The least glamorous thing I have built and possibly the one I use most. Live at
kvdrt.maqsudjon.com.
You type three numbers — width, height, and how far the grille projects from the wall — and it returns the front face plus all four side faces, which is the area you actually price.
The whole design problem was the input order. 120 130 30 has to mean width 120, height 130, projection 30, every single time, for someone holding a tape measure in one hand and a phone in the other. Making that impossible to misread took longer than the arithmetic did.