Marketers frequently need to estimate the size of their markets -- both for existing products so that sales forecasts can be developed, and for new products so that market opportunities can be assessed. This toolkit enables students to size a market and generate a sales forecast using a market build-up methodology. Students learn to measure market demand and company demand and calculate market and product penetration rates and market share. The note gives students a foundation for analyzing marketing cases, as well as providing an analytical structure and process for completing a marketing plan. The note is accompanied by a free Excel worksheet (available only to authorized faculty) which contains sample problems, pre-built Excel models to calculate market size, market penetration, and market share, and charts and graphs which help visualize the results.