diff --git a/README.md b/README.md
index 4042f11..2fdaea8 100644
--- a/README.md
+++ b/README.md
@@ -201,6 +201,13 @@ By default, files are compared after gzip compression, but it's possible to use
 compression: "none"
 ```
 
+### Specifying the base ref
+
+Use the `base-ref` option to compare against a specific ref. Otherwise, the action compares against the PR's base branch.
+
+
+For example, a project could set `base-ref` conditionally to use the PR target branch for feature PRs but compare against `production` for release PRs. Comparing against the previous release tag could show cumulative size change across all changes going into the release.
+
 ### Checking multiple bundles
 
 The action reuses the same comment each time it runs on a PR. In order to run the action multiple times against separate bundles for a single PR, you must provide a `comment-key` option, which the action will use to determine which comment to add or update for the run. The example below demonstrates this for separate "modern" and "legacy" bundles:
diff --git a/action.yml b/action.yml
index aa4dae3..a88f084 100644
--- a/action.yml
+++ b/action.yml
@@ -9,6 +9,9 @@ inputs:
     description: 'The GITHUB_TOKEN secret'
     required: false
     default: ${{ github.token }}
+  base-ref:
+    description: 'A specific git ref (branch, tag, or SHA) to compare against instead of the PR base branch'
+    required: false
   clean-script:
     description: 'An npm-script that cleans/resets state between branch builds'
   install-script:
diff --git a/src/index.js b/src/index.js
index b99602f..2ad852a 100644
--- a/src/index.js
+++ b/src/index.js
@@ -37,6 +37,14 @@ async function run(octokit, context, token) {
 		);
 	}
 
+	const inputBaseRef = getInput('base-ref');
+	if (inputBaseRef) {
+		console.log(`Setting base ref to: "${inputBaseRef}"`);
+		baseRef = inputBaseRef;
+		// fallback base sha doesn't make sense if given an explicit base ref:
+		baseSha = null;
+	}
+
 	if (getInput('cwd')) process.chdir(getInput('cwd'));
 
 	const plugin = new SizePlugin({
@@ -76,15 +84,19 @@ async function run(octokit, context, token) {
 		console.log('successfully fetched base.ref');
 	} catch (e) {
 		console.log('fetching base.ref failed', e.message);
-		try {
-			await exec(`git fetch -n origin ${baseSha}`);
-			console.log('successfully fetched base.sha');
-		} catch (e) {
-			console.log('fetching base.sha failed', e.message);
+		if (baseSha === null) {
+			throw new Error('base.ref fetch failed and no base.sha as fallback');
+		} else {
 			try {
-				await exec(`git fetch -n`);
+				await exec(`git fetch -n origin ${baseSha}`);
+				console.log('successfully fetched base.sha');
 			} catch (e) {
-				console.log('fetch failed', e.message);
+				console.log('fetching base.sha failed', e.message);
+				try {
+					await exec(`git fetch -n`);
+				} catch (e) {
+					console.log('fetch failed', e.message);
+				}
 			}
 		}
 	}
@@ -101,6 +113,7 @@ async function run(octokit, context, token) {
 		if (!baseRef) throw Error('missing context.payload.base.ref');
 		await exec(`git reset --hard ${baseRef}`);
 	} catch (e) {
+		if (!baseSha) throw e;
 		await exec(`git reset --hard ${baseSha}`);
 	}
 	endGroup();