Whether you work for a startup or a large company, there have never been so many metrics to help you understand how your business is doing. But I would argue that one metric rules them all: the net promoter score (NPS).

NPS represents the willingness of consumers to recommend your product to someone they know. For most businesses, especially startups and small businesses with tiny marketing budgets, NPS is crucial. Why? Because word of mouth is their most important marketing channel.

Word-of-mouth fundamentally comes from the willingness of customers to recommend your business, which is what NPS measures. According to a survey conducted by Verizon and Small Business Trends, 85 percent of small-business owners credit word of mouth as the main avenue for customers to find out about them.

If you have a low NPS, positive word of mouth eludes your business. In fact, potential customers might hear or read instead: “Stay away from Bruno’s Beefeaters. You don’t want to know what I think I found in my last hot dog....”

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