How to Calculate a Stock Split: A Step-by-Step Guide

What a stock split actually does

A stock split is the company cutting each share into pieces. You own 100 shares at $120.00, the company announces a 2-for-1 split, and the next morning you own 200 shares at $60.00. Nothing was bought or sold: the company is the same, and your $12,000.00 is still $12,000.00. The split changes two numbers — the share count and the price — and leaves two alone — the total value and the gain. That is the whole idea of this guide.

Step 1: Find the split factor

The split factor is the first number of the split ratio divided by the second: split factor = a / b. A 2-for-1 split has a factor of 2, a 4-for-1 split a factor of 4, a 3-for-2 split 1.5, and a 1-for-2 reverse split 0.5. Here the split is 2-for-1, so the factor is 2: the share count multiplies by it, and the price divides by it.

Step 2: Count the new shares

Multiply your shares by the split factor: new shares = shares owned × split factor. Here 100 × 2 = 200 shares. The count scales: 50 shares become 100, and 250 become 500. A 3-for-1 split would give 300 shares, a 4-for-1 split 400, and a 3-for-2 split 150. A reverse split runs the same formula backwards: a 1-for-2 reverse split gives 50 shares, and a 1-for-10 reverse split gives 10.

Step 3: Work out the new price

Divide the old price by the split factor: new price = price before / split factor. Here $120.00 / 2 = $60.00. A 3-for-1 split would give $40.00, a 4-for-1 split $30.00, and a 3-for-2 split $80.00. A 1-for-2 reverse split would give $240.00, and a 1-for-10 reverse split $1,200.00. The price always moves opposite the share count, by the same factor — that keeps the value still.

Step 4: Check the total value

Multiply the new shares by the new price: total value after = new shares × new price. Here 200 × $60.00 = $12,000.00 — exactly what the holding was worth before, because 100 × $120.00 is also $12,000.00. The value after equals the value before for every split, because (N × a / b) × (P × b / a) = N × P.

Step 5: Adjust the cost basis

Divide your cost basis per share by the split factor: cost basis after = cost basis before / split factor. Here $80.00 / 2 = $40.00 a share. Your total cost basis does not change: 100 × $80.00 = $8,000.00 before, and 200 × $40.00 = $8,000.00 after. A 3-for-1 split gives $26.67 (exact 26.6667), and a 3-for-2 split gives $53.33 (exact 53.3333).

Step 6: Check the gain

Subtract the cost basis from the price and multiply by the shares: unrealized gain = shares owned × (price before − cost basis per share). Here 100 × ($120.00 − $80.00) = $4,000.00. The gain is the same before and after, because the price and the basis move together: after the split it is 200 × ($60.00 − $40.00), also $4,000.00. If your basis is above the price, the holding is at a loss — with a $150.00 basis the gain is -$3,000.00 — and the split does not fix it. If the basis equals the price ($120.00), the gain is $0.00.

Step 7: Check the rounding

Most splits divide evenly, but some do not: a 3-for-1 split gives a basis of $26.6667 (rounds to $26.67), and a 3-for-2 split $53.3333 (rounds to $53.33). Rounding convention: every figure here is rounded from the exact unrounded value — to the nearest cent, or to the nearest ten-thousandth for the exact basis — and totals are computed from unrounded values.

The worked example

Shares owned 100; price $120.00; split 2-for-1; cost basis $80.00. Split factor 2; new shares 200; new price $60.00; total value $12,000.00 before and after; cost basis per share after $40.00; total cost basis $8,000.00; unrealized gain $4,000.00. Every figure matches the calculator.

Working backwards

To hold 500 shares after a 2-for-1 split, own 250 shares before it (exact 250; 250 × 2 = 500, while 249 gives 498). To get a $30.00 share price, the ratio must be 4-for-1 (exact 4; $120.00 / 4 = $30.00). To hold $15,000.00 of stock, the price before must be $150.00 (exact $150.0000; 100 × $150.00 = $15,000.00). To have a $5,000.00 gain, the cost basis must be $70.00 (exact $70.0000; 100 × ($120.00 − $70.00) = $5,000.00). To hold $20,000.00 of stock at $120.00 a share, own 167 shares (exact 166.6667; 167 × $120.00 = $20,040.00, while 166 gives $19,920.00).

What changes the answer most

The price before the split moves the value most: $200.00 → value $20,000.00 (gain $12,000.00); $90.00 → value $9,000.00 (gain $1,000.00). The share count is next: 250 shares → value $30,000.00; 50 shares → value $6,000.00. The cost basis is third: $50.00 → gain $7,000.00; $120.00 → gain $0.00; $150.00 → gain -$3,000.00. The split ratio moves nothing but the count and the price: 3-for-1 → 300 shares at $40.00; 4-for-1 → 400 at $30.00; 3-for-2 → 150 at $80.00; 1-for-2 reverse → 50 at $240.00; 1-for-10 reverse → 10 at $1,200.00. The value is $12,000.00 and the gain is $4,000.00 in every one of them.

What not to do

Do not count the new shares as new money — 200 shares at $60.00 are worth the same $12,000.00 as 100 at $120.00. Do not forget to divide the cost basis: entering $80.00 a share after a 2-for-1 split overstates your basis by $40.00 a share. Do not assume a reverse split fixed a loss — it leaves the -$3,000.00 loss exactly where it was.

How this connects

A split is one way a holding changes shape without changing value. If the shares came from an ESPP, the ESPP calculator works out the discount and the tax on them — and a split after purchase divides the ESPP purchase price the same way it divides any other basis. And the RSU calculator works out the tax on restricted stock the same way.