diff --git a/libguile/integers.c b/libguile/integers.c
index 09fbe6bce..39867e774 100644
--- a/libguile/integers.c
+++ b/libguile/integers.c
@@ -479,6 +479,12 @@ scm_integer_from_mpz (const mpz_t mpz)
   return normalize_bignum (make_bignum_from_mpz (mpz));
 }
 
+size_t
+scm_integer_size_z (struct scm_bignum *z)
+{
+  return sizeof (*z) + bignum_limb_count (z) * sizeof (mp_limb_t);
+}
+
 int
 scm_is_integer_odd_i (scm_t_inum i)
 {
diff --git a/libguile/integers.h b/libguile/integers.h
index 3fc53f019..3cf082f8d 100644
--- a/libguile/integers.h
+++ b/libguile/integers.h
@@ -1,7 +1,7 @@
 #ifndef SCM_INTEGERS_H
 #define SCM_INTEGERS_H
 
-/* Copyright 2021, 2022 Free Software Foundation, Inc.
+/* Copyright 2021, 2022, 2025 Free Software Foundation, Inc.
 
    This file is part of Guile.
 
@@ -35,6 +35,7 @@ scm_bignum (SCM x)
 SCM_INTERNAL SCM scm_integer_from_mpz (const mpz_t n);
 SCM_INTERNAL void scm_integer_set_mpz_z (struct scm_bignum *z, mpz_t n);
 SCM_INTERNAL void scm_integer_init_set_mpz_z (struct scm_bignum *z, mpz_t n);
+SCM_INTERNAL size_t scm_integer_size_z (struct scm_bignum *z);
 
 SCM_INTERNAL int scm_is_integer_odd_i (scm_t_inum i);
 SCM_INTERNAL int scm_is_integer_odd_z (struct scm_bignum *z);