The answer to 'my bathtub smells bad' is that a smelly bathtub is usually caused by a buildup of hair, soap scum, and bacteria in the drain or overflow. This can lead to unpleasant odors, especially if the tub isn't used regularly. The good news is that this is a common problem with some easy solutions, though you may need to call a professional plumber in some cases.
Some of the most common causes of a smelly bathtub include a clogged drain, a dirty overflow drain, mold or mildew growth, and stagnant water. You can often fix these issues yourself with some basic cleaning and maintenance. Start by removing any visible debris from the drain, then use a drain cleaner or baking soda and vinegar to help break down the buildup. You may also need to clean the overflow drain if it's collecting gunk. If the smell persists, it could be due to mold or mildew growth, which will require more thorough cleaning and possibly sealing any cracks or gaps.