diff --git a/canister/src/state.rs b/canister/src/state.rs
index 734d13ab..6c0e63a0 100644
--- a/canister/src/state.rs
+++ b/canister/src/state.rs
@@ -177,7 +177,8 @@ pub fn ingest_stable_blocks_into_utxoset(state: &mut State) -> bool {
 }
 
 pub fn main_chain_height(state: &State) -> Height {
-    unstable_blocks::get_main_chain(&state.unstable_blocks).len() as u32 + state.utxos.next_height()
+    unstable_blocks::get_main_chain_length(&state.unstable_blocks) as u32
+        + state.utxos.next_height()
         - 1
 }
 
diff --git a/canister/src/unstable_blocks.rs b/canister/src/unstable_blocks.rs
index a9f27feb..2859017f 100644
--- a/canister/src/unstable_blocks.rs
+++ b/canister/src/unstable_blocks.rs
@@ -228,6 +228,22 @@ pub fn get_main_chain(blocks: &UnstableBlocks) -> BlockChain {
     main_chain
 }
 
+/// Returns the length of the "main chain".
+/// See `get_main_chain` for what defines a main chain.
+pub fn get_main_chain_length(blocks: &UnstableBlocks) -> usize {
+    let blocks_by_height = blocks.blocks_with_depths_by_heights();
+
+    // Traverse the heights in reverse order. The highest height with a single block corresponds to
+    // the tip of the main chain.
+    for height in (0..blocks_by_height.len()).rev() {
+        if blocks_by_height[height].len() == 1 {
+            return height + 1;
+        }
+    }
+
+    unreachable!("There must be at least one height with exactly one block.");
+}
+
 pub fn get_blocks(blocks: &UnstableBlocks) -> Vec<&Block> {
     blocktree::blockchains(&blocks.tree)
         .into_iter()