diff --git a/README.md b/README.md
index f753f10..cb9d67d 100644
--- a/README.md
+++ b/README.md
@@ -26,7 +26,9 @@ cmake --build --preset attiny85-generated
 
 Presets cover attiny85/atmega328p in both libavr modes (generated and
 reflect); `ctest` runs the consteval battery over the BCD, the hours
-register's two formats, the weekday and the alarm rate encodings. The legacy
+register's two formats, the weekday and the alarm rate encodings, and holds
+the example to its stated size — **1480 B** on the mega328P and **1238 B**
+on the tiny85, byte-identical across the two modes. The legacy
 yazoalfa-based driver lives on the `master` branch.
 
 The weekday register is derived from the date by default. A program that
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index 373b8b6..7274c3f 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -1,5 +1,13 @@
 libavr_format_test()
 
+# The example's size is stated in the README, per chip, and a library advance
+# that moves it silently is what this catches.
+if(LIBAVR_MCU STREQUAL "atmega328p")
+	libavr_size_claim_test(clock 1480)
+elseif(LIBAVR_MCU STREQUAL "attiny85")
+	libavr_size_claim_test(clock 1238)
+endif()
+
 # The battery is a compile: a static_assert that fails is the failure. It is a
 # target rather than only a ctest so a plain build catches a regression too.
 add_library(consteval_tests OBJECT consteval.cpp)