diff --git a/README.md b/README.md
index 828cb57..908a5b5 100644
--- a/README.md
+++ b/README.md
@@ -8,3 +8,8 @@
 - Circle: P = 2πR
 - Rectangle: P = 2a + 2b
 - Square: P = 4a
+
+# Diagonal length
+- Circle: D = 2R
+- Rectangle: sqrt(a^2 + b^2)
+- Square: a * sqrt(2)
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