Inventory quantities are a lot like basic FIFO accounting. However, when you step into rentals, you’ve got another thing coming. The rental business depends upon accurate availability calculations. Unlike basic stock, where you can keep a tally of what’s in stock, you have to be able to predict the what’s going to be available, for a set amount of time, between two points of time in the future. Worse yet, rental companies tend to be very dependant on custom business processes, so you can’t just keep a simple shipment registry. So, how do you calculate these numbers on the fly? Well, here’s how we do it.