You have a promising idea for a new product — maybe a better energy bar, a clever kitchen gadget, or a mobile app. Before anyone invests serious money, you need a sensible guess of how many people will buy it. But since the product does not exist yet, you cannot look at past sales. This chapter shows you how to build a demand forecast from scratch, using a few simple assumptions about how customers go from never hearing of you to buying regularly. You will also learn a simple way to test those assumptions, so you can make smarter choices even when many numbers feel uncertain.