简短的回答
按 8.25% 的税率计算，100 美元的商品含税后售价为 108.25 美元。向后推算，108.25 美元的总额按 8.25% 分解为税前 100.00 美元和税后 8.25 美元。如果您支付 108.25 美元购买价格为 100 美元的商品，则有效税率为 8.25%。所有三种模式都描述相同的关系 - 您只需要知道您已经拥有哪两个数字即可。
要点
- 加税乘数：总计=价格×（1+税率）。提取税收划分：税前价格 = 总额 ÷ (1 + 税率) — 除以税率本身，而不是 (1 + 税率) 因素，是人们最常犯的错误。
- Sales tax rates in the US stack: a state rate plus city and county add-ons combine into one "combined" or "effective" rate, which is what this calculator's rate field expects.
- 并非每个州都征收销售税——特拉华州、蒙大拿州、新罕布什尔州和俄勒冈州没有全州销售税，因此 0% 的税率在这些州是合法的。
- 美国销售税在最终零售销售时征收一次——与增值税不同，增值税在每个生产阶段征收，但最终由最终消费者承担。
三个公式并列
加税：总计 = 100 美元 × (1 + 0.0825) = 108.25 美元
提取税：税前 = $108.25 ÷ (1 + 0.0825) = $100.00
查找费率：费率 = ($108.25 − $100.00) ÷ $100.00 = 8.25%
All three modes work off the same relationship between price, tax, and total — they just start from different known values. Use "Add Tax" when you know the sticker price and the local rate and want the final total. Use "Extract Tax" when a receipt or invoice only shows a tax-included total and you need to know how much was pre-tax versus tax (common for expense reports or accounting). Use "Find Rate" when you have both the before-tax price and the total and want to confirm or discover the rate that was actually applied — useful for checking a receipt against the rate your locality is supposed to charge.
最常见的提取错误
一个常见的错误是直接从总额中减去税率，而不是除以（1 + 税率）。总额为 108.25 美元，利率为 8.25%：
错误：108.25 美元 × (1 − 0.0825) = 99.32 美元（低估了真实的税前价格）
右：$108.25 ÷ (1 + 0.0825) = $100.00（正确的税前价格）
The tax rate applies to the pre-tax price, not the tax-included total, so simply knocking a percentage off the total overcorrects and understates the real pre-tax amount. This is exactly the calculation this calculator's "Extract Tax" mode automates correctly, so it's a good way to double-check manual math on a receipt or invoice.