Stock Split Adjuster
Back-adjust a price series for a forward or reverse stock split.
Overview
A stock split adjuster rewrites a price series so that pre-split prices are directly comparable to post-split prices. When a company splits its stock 2-for-1, every existing share becomes two shares at half the price; the market capitalization is unchanged but the historical chart suddenly drops by 50% if the prices are taken at face value. Reverse splits (1-for-10 is common for distressed micro-caps) do the opposite, pushing the apparent historical price up by ten times.
The adjuster scales pre-split prices so the entire history reads in current share-equivalent terms. This is essential for charting, backtesting, technical analysis, and computing returns across split events. Volumes go in the opposite direction — when shares split, the same dollar volume corresponds to more share volume, so historical share volumes must also be multiplied by the split ratio for consistency.
How it works
A split is expressed as a ratio new : old. For a 2-for-1 split, every old share becomes 2 new shares, so pre-split prices are divided by 2 and pre-split volumes are multiplied by 2. For a 1-for-10 reverse split, pre-split prices are multiplied by 10 and pre-split volumes divided by 10. The general rule for any split factor f = new / old: adjusted_price = original_price / f, adjusted_volume = original_volume × f. Multiple splits compound — apply the cumulative product of all post-date split factors to any given pre-event date.
Examples
- Apple's 4-for-1 split on Aug 31, 2020. A $500 close on Aug 28, 2020 becomes $125 on a split-adjusted chart. A 100-share lot becomes 400 shares; reported daily volume for prior dates multiplies by 4.
- Tesla's 3-for-1 split in 2022 after an earlier 5-for-1 in 2020. A 2019 price needs to be divided by 15 to compare against a 2023 quote.
- A 1-for-10 reverse split on a $0.20 penny stock raises the apparent price to $2.00; a previously held 1,000-share lot becomes 100 shares.
- An IPO 2 years ago at $20, then a 2-for-1 split, then a 3-for-1 split. The split-adjusted IPO price is
20 / 2 / 3 = $3.33.
FAQ
Why adjust at all?
Without adjustment, charts and indicators (moving averages, channel breaks) would generate false signals at the split date.
Do dividends require similar adjustment?
Total-return charts back-adjust for dividends as well, but ordinary price charts do not.
What about share-buyback effects?
Buybacks reduce share count but do not change the per-share price by definition, so no historical adjustment is needed.
How do data providers handle this?
Most reputable feeds publish adjusted-close prices alongside raw close. Always confirm which series you are pulling.
Does the split change my cost basis?
Per share, yes — the total dollar basis stays the same but spreads across more (or fewer) shares.