diff --git a/core/src/main/groovy/com/muwire/core/FileHasher.groovy b/core/src/main/groovy/com/muwire/core/FileHasher.groovy
new file mode 100644
index 00000000..18ce2503
--- /dev/null
+++ b/core/src/main/groovy/com/muwire/core/FileHasher.groovy
@@ -0,0 +1,24 @@
+package com.muwire.core
+
+class FileHasher {
+
+	/** max size of shared file is 128 GB */
+	public static final long MAX_SIZE = 0x1 << 37
+	
+	/**
+	 * @param size of the file to be shared
+	 * @return the size of each piece in power of 2
+	 */
+	static int getPieceSize(long size) {
+		if (size <= 0x1 << 25)
+			return 18
+		
+		for (int i = 26; i <= 37; i++) {
+			if (size <= 0x1 << i) {
+				return i-7
+			}
+		}
+		
+		throw new IllegalArgumentException("File too large $size")
+	}
+}
diff --git a/core/src/test/groovy/com/muwire/core/FileHasherTest.groovy b/core/src/test/groovy/com/muwire/core/FileHasherTest.groovy
new file mode 100644
index 00000000..61cc2409
--- /dev/null
+++ b/core/src/test/groovy/com/muwire/core/FileHasherTest.groovy
@@ -0,0 +1,18 @@
+package com.muwire.core
+
+import static org.junit.jupiter.api.Assertions.assertEquals
+
+import org.junit.Test
+
+class FileHasherTest extends GroovyTestCase {
+
+	@Test
+	void testPieceSize() {
+		assert 18 == FileHasher.getPieceSize(1000000)
+		assert 20 == FileHasher.getPieceSize(100000000)
+		shouldFail IllegalArgumentException, {
+			FileHasher.getPieceSize(Long.MAX_VALUE)
+		}
+		
+	}
+}