There is not a simple answer to this question. How to charge is a common issue among freelancers who work in creative fields such as design, music, writing or app building. To answer the question, you need to figure out how much your time is worth. Even if you charge by the project, you still need to ensure that you are being paid for how much your time is worth. How much you choose to charge also needs to make sense. You should research your industry and see what going rates are for your skill level. Since creative work often involves multiple versions, thinking time, and trial-and-error time, it may be best to configure your hourly rate and use that number to estimate your project rate. For example, if your time is worth about $40 per hour and you think the project will take about ten hours to complete, you may choose to set the project rate at $400. Your client doesn't need to know the actual time it took to complete the project. If your client insists on paying by the hour, make sure to charge a suitable hourly rate. You'll also want to ensure that the hours they want to pay you for are worth fitting into your schedule. A client who will pay in blocks of ten hours is more valuable than one who only wants to pay for a few hours per month. As you gain experience, you'll find that billing becomes easier.