diff --git a/client/rpc/src/state/mod.rs b/client/rpc/src/state/mod.rs
index ded87c73dc8f7..01c7c5f1eb40c 100644
--- a/client/rpc/src/state/mod.rs
+++ b/client/rpc/src/state/mod.rs
@@ -97,14 +97,14 @@ pub trait StateBackend<Block: BlockT, Client>: Send + Sync + 'static
 	) -> FutureResult<Option<Block::Hash>>;
 
 	/// Returns the size of a storage entry at a block's state.
+	///
+	/// If data is available at `key`, it is returned. Else, the sum of values who's key has `key`
+	/// prefix is returned, i.e. all the storage (double) maps that have this prefix.
 	fn storage_size(
 		&self,
 		block: Option<Block::Hash>,
 		key: StorageKey,
-	) -> FutureResult<Option<u64>> {
-		Box::new(self.storage(block, key)
-			.map(|x| x.map(|x| x.0.len() as u64)))
-	}
+	) -> FutureResult<Option<u64>>;
 
 	/// Returns the runtime metadata as an opaque blob.
 	fn metadata(&self, block: Option<Block::Hash>) -> FutureResult<Bytes>;
diff --git a/client/rpc/src/state/state_full.rs b/client/rpc/src/state/state_full.rs
index 3a5580c539a4c..fda73cea27110 100644
--- a/client/rpc/src/state/state_full.rs
+++ b/client/rpc/src/state/state_full.rs
@@ -298,6 +298,36 @@ impl<BE, Block, Client> StateBackend<Block, Client> for FullState<BE, Block, Cli
 				.map_err(client_err)))
 	}
 
+	fn storage_size(
+		&self,
+		block: Option<Block::Hash>,
+		key: StorageKey,
+	) -> FutureResult<Option<u64>> {
+		let block = match self.block_or_best(block) {
+			Ok(b) => b,
+			Err(e) => return Box::new(result(Err(client_err(e)))),
+		};
+
+		match self.client.storage(&BlockId::Hash(block), &key) {
+			Ok(Some(d)) => return Box::new(result(Ok(Some(d.0.len() as u64)))),
+			Err(e) => return Box::new(result(Err(client_err(e)))),
+			Ok(None) => {},
+		}
+
+		Box::new(result(
+			self.client.storage_pairs(&BlockId::Hash(block), &key)
+				.map(|kv| {
+					let item_sum = kv.iter().map(|(_, v)| v.0.len() as u64).sum::<u64>();
+					if item_sum > 0 {
+						Some(item_sum)
+					} else {
+						None
+					}
+				})
+				.map_err(client_err)
+		))
+	}
+
 	fn storage_hash(
 		&self,
 		block: Option<Block::Hash>,
diff --git a/client/rpc/src/state/state_light.rs b/client/rpc/src/state/state_light.rs
index 01fd1c16018ff..8f4dce08b3fb6 100644
--- a/client/rpc/src/state/state_light.rs
+++ b/client/rpc/src/state/state_light.rs
@@ -214,6 +214,14 @@ impl<Block, F, Client> StateBackend<Block, Client> for LightState<Block, F, Clie
 		Box::new(result(Err(client_err(ClientError::NotAvailableOnLightClient))))
 	}
 
+	fn storage_size(
+		&self,
+		_: Option<Block::Hash>,
+		_: StorageKey,
+	) -> FutureResult<Option<u64>> {
+		Box::new(result(Err(client_err(ClientError::NotAvailableOnLightClient))))
+	}
+
 	fn storage(
 		&self,
 		block: Option<Block::Hash>,
diff --git a/client/rpc/src/state/tests.rs b/client/rpc/src/state/tests.rs
index 0cc16ce8d5e92..b6677a1f2ffb4 100644
--- a/client/rpc/src/state/tests.rs
+++ b/client/rpc/src/state/tests.rs
@@ -53,6 +53,9 @@ fn should_return_storage() {
 	let client = TestClientBuilder::new()
 		.add_extra_storage(KEY.to_vec(), VALUE.to_vec())
 		.add_extra_child_storage(&child_info, KEY.to_vec(), CHILD_VALUE.to_vec())
+		// similar to a map with two keys
+		.add_extra_storage(b":map:acc1".to_vec(), vec![1, 2])
+		.add_extra_storage(b":map:acc2".to_vec(), vec![1, 2, 3])
 		.build();
 	let genesis_hash = client.genesis_hash();
 	let (client, child) = new_full(Arc::new(client), SubscriptionManager::new(Arc::new(TaskExecutor)));
@@ -72,6 +75,10 @@ fn should_return_storage() {
 		client.storage_size(key.clone(), None).wait().unwrap().unwrap() as usize,
 		VALUE.len(),
 	);
+	assert_eq!(
+		client.storage_size(StorageKey(b":map".to_vec()), None).wait().unwrap().unwrap() as usize,
+		2 + 3,
+	);
 	assert_eq!(
 		executor::block_on(
 			child.storage(prefixed_storage_key(), key, Some(genesis_hash).into())
@@ -80,7 +87,6 @@ fn should_return_storage() {
 		).unwrap().unwrap() as usize,
 		CHILD_VALUE.len(),
 	);
-
 }
 
 #[test]