diff --git a/common/sphinx.c b/common/sphinx.c
index ba730edbd..5163c3188 100644
--- a/common/sphinx.c
+++ b/common/sphinx.c
@@ -83,7 +83,7 @@ static size_t sphinx_hop_size(const struct sphinx_hop *hop)
 	return tal_bytelen(hop->raw_payload) + HMAC_SIZE;
 }
 
-static size_t sphinx_path_payloads_size(const struct sphinx_path *path)
+size_t sphinx_path_payloads_size(const struct sphinx_path *path)
 {
 	size_t size = 0;
 	for (size_t i=0; i<tal_count(path->hops); i++)
diff --git a/common/sphinx.h b/common/sphinx.h
index 30078b929..945dffe32 100644
--- a/common/sphinx.h
+++ b/common/sphinx.h
@@ -221,4 +221,9 @@ struct sphinx_path *sphinx_path_new_with_key(const tal_t *ctx,
 void sphinx_add_hop(struct sphinx_path *path, const struct pubkey *pubkey,
 		    const u8 *payload TAKES);
 
+/**
+ * Compute the size of the serialized payloads.
+ */
+size_t sphinx_path_payloads_size(const struct sphinx_path *path);
+
 #endif /* LIGHTNING_COMMON_SPHINX_H */