diff --git a/index.js b/index.js
index 465f761..ece0cfa 100644
--- a/index.js
+++ b/index.js
@@ -1093,7 +1093,13 @@ class Packed {
     }
 
     estimateLength() {
-        return this.firstLineLengthInBytes + estimateDeflatedSize(this.secondLine);
+        // the first line is mostly 6-bit code, but there are additional characters
+        // so one of the 6-bit code literal has to be encoded in 7 bits.
+        // if the original data is N bytes the first line will contain about 4/3 N codes,
+        // 1/64 of which will have to use one additional bit.
+        // therefore the estimated overhead is 4/3 N * 1/64 [bits] = N/384 [bytes].
+        // the additional 35 bytes are experimentally derived from the Huffman tree coding.
+        return 35 + Math.ceil(this.firstLineLengthInBytes * (1 + 1/384)) + estimateDeflatedSize(this.secondLine);
     }
 }
 
diff --git a/tools/compare-size.js b/tools/compare-size.js
index c341f02..f55cf7e 100644
--- a/tools/compare-size.js
+++ b/tools/compare-size.js
@@ -36,7 +36,7 @@ function twoPartDeflate(buf1, buf2, options) {
 }
 
 function analyze(buf) {
-    console.log([...analyzeDeflate(1, buf)].map(i => i[0] / 8));
+    console.log([...analyzeDeflate('zlib', buf)].map(i => [i[0] / 8, i.slice(1).reduce((x,[y]) => x + y, 0) / 8]));
     return buf.length;
 }