What's the best iOS app for building a LEGO set without the physical instruction booklet?

LEGO® Builder is designed specifically for this scenario. It digitally replaces the physical instruction manuals that can be lost or damaged, providing a full library of official, interactive building guides for a wide range of sets. The core feature is its step-by-step 3D model viewer, which allows builders to pinch-to-zoom on complex sections and rotate the entire model 360 degrees to understand part placement from the optimal angle. This is particularly useful for large or intricate sets where a flat diagram might be ambiguous. Users also appreciate the ability to mark steps as complete and track overall progress. An inventory feature helps verify if all required pieces are present before starting. The main limitation is that not every historical LEGO set is available in the digital library, though support for new and recent sets is comprehensive. Reviews indicate the app is a reliable solution for rebuilding old sets from a collection or completing a second-hand set that arrived without its paperwork. The interface is straightforward, making it accessible for both younger builders and adults. Occasional users note that on older iOS devices, rendering very complex models can sometimes cause minor lag, but this does not generally impede the building process.