关键在于：下跌和上涨，用来计算百分比的基数变了。
小案例：假设某只股票买入价为100元，后来跌到80元。少了20元，相对于原来的100元，跌幅就是20÷100＝20%。
要从80元回到100元，需要再涨20元。这次的起点是80元，所以所需涨幅是20÷80＝25%。如果只上涨20%，结果是80×1.20＝96元，离原价还差4元。
可以记住这个公式：
回本所需涨幅＝（原买入价÷当前价−1）×100%。
代入案例：（100÷80−1）×100%＝25%。这里的“回本”，仅指股价回到原买入价，假设当前价大于0、持股数量不变，并忽略交易税费和分红。实际投资盈亏还需计入相关成本和收入。
同样的涨跌百分比，计算基数可能不同，不能直接相加抵消。看价格变化时，先问一句：这个百分比，是相对于哪个价格算的？
最后提醒：这只是算术关系，不是反弹预测。跌了20%，并不意味着随后一定会涨25%，也不保证能回到原价。本文仅作投资知识教育，不构成买卖建议。
参考资料
English version
Why Does a 20% Fall Require a 25% Rise to Break Even?
Illustration: An educational card in cream and dark green. A 20% fall takes 100 yuan down to 80 yuan, and a subsequent 25% rise is needed to return to 100 yuan. The formula is (100 ÷ 80 − 1) × 100% = 25%.
智灵 | Daily Stock Investing Tips: A fall from 100 yuan to 80 yuan is a decline of 20%; returning from 80 yuan to 100 yuan requires a rise of 25%. Hypothetical example, excluding taxes, fees, and dividends.
The key is that the base used to calculate the percentage changes between the fall and the rise.
A small example: suppose you buy a stock at 100 yuan, and it later falls to 80 yuan. It has lost 20 yuan. Relative to the original 100 yuan, the decline is 20 ÷ 100 = 20%.
To return from 80 yuan to 100 yuan, it needs to rise by 20 yuan. This time, the starting point is 80 yuan, so the required gain is 20 ÷ 80 = 25%. If it rises by only 20%, the result is 80 × 1.20 = 96 yuan, still 4 yuan below the original price.
You can remember this formula:
Percentage gain needed to break even = (Original purchase price ÷ Current price − 1) × 100%.
Using the example: (100 ÷ 80 − 1) × 100% = 25%. Here, “breaking even” means only that the share price returns to the original purchase price, assuming the current price is greater than 0, the number of shares held is unchanged, and trading taxes, fees, and dividends are ignored. Actual investment gains and losses must also account for the relevant costs and income.
The same percentage rise and fall may be calculated from different bases, so they cannot simply be added together to cancel each other out. When looking at a price change, first ask: which price is this percentage calculated against?
A final reminder: this is only an arithmetic relationship, not a prediction of a rebound. A fall of 20% does not mean a rise of 25% will necessarily follow, nor does it guarantee a return to the original price. This article is for investment education only and does not constitute advice to buy or sell.