The composition of regular conditioners varies significantly, depending on the brand, so it’s difficult to generalize that a product must always be used as a regular conditioner or a leave-in conditioner. In general, it’s not a problem to use a regular conditioner as a leave-in conditioner, but not necessarily the efficacy on hair will be the same.
Most regular conditioners are designed to be rinse-off products only, meaning that they are not tested for leave-in applications. Although it’s true that regular conditioners and leave-in conditioners have many ingredients in common, the amount of the ingredients Inside Hair Products are quite different. And it impacts on the performance on hair. For example, a regular conditioner can leave your hair heavy and with low manageability if applied as a leave-in conditioner. Also, leave-in conditioners may contain ingredients in higher concentrations that perform better when in contact with hair for a long time.
Based on the explanation above, it’s easy to understand that adding water to a regular conditioner does not turn that into a leave-in conditioner. In fact, if you want to use a regular conditioner as a leave-in conditioner, adding water to the regular product is not recommended. Doing that, you’re changing the composition of your regular conditioner (now you have a diluted version, that has an efficacy different – and in general worse – than the original version). But more important is that cosmetic products contain ingredients that guarantee the formula preservation, protecting against microorganism growth in your product. To understand more about the different steps involved in the creation of a cosmetic product, you can check this Free Ultimate Hair Care Product Creation Checklist.
Summarizing, if you want to use a regular conditioner as a leave-in conditioner, apply the regular conditioner as is (do not add water or change its composition). The good side is that you know already the performance of your regular conditioner. So, if you apply that as a leave-in conditioner you can directly compare the performance. Maybe you will discover a new application for your regular conditioner!