diff --git a/_posts/2019-01-14-padsize.md b/_posts/2019-01-14-padsize.md
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+++ b/_posts/2019-01-14-padsize.md
@@ -0,0 +1,30 @@
+---
+title: padStartにおけるpadSizeの求め方
+---
+
+$$
+\textrm{padSize} = \lceil \log_{10}(\mathbf{arraySize} + 1) \rceil
+$$
+
+```js
+const padSize = Math.ceil(Math.log10(arr.length + 1))
+
+arr.forEach(async (item, index) => {
+  console.log(`${index.padStart(padSize, '0')}: ${item}`)
+})
+```
+
+結果は以下のようになる。
+
+```
+01: item1
+02: item2
+03: item3
+04: item4
+05: item5
+06: item6
+07: item7
+08: item8
+09: item9
+10: item10
+```