diff --git a/include/NeuralNetwork/FeedForward/Layer.h b/include/NeuralNetwork/FeedForward/Layer.h
index b6a7a76..3c07750 100644
--- a/include/NeuralNetwork/FeedForward/Layer.h
+++ b/include/NeuralNetwork/FeedForward/Layer.h
@@ -72,7 +72,7 @@ namespace FeedForward {
 			/**
 			 * @returns Size of layer
 			 */
-			std::size_t size() {
+			std::size_t size() const {
 				return neurons.size();
 			}