How to use the stock split calculator
- Enter the split ratio as two numbers, new shares for old, as announced: 4-for-1 is 4 and 1, 3-for-2 is 3 and 2, a 1-for-10 reverse split is 1 and 10.
- Enter the shares you hold before the split takes effect.
- Enter the share price before the split, normally the last close before the effective date.
- Add your cost basis per share if you want the adjusted figure for your tax records. It is optional.
When the numbers do not divide evenly, the results also show the fractional share and the cash you would receive for it.
How a split is calculated
Shares after = Shares before × Split ratio
Price after = Price before ÷ Split ratio
Cost basis per share after = Cost basis per share before ÷ Split ratioFor a 4-for-1 split the ratio is 4; for a 1-for-10 reverse split it is 0.1. Shares × price, and total cost basis, are the same before and after.
When the shares after are not a whole number, the fraction is either kept (common with forward splits at brokers that support fractional shares) or paid out in cash:
A worked example
You own 150 shares of a company trading at $680, bought at $412 a share. The company announces a 4-for-1 split.
After the split you hold 600 shares, and the price adjusts to $170.00. The position was worth $102,000.00 and is still worth $102,000.00.
Your cost basis per share becomes $103.00; the total basis stays at $61,800.00. When you eventually sell, your gain is measured against that adjusted figure.
Forward splits, odd ratios and reverse splits
The arithmetic is the same in every case; what changes is whether you end up with a fraction.
| Split | Shares before | Price before | Shares after | Price after | Cash in lieu |
|---|---|---|---|---|---|
| 4-for-1 | 150 shares | $680 | 600 | $170.00 | none |
| 3-for-2 | 75 shares | $90 | 112.5 | $60.00 | $30.00 if not kept |
| 1-for-10 reverse | 1,234 shares | $1.85 | 123.4 | $18.50 | $7.40 |
A 3-for-2 split turns an odd number of shares into a half share over, which many brokers now hold and some pay out.
In a 1-for-10 reverse split, every ten shares become one. 1,234 shares divide into 123 whole new shares with 0.4 of a share left over, which is typically sold for you. If your adjusted basis is $42.00 a share, the fraction carries $16.80 of basis, and the cash payment is compared with that figure to work out a small gain or loss.
A shareholder with very few shares can be cashed out completely. With 15 shares going through a 1-for-20 reverse split, you would end up with 0 whole shares and receive about $7.50 instead.
Why companies split their shares
A forward split makes each share cheaper without changing the company’s value. Companies usually cite affordability for small investors and trading liquidity; with fractional shares now widespread, that effect is smaller than it once was.
Earnings per share, dividends per share and the share price all divide by the same ratio, so valuation ratios such as the P/E do not change. Historical prices are restated on a split-adjusted basis.
A reverse split is common among companies whose price has fallen far, often to stay above an exchange’s minimum price. It neither helps nor harms holders, and it does not change the business behind the low price.
What this calculator does not include
- Price moves around the split. The “after” price is the mathematical equivalent of the last pre-split price, not a forecast.
- Broker-specific rounding. Brokers and transfer agents handle fractions differently, and the cash in lieu is based on the price they actually get.
- Multiple tax lots. If you bought at several prices, each lot is adjusted by the same ratio. Enter one lot at a time, or adjust your average cost with the stock average calculator.
- Spin-offs and stock dividends, which have their own basis rules.