diff --git a/CHANGELOG.md b/CHANGELOG.md
index d552b8d..a19a426 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- Use `out_<runid>.csv` as default output path
+
 ## [1.0.1]
 
 ### Fixed
diff --git a/src/main.rs b/src/main.rs
index 73f24c6..74abb8f 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -11,7 +11,7 @@ use anyhow::{anyhow, Context, Result};
 use chrono::{DateTime, Utc};
 use clap::{Parser, Subcommand};
 use log::{debug, error, info, trace, warn};
-use masterlog::MasterLogResult;
+use masterlog::{get_output_filename, MasterLogResult};
 use semver::Version;
 use serde::{Deserialize, Serialize};
 use serde_with::{serde_as, DisplayFromStr};
@@ -36,8 +36,9 @@ pub enum Commands {
         /// Master log file.
         master_log: path::PathBuf,
         /// Output CSV file.
+        /// Default is `out_<runid>.csv`
         // #[arg(default_value = get_default_log_path().into_os_string())]
-        outfile: PathBuf,
+        outfile: Option<PathBuf>,
     },
     /// Check CSV format
     CheckDB {
@@ -377,6 +378,8 @@ fn main() -> Result<()> {
                 }
             }
 
+            let outfile = outfile.unwrap_or(get_output_filename(&result));
+
             let expanded_results = PsbQaqcResult::from_masterlogresult(result);
 
             let mut wtr = match outfile.exists() {
diff --git a/src/masterlog.rs b/src/masterlog.rs
index 67d684b..8052778 100644
--- a/src/masterlog.rs
+++ b/src/masterlog.rs
@@ -1,4 +1,4 @@
-use std::{collections::BTreeMap, io::BufRead, str::FromStr};
+use std::{collections::BTreeMap, io::BufRead, path::PathBuf, str::FromStr};
 
 use anyhow::{anyhow, Context, Result};
 use chrono::{DateTime, Utc};
@@ -6,7 +6,7 @@ use log::{debug, info, trace};
 use regex::Regex;
 use semver::Version;
 
-use crate::{Position, PositionLayer, PsbId};
+use crate::{Position, PositionLayer, PsbId, PsbQaqcResult};
 
 /// QAQC results for each boards extracted from master log.
 ///
@@ -303,6 +303,10 @@ impl MasterLogResult {
     }
 }
 
+pub fn get_output_filename(result: &MasterLogResult) -> PathBuf {
+    PathBuf::from(format!("out_{}.csv", result.runid))
+}
+
 #[cfg(test)]
 mod test {
     use semver::Version;
diff --git a/tests/cli.rs b/tests/cli.rs
index cae07cf..1ed54d2 100644
--- a/tests/cli.rs
+++ b/tests/cli.rs
@@ -1,11 +1,13 @@
 mod integrated_test {
     use std::{
-        fs::File,
+        env::current_dir,
+        ffi::OsString,
+        fs::{self, read_dir, File},
         io::{BufRead, BufReader},
         path::PathBuf,
     };
 
-    use anyhow::Result;
+    use anyhow::{Context, Result};
     use assert_cmd::Command;
     use itertools::Itertools;
     use predicates::prelude::*;
@@ -67,17 +69,28 @@ mod integrated_test {
     #[test]
     fn partial_log() -> Result<()> {
         let test_out_dir = assert_fs::TempDir::new()?;
-        let test_out = PathBuf::new().join(&test_out_dir).join("out.csv");
+        fs::copy(
+            "./tests/example_logs/valid/84.log",
+            test_out_dir.join("84.log"),
+        )
+        .context("Error preparing source file")?;
 
         let mut cmd = Command::cargo_bin("psb-qaqc")?;
-        cmd.current_dir("tests")
+        cmd.current_dir(&test_out_dir)
             .arg("add-master-log")
-            .arg("./example_logs/valid/84.log")
-            .arg(test_out.as_path())
+            .arg("84.log")
             .assert()
             .success()
             .stdout(predicate::str::contains("Creating new file"));
 
+        assert!(read_dir(&test_out_dir)
+            .context("Failed to read dir")?
+            .any(|entry| {
+                entry
+                    .map_or(OsString::from(""), |entry| entry.file_name())
+                    .eq("out_84.csv")
+            }));
+
         Ok(())
     }
 }