diff --git a/include/olm/account.hh b/include/olm/account.hh
index 7002f51..88302ba 100644
--- a/include/olm/account.hh
+++ b/include/olm/account.hh
@@ -139,7 +139,7 @@ struct Account {
         std::uint8_t const * random, std::size_t random_length
     );
 
-    /** Number of bytes needed to output the one time keys for this account */
+    /** Number of bytes needed to output the fallback keys for this account */
     std::size_t get_fallback_key_json_length() const;
 
     /** Deprecated: use get_unpublished_fallback_key_json instead */
@@ -147,6 +147,10 @@ struct Account {
         std::uint8_t * fallback_json, std::size_t fallback_json_length
     );
 
+    /** Number of bytes needed to output the unpublished fallback keys for this
+     * account */
+    std::size_t get_unpublished_fallback_key_json_length() const;
+
     /** Output the fallback key as JSON:
      *
      *  {"curve25519":
diff --git a/include/olm/olm.h b/include/olm/olm.h
index 0e98e77..cdb35a2 100644
--- a/include/olm/olm.h
+++ b/include/olm/olm.h
@@ -297,6 +297,12 @@ OLM_EXPORT size_t olm_account_fallback_key(
     void * fallback_key, size_t fallback_key_size
 );
 
+/** The number of bytes needed to hold the unpublished fallback key as returned
+ * by olm_account_unpublished fallback_key. */
+OLM_EXPORT size_t olm_account_unpublished_fallback_key_length(
+    OlmAccount const * account
+);
+
 /** Returns the fallback key (if present, and if unpublished) into the
  * fallback_key buffer */
 OLM_EXPORT size_t olm_account_unpublished_fallback_key(
diff --git a/src/account.cpp b/src/account.cpp
index cebd89f..80768ae 100644
--- a/src/account.cpp
+++ b/src/account.cpp
@@ -356,11 +356,25 @@ std::size_t olm::Account::get_fallback_key_json(
     return pos - fallback_json;
 }
 
+std::size_t olm::Account::get_unpublished_fallback_key_json_length(
+) const {
+    std::size_t length = 4 + sizeof(KEY_JSON_CURVE25519) - 1; /* {"curve25519":{}} */
+    const OneTimeKey & key = current_fallback_key;
+    if (num_fallback_keys >= 1 && !key.published) {
+        length += 1; /* " */
+        length += olm::encode_base64_length(_olm_pickle_uint32_length(key.id));
+        length += 3; /* ":" */
+        length += olm::encode_base64_length(sizeof(key.key.public_key));
+        length += 1; /* " */
+    }
+    return length;
+}
+
 std::size_t olm::Account::get_unpublished_fallback_key_json(
     std::uint8_t * fallback_json, std::size_t fallback_json_length
 ) {
     std::uint8_t * pos = fallback_json;
-    if (fallback_json_length < get_fallback_key_json_length()) {
+    if (fallback_json_length < get_unpublished_fallback_key_json_length()) {
         last_error = OlmErrorCode::OLM_OUTPUT_BUFFER_TOO_SMALL;
         return std::size_t(-1);
     }
diff --git a/src/olm.cpp b/src/olm.cpp
index 2f3e777..f7fbc42 100644
--- a/src/olm.cpp
+++ b/src/olm.cpp
@@ -495,6 +495,13 @@ size_t olm_account_fallback_key(
 }
 
 
+size_t olm_account_unpublished_fallback_key_length(
+    OlmAccount const * account
+) {
+    return from_c(account)->get_unpublished_fallback_key_json_length();
+}
+
+
 size_t olm_account_unpublished_fallback_key(
     OlmAccount * account,
     void * fallback_key_json, size_t fallback_key_json_length