The basis of determining which stocks are the biggest advancers or biggest decliners is based on the percent movement in stock price. For the purposes of this article, we're going to focus on the biggest losers or percentage decliners. The securities seeing a substantial decline in price are the biggest stock losers.
The formula for calculating a percentage loss—in terms of daily performance—is as follows:
(Current stock price – Previous closing price) / Previous closing price = Percentage loss
So, a stock that is currently trading at $25 after a previous day’s close of $28 would be showing a percentage loss of -10.7%.