需求价格弹性（PEoD）=（需求数量变化百分比）÷（价格变化百分比）该公式将给定需求量化为所需商品数量的百分比变化除以其价格变化百分比。例如，如果产品是阿司匹林，这种产品可以从许多不同的制造商那里获得，那么一个制造商的价格变化很小，比如增加5％，可能会对产品的需求产生很大的影响。让我们假设需求减少了20％，即-20％。将减少的需求（-20％）除以增加的价格（+ 5％）得到-4的结果。阿司匹林需求的价格弹性很高 – 价格的微小差异导致需求大幅减少。您可以通过观察它表达两个变量（需求和价格）之间的关系来推广公式。类似的公式表达了另一种关系，即给定产品的需求与消费者收入之间的关系。另一方面，在同一次经济衰退中，我们可能会发现家庭收入下降7％导致婴儿配方奶粉销售额下降3％。在这种情况下的计算是3÷7或约0.43。例如，在经济衰退中，美国家庭收入可能会下降7％，但用于外出就餐的家庭资金可能会下降12％。在这种情况下，需求的收入弹性计算为12÷7或约1.7。换句话说，收入的适度下降会导致需求下降。
您可以从中得出的结论是，在餐馆外出就餐不是美国家庭必不可少的经济活动 – 需求弹性为1.7，远大于1.0 – 但购买婴儿配方奶粉，需求收入弹性为0.43 ，相对重要，即使收入下降，需求也会持续存在。 需求的收入弹性用于了解对商品的需求对收入变化的敏感程度。收入弹性越高，对收益变化的需求就越敏感。非常高收入的弹性表明，当消费者的收入增加时，消费者会购买更多的这种商品，相反，当收入下降时，消费者会将购买的商品减少到更大的程度。极低的价格弹性意味着恰恰相反，消费者收入的变化对需求几乎没有影响。
Price Elasticity of Demand (PEoD) = (% Change in Quantity Demanded) ÷ (% Change in Price) The formula quantifies the demand for a given as the percentage change in the quantity of the good demanded divided by the percentage change in its price. If the product, for example, is aspirin, which is widely available from many different manufacturers, a small change in one manufacturer’s price, let’s say a 5 percent increase, might make a big difference in the demand for the product. Let’s suppose that the decreased demand was a minus 20 percent, or -20%. Dividing the decreased demand (-20%) by the increased price (+5 percent) gives a result of -4. The price elasticity of demand for aspirin is high — a small difference in price produces a significant decrease in demand. You can generalize the formula by observing that it expresses the relationship between two variables, demand and price. A similar formula expresses another relationship, that between the demand for a given product and consumer income. In the same recession, on the other hand, we might discover that the 7 percent drop in household income produced only a 3 percent drop in baby formula sales. The calculation in this instance is 3 ÷ 7 or about 0.43. In an economic recession, for example, U.S. household income might drop by 7 percent, but the household money spent on eating out might drop by 12 percent. In this case, the income elasticity of demand is calculated as 12 ÷ 7 or about 1.7. In other words, a moderate drop in income produces a greater drop in demand. What you can conclude from this is that eating out in restaurants is not an essential economic activity for U.S. households — the elasticity of demand is 1.7, considerably great than 1.0 — but that buying baby formula, with an income elasticity of demand of 0.43, is relatively essential and that demand will persist even when income drops.
Income elasticity of demand is used to see how sensitive the demand for a good is to an income change. The higher the income elasticity, the more sensitive demand for a good is to income changes. A very high-income elasticity suggests that when a consumer’s income goes up, consumers will buy a great deal more of that good and, conversely, that when income goes down consumers will cut back their purchases of that good to an even greater degree. A very low price elasticity implies just the opposite, that changes in a consumer’s income have little influence on demand.