diff --git a/hledger-ui/hledger-ui.m4.md b/hledger-ui/hledger-ui.m4.md
index a78e8f11a..b559f9d15 100644
--- a/hledger-ui/hledger-ui.m4.md
+++ b/hledger-ui/hledger-ui.m4.md
@@ -355,6 +355,11 @@ CPU and memory usage can sometimes gradually increase, if `hledger-ui --watch` i
 To work around, `q`uit and restart it,
 or (where supported) suspend (`CTRL-z`) and restart it (`fg`).
 
+## Debug output
+
+You can add `--debug[=N]` to the command line to log debug output.
+This will be logged to the file `hledger-ui.log` in the current directory.
+N ranges from 1 (least output, the default) to 9 (maximum output).
 
 # ENVIRONMENT
 
diff --git a/hledger-web/hledger-web.m4.md b/hledger-web/hledger-web.m4.md
index 946eee9a6..c4fd05ad3 100644
--- a/hledger-web/hledger-web.m4.md
+++ b/hledger-web/hledger-web.m4.md
@@ -401,6 +401,17 @@ And here's how to test adding it with curl. This should add a new entry to your
 $ curl http://127.0.0.1:5000/add -X PUT -H 'Content-Type: application/json' --data-binary @txn.json
 ```
 
+# DEBUG OUTPUT
+
+## Debug output
+
+You can add `--debug[=N]` to the command line to log debug output.
+N ranges from 1 (least output, the default) to 9 (maximum output).
+Typically you would start with 1 and increase until you are seeing enough.
+Debug output goes to stderr, interleaved with the requests logged on stdout.
+To capture debug output in a log file instead, you can usually redirect stderr, eg:\
+`hledger-web --debug=3 2>hledger-web.log`.
+
 # ENVIRONMENT
 
 _LEDGER_FILE_