We are taught that arithmetic should be precise. But precision is rarely a make-or-break for most decisions. Back-of-envelope math, or napkin math, is a tried and true method to quickly assess value. Having a good-enough estimate, and being able to assess values in real time helps in countless situations, from quickly summing a check, to determining the total addressable market for a new product line. Envelope Math can help you practice your estimation skills and number sense with numbers both large and small, so youre ready to evaluate the nitty gritty whenever you need to.