diff --git a/CHANGELOG.md b/CHANGELOG.md
index e757a8fdf7a..8abf6416d01 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -13,6 +13,9 @@ release.
 
 ### Metrics
 
+- Clarify that units should use UCUM case sensitive variant.
+  ([#3306](https://github.com/open-telemetry/opentelemetry-specification/pull/3306))
+
 ### Logs
 
 ### Resource
diff --git a/specification/metrics/semantic_conventions/README.md b/specification/metrics/semantic_conventions/README.md
index 27dd44f9d75..a7d5db37483 100644
--- a/specification/metrics/semantic_conventions/README.md
+++ b/specification/metrics/semantic_conventions/README.md
@@ -188,6 +188,8 @@ total) are dimensionless and SHOULD use the default unit `1` (the unity).
 [annotations](https://ucum.org/ucum.html#para-curly) with curly braces to
 give additional meaning *without* the leading default unit (`1`). For example,
 use `{packet}`, `{error}`, `{fault}`, etc.
+- Instrument units SHOULD be specified using the UCUM case sensitive ("c/s")
+  variant. For example, "Cel" for the unit with full name "degree Celsius".
 
 ### Instrument Types