diff --git a/CHANGELOG.md b/CHANGELOG.md
index 31f2996..4d2f5c2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
+## [Unreleased]
+
+### Fixed
+
+- Fix panic in `tlsf::Heap::used`.
+
 ## [v0.7.0] - 2026-01-03
 
 ### Added
diff --git a/examples/tlsf_integration_test.rs b/examples/tlsf_integration_test.rs
index 5bf5601..7a911e7 100644
--- a/examples/tlsf_integration_test.rs
+++ b/examples/tlsf_integration_test.rs
@@ -33,6 +33,8 @@ pub type TestTable<'a> = &'a [(fn() -> (), &'static str)];
 
 fn test_global_heap() {
     const ELEMS: usize = 250;
+    assert_eq!(HEAP_SIZE, HEAP.free() + HEAP.used());
+    let initial_free = HEAP.free();
 
     let mut allocated = LinkedList::new();
     for _ in 0..ELEMS {
@@ -51,6 +53,8 @@ fn test_global_heap() {
     for i in 0..ELEMS {
         assert_eq!(allocated.pop_front().unwrap(), i as i32);
     }
+    assert_eq!(HEAP_SIZE, HEAP.free() + HEAP.used());
+    assert_eq!(initial_free, HEAP.free());
 }
 
 fn test_allocator_api() {
diff --git a/src/tlsf.rs b/src/tlsf.rs
index 7a5017f..7155aaa 100644
--- a/src/tlsf.rs
+++ b/src/tlsf.rs
@@ -101,7 +101,8 @@ impl Heap {
     /// Get the amount of bytes used by the allocator.
     pub fn used(&self) -> usize {
         critical_section::with(|cs| {
-            self.heap.borrow_ref_mut(cs).raw_block_size - self.free_with_cs(cs)
+            let free = self.free_with_cs(cs);
+            self.heap.borrow_ref_mut(cs).raw_block_size - free
         })
     }