diff --git a/include/osmocom/gsm/gsm0808_utils.h b/include/osmocom/gsm/gsm0808_utils.h
index 76db2b6..ccdf5ed 100644
--- a/include/osmocom/gsm/gsm0808_utils.h
+++ b/include/osmocom/gsm/gsm0808_utils.h
@@ -95,6 +95,7 @@
 int gsm0808_cell_id_to_cgi(struct osmo_cell_global_id *cgi, const struct gsm0808_cell_id *cid);
 void gsm0808_msgb_put_cell_id_u(struct msgb *msg, enum CELL_IDENT id_discr, const union gsm0808_cell_id_u *u);
 int gsm0808_decode_cell_id_u(union gsm0808_cell_id_u *out, enum CELL_IDENT discr, const uint8_t *buf, unsigned int len);
+int gsm0808_cell_id_size(enum CELL_IDENT discr);
 
 uint8_t gsm0808_enc_cause(struct msgb *msg, uint16_t cause);
 uint8_t gsm0808_enc_aoip_trasp_addr(struct msgb *msg,