When it comes time to replace your bathtub, you aren't going to be able to do any shopping until you know the size of the tub you are trying to replace. While this process may seem simple at first, you have to remember that many bathtubs are partially hidden under surrounds. A true bathtub measurement will come from measuring the exact space where the bathtub will fit, which may require a little guesswork on your part. Luckily, almost all bathtubs follow a fairly small set of standard measurements, so if you can get within an inch of one of those you should be set.
Stand in front of the bathtub, like you were about to get in. If your are measuring a bathtub with multiple entrance points, like a corner unit, simple pick one side.
Measure the distance from the left side of the bathtub to the right. If your bathtub is a stand-alone unit, like a cast iron tub, measure down the middle of the tub so you are measuring the two furthest points. If your bathtub is enclosed in an alcove measure the distance from one end of the alcove to the other. If your bathtub has a surround, which is just a layer of material surrounding three sides of the tub, add at least an inch to your measurement for each wall you encounter. To get an exact measurement you should measure from one wall stud to the opposite wall stud, but that would require demolishing the surround, so one inch is a fair approximation. Write down this measurement as the length.
Repeat the measuring process going from the back wall, or back of the tub, to the front. Make the same additions to the measurements as you did before. Write down this measurement as the width.
Measure the height of the bath tub from the floor of your bathroom to the top of the bathtub. Do not make this measurement on the inside of the bathtub. Record this as the height.
Make minor adjustments if your bathtub is close to one of these standard measurements: 32-by-60, 30-by-60, 27-by-54, 60-by-60, 42-by-72, 42-by-60, 32-by-60. Not every bathtub will fall into these dimensions, but if you measured 32.5-by-60.25, it is very likely that your tub is really 32-by-60.