diff --git a/lib/os/spsc_pbuf.c b/lib/os/spsc_pbuf.c
index 967869fb2e8..afe8c206bf9 100644
--- a/lib/os/spsc_pbuf.c
+++ b/lib/os/spsc_pbuf.c
@@ -169,7 +169,7 @@ int spsc_pbuf_alloc(struct spsc_pbuf *pb, uint16_t len, char **buf)
 			/* Packet will fit at the end */
 			free_space = remaining - ((rd_idx > 0) ? 0 : sizeof(uint32_t));
 		} else {
-			if (rd_idx > remaining) {
+			if (rd_idx > space) {
 				/* Padding must be added. */
 				data_loc[wr_idx] = PADDING_MARK;
 				__sync_synchronize();
diff --git a/tests/lib/spsc_pbuf/src/main.c b/tests/lib/spsc_pbuf/src/main.c
index ba7a413a8a9..1e39ba94954 100644
--- a/tests/lib/spsc_pbuf/src/main.c
+++ b/tests/lib/spsc_pbuf/src/main.c
@@ -305,7 +305,7 @@ ZTEST(test_spsc_pbuf, test_0cpy_corner2)
 	pb = spsc_pbuf_init(buffer, sizeof(buffer), 0);
 	capacity = spsc_pbuf_capacity(pb);
 
-	/* Commit 5 byte packet. */
+	/* Commit 16 byte packet. */
 	len1 = 16;
 	PACKET_WRITE(pb, len1, len1, 0, len1);
 
@@ -349,10 +349,10 @@ ZTEST(test_spsc_pbuf, test_largest_alloc)
 	PACKET_WRITE(pb, len1, len1, 0, len1);
 	PACKET_CONSUME(pb, len1, 0);
 
-	len2 = capacity - ROUND_UP(len1, sizeof(uint32_t)) - 2 * sizeof(uint32_t) - 10;
+	len2 = capacity - TLEN(len1) - TLEN(10);
 	PACKET_WRITE(pb, len2, len2, 1, len2);
 
-	PACKET_WRITE(pb, SPSC_PBUF_MAX_LEN, 0, 1, 8);
+	PACKET_WRITE(pb, SPSC_PBUF_MAX_LEN, 0, 1, 12);
 
 	PACKET_WRITE(pb, SPSC_PBUF_MAX_LEN - 1, 0, 1, 12);
 
@@ -363,7 +363,7 @@ ZTEST(test_spsc_pbuf, test_largest_alloc)
 	PACKET_WRITE(pb, len1, len1, 0, len1);
 	PACKET_CONSUME(pb, len1, 0);
 
-	len2 = capacity - ROUND_UP(len1, sizeof(uint32_t)) - 2 * sizeof(uint16_t) - 8;
+	len2 = capacity - TLEN(len1) - TLEN(12);
 	PACKET_WRITE(pb, len2, len2, 1, len2);
 
 	PACKET_WRITE(pb, SPSC_PBUF_MAX_LEN - 1, 0, 1, 12);