diff --git a/services/backup/src/constants.rs b/services/backup/src/constants.rs
--- a/services/backup/src/constants.rs
+++ b/services/backup/src/constants.rs
@@ -4,6 +4,11 @@
 pub const MPSC_CHANNEL_BUFFER_CAPACITY: usize = 1;
 pub const ID_SEPARATOR: &str = ":";
 
+// 400KiB limit (in docs there is KB but they mean KiB) -
+// https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/ServiceQuotas.html
+// This includes both attribute names' and values' lengths
+pub const LOG_DATA_SIZE_DATABASE_LIMIT: usize = 1024 * 400;
+
 // Configuration defaults
 
 pub const DEFAULT_GRPC_SERVER_PORT: u64 = 50051;
diff --git a/services/backup/src/database.rs b/services/backup/src/database.rs
--- a/services/backup/src/database.rs
+++ b/services/backup/src/database.rs
@@ -50,6 +50,17 @@
   }
 }
 
+static LOG_ITEM_HEADERS_SIZE: usize = {
+  let mut size: usize = 0;
+  size += LOG_TABLE_FIELD_BACKUP_ID.as_bytes().len();
+  size += LOG_TABLE_FIELD_LOG_ID.as_bytes().len();
+  size += LOG_TABLE_FIELD_PERSISTED_IN_BLOB.as_bytes().len();
+  size += LOG_TABLE_FIELD_VALUE.as_bytes().len();
+  size += LOG_TABLE_FIELD_ATTACHMENT_HOLDERS.as_bytes().len();
+  size += LOG_TABLE_FIELD_DATA_HASH.as_bytes().len();
+  size
+};
+
 #[derive(Clone, Debug)]
 pub struct LogItem {
   pub backup_id: String,
@@ -60,6 +71,43 @@
   pub data_hash: String,
 }
 
+impl LogItem {
+  /// Calculates size based on raw log item components,
+  /// without allocating a new item
+  pub fn size_from_components(
+    backup_id: &str,
+    log_id: &str,
+    log_hash: &str,
+    data: &[u8],
+  ) -> usize {
+    let mut size: usize = LOG_ITEM_HEADERS_SIZE;
+    size += backup_id.as_bytes().len();
+    size += log_id.as_bytes().len();
+    size += data.len();
+    size += log_hash.as_bytes().len();
+
+    // persistent in blob, attachment holders, use defaults here
+    size += false.to_string().as_bytes().len();
+    size += "".as_bytes().len();
+
+    size
+  }
+
+  /// Total size of this item in the DynamoDB table. This value must be
+  /// smaller than [`crate::constants::LOG_DATA_SIZE_DATABASE_LIMIT`]
+  /// in order to successfully put this item into a DynamoDB database.
+  pub fn total_size(&self) -> usize {
+    let mut size: usize = LOG_ITEM_HEADERS_SIZE;
+    size += self.backup_id.as_bytes().len();
+    size += self.log_id.as_bytes().len();
+    size += self.persisted_in_blob.to_string().as_bytes().len();
+    size += self.value.as_bytes().len();
+    size += self.attachment_holders.as_bytes().len();
+    size += self.data_hash.as_bytes().len();
+    size
+  }
+}
+
 #[derive(Clone)]
 pub struct DatabaseClient {
   client: Arc<aws_sdk_dynamodb::Client>,