diff --git a/src/libraries/Common/tests/System/Security/Cryptography/AlgorithmImplementations/ECDsa/ECDsaSignatureFormatTests.cs b/src/libraries/Common/tests/System/Security/Cryptography/AlgorithmImplementations/ECDsa/ECDsaSignatureFormatTests.cs
index 5451458f117a8..2370909f32863 100644
--- a/src/libraries/Common/tests/System/Security/Cryptography/AlgorithmImplementations/ECDsa/ECDsaSignatureFormatTests.cs
+++ b/src/libraries/Common/tests/System/Security/Cryptography/AlgorithmImplementations/ECDsa/ECDsaSignatureFormatTests.cs
@@ -331,7 +331,9 @@ public void Rfc23279TrySignHashUnderMax()
             ECDsa key = (ECDsa)keyDescription.Key;
 
             const DSASignatureFormat SignatureFormat = DSASignatureFormat.Rfc3279DerSequence;
-            const int RetryCount = 10;
+            // Make secp521r1 (7/16 chance of being smaller) and mod-8 keys (3/4 chance of being smaller)
+            // have the same 1-in-a-billion chance of failure.
+            int retryCount = keyDescription.FieldSizeInBits % 8 == 1 ? 36 : 15;
             byte[] hash = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 };
 
             int expectedSize = GetExpectedSize(keyDescription.FieldSizeInBits);
@@ -339,7 +341,7 @@ public void Rfc23279TrySignHashUnderMax()
             Assert.True(expectedSize < maxSize, "expectedSize < maxSize");
             byte[] signature = new byte[expectedSize];
 
-            for (int i = 0; i < RetryCount; i++)
+            for (int i = 0; i < retryCount; i++)
             {
                 if (key.TrySignHash(hash, signature, SignatureFormat, out int written))
                 {
@@ -359,7 +361,9 @@ public void Rfc23279TrySignDataUnderMax()
             ECDsa key = (ECDsa)keyDescription.Key;
 
             const DSASignatureFormat SignatureFormat = DSASignatureFormat.Rfc3279DerSequence;
-            const int RetryCount = 10;
+            // Make secp521r1 (7/16 chance of being smaller) and mod-8 keys (3/4 chance of being smaller)
+            // have the same 1-in-a-billion chance of failure.
+            int retryCount = keyDescription.FieldSizeInBits % 8 == 1 ? 36 : 15;
             HashAlgorithmName hashAlgorithm = HashAlgorithmName.SHA1;
 
             int expectedSize = GetExpectedSize(keyDescription.FieldSizeInBits);
@@ -367,7 +371,7 @@ public void Rfc23279TrySignDataUnderMax()
             Assert.True(expectedSize < maxSize, "expectedSize < maxSize");
             byte[] signature = new byte[expectedSize];
 
-            for (int i = 0; i < RetryCount; i++)
+            for (int i = 0; i < retryCount; i++)
             {
                 if (key.TrySignData(Array.Empty<byte>(), signature, hashAlgorithm, SignatureFormat, out int written))
                 {