diff --git a/ch06.asciidoc b/ch06.asciidoc
index 5501f98d..8a233f3f 100644
--- a/ch06.asciidoc
+++ b/ch06.asciidoc
@@ -304,7 +304,7 @@ Over time, the way transaction fees are calculated and the effect they have on t
 
 Fee estimation algorithms calculate the appropriate fee, based on capacity and the fees offered by "competing" transactions. These algorithms range from simplistic (average or median fee in the last block) to sophisticated (statistical analysis). They estimate the necessary fee (in satoshis per byte) that will give a transaction a high probability of being selected and included within a certain number of blocks. Most services offer users the option of choosing high, medium, or low priority fees. High priority means users pay higher fees but the transaction is likely to be included in the next block. Medium and low priority means users pay lower transaction fees but the transactions may take much longer to confirm.
 
-((("bitcoinfees (third-party service)")))Many wallet applications use third-party services for fee calculations. One popular service is http://bitcoinfees.21.co/[_http://bitcoinfees.21.co_], which provides an API and a visual chart showing the fee in satoshi/byte for different priorities.
+((("bitcoinfees (third-party service)")))Many wallet applications use third-party services for fee calculations. One popular service is https://bitcoinfees.21.co/[_https://bitcoinfees.21.co/_], which provides an API and a visual chart showing the fee in satoshi/byte for different priorities.
 
 [TIP]
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