap());
+    assert_eq!(10, encoded_len(7, false).unwrap());
+}
+
+#[test]
+fn encoded_len_padded() {
+    assert_eq!(0, encoded_len(0, true).unwrap());
+    assert_eq!(4, encoded_len(1, true).unwrap());
+    assert_eq!(4, encoded_len(2, true).unwrap());
+    assert_eq!(4, encoded_len(3, true).unwrap());
+    assert_eq!(8, encoded_len(4, true).unwrap());
+    assert_eq!(8, encoded_len(5, true).unwrap());
+    assert_eq!(8, encoded_len(6, true).unwrap());
+    assert_eq!(12, encoded_len(7, true).unwrap());
+}
+#[test]
+fn encoded_len_overflow() {
+    let max_size = usize::MAX / 4 * 3 + 2;
+    assert_eq!(2, max_size % 3);
+    assert_eq!(Some(usize::MAX), encoded_len(max_size, false));
+    assert_eq!(None, encoded_len(max_size + 1, false));
+}