Stock Split Calculator: Shares, Price & Cost Basis 2026

A stock split changes the share count and the price — never the value

A stock split sounds like free money: your 100 shares become 200 shares overnight, and the number in your account doubles. It is not free money, and it is not a loss either — it is a reshuffle. The company takes each share and cuts it into pieces, so you own more pieces of the same company. The price of each piece falls by the same factor the count rises by, and the total value of your holding does not move. The formula is short: new shares = shares owned × split factor; new price = price before ÷ split factor; total value before = shares owned × price before; total value after = new shares × new price — equal by construction; cost basis per share after = cost basis per share before ÷ split factor; total cost basis = shares owned × cost basis per share (unchanged); unrealized gain = shares owned × (price before − cost basis per share) (unchanged); split factor = a / b. On the defaults — 100 shares, $120.00 a share, a 2-for-1 split, $80.00 cost basis a share — the new shares are 200, the new price is $60.00, the split factor is 2, the total value is $12,000.00 before and $12,000.00 after, the cost basis per share after is $40.00, the total cost basis is $8,000.00, and the unrealized gain is $4,000.00.

At the defaults — shares owned 100, price $120.00, split 2-for-1, cost basis $80.00 — the new shares are 200, the new price is $60.00, the split factor is 2, the total value before is $12,000.00, the total value after is $12,000.00, the cost basis per share after is $40.00, the total cost basis is $8,000.00, and the unrealized gain is $4,000.00. Check the identities: new shares = N × a / b; new price = P × b / a; value before = N × P; value after = new shares × new price; new basis = B × b / a; total basis = N × B; gain = N × (P − B). The value after equals the value before for every split, because (N × a / b) × (P × b / a) = N × P — the a and the b cancel. A reverse split is the same formula with a < b: a 1-for-2 reverse split halves the shares and doubles the price, and the value still does not move. Rounding convention: totals are computed from unrounded values, so rounded parts may differ by a cent from their rounded sum.

The worked example, line by line

Setup: shares owned 100, price $120.00, split 2-for-1, cost basis $80.00.

Line 1 — the split factor. 2 / 1 = 2.

Line 2 — the new shares. 100 × 2 = 200 shares.

Line 3 — the new price. $120.00 / 2 = $60.00.

Line 4 — the total value before. 100 × $120.00 = $12,000.00.

Line 5 — the total value after. 200 × $60.00 = $12,000.00 — the same as before, by construction.

Line 6 — the cost basis per share after. $80.00 / 2 = $40.00.

Line 7 — the total cost basis. 100 × $80.00 = $8,000.00 — unchanged, because the basis per share halves and the share count doubles.

Line 8 — the unrealized gain. 100 × ($120.00 − $80.00) = $4,000.00 — unchanged, because the price and the basis move together.

Reverse: the shares owned for 500 post-split shares are 250 shares (exact 250); at 250 shares the post-split shares are 500, at 249 they are 498 — 250 is the nearest. The split ratio for a $30.00 post-split price is 4-for-1 (exact 4); at 4-for-1 the new price is $30.00, at 3-for-1 it is $40.00 — 4-for-1 is the nearest. The pre-split price for a $15,000.00 total value is $150.00 (exact $150.0000); at $150.00 the value is $15,000.00, at $149.99 it is $14,999.00 — $150.00 is the nearest. The cost basis for a $5,000.00 unrealized gain is $70.00 (exact $70.0000); at $70.00 the gain is $5,000.00, at $69.99 it is $5,001.00 — $70.00 is the nearest. The shares owned for a $20,000.00 total value are 167 shares (exact 166.6667); at 167 shares the value is $20,040.00, at 166 it is $19,920.00 — 167 is the nearest.

Every branch, checked

Shares owned 50: new shares 100, new price $60.00, value $6,000.00, new basis $40.00, total basis $4,000.00, gain $2,000.00. Half the shares, half the money — the split scales exactly with the holding.

Shares owned 250: new shares 500, new price $60.00, value $30,000.00, new basis $40.00, total basis $20,000.00, gain $10,000.00. Two and a half times the shares, two and a half times the value — the price and the basis do not move.

Price $90: new shares 200, new price $45.00, value $9,000.00, new basis $40.00, total basis $8,000.00, gain $1,000.00. A lower price before the split means a lower price after it — and a smaller gain, because the basis did not move.

Price $200: new shares 200, new price $100.00, value $20,000.00, new basis $40.00, total basis $8,000.00, gain $12,000.00. A higher price means a bigger gain — the split itself adds nothing.

3-for-1 split: new shares 300, new price $40.00, value $12,000.00, new basis $26.67 (exact 26.6667), total basis $8,000.00, gain $4,000.00. The basis per share is a third of $80.00 — $26.67, not $26.66, because 80 / 3 = 26.6667 rounds up.

4-for-1 split: new shares 400, new price $30.00, value $12,000.00, new basis $20.00, total basis $8,000.00, gain $4,000.00. Four times the shares at a quarter of the price — the value and the gain are untouched.

3-for-2 split: new shares 150, new price $80.00, value $12,000.00, new basis $53.33 (exact 53.3333), total basis $8,000.00, gain $4,000.00. An uneven split still leaves the value alone — the basis per share is $80.00 × 2 / 3 = 53.3333, which rounds to $53.33.

1-for-2 reverse split: new shares 50, new price $240.00, value $12,000.00, new basis $160.00, total basis $8,000.00, gain $4,000.00. A reverse split runs the same formula backwards: half the shares at twice the price, same $12,000.00.

1-for-10 reverse split: new shares 10, new price $1,200.00, value $12,000.00, new basis $800.00, total basis $8,000.00, gain $4,000.00. Ten shares at $1,200.00 are worth exactly what a hundred at $120.00 were.

Cost basis $50: new shares 200, new price $60.00, value $12,000.00, new basis $25.00, total basis $5,000.00, gain $7,000.00. A lower basis means a bigger gain — the split does not change either one.

Cost basis $120: new shares 200, new price $60.00, value $12,000.00, new basis $60.00, total basis $12,000.00, gain $0.00. When the basis equals the price, there is no gain — and no loss — to carry through the split.

Cost basis $150: new shares 200, new price $60.00, value $12,000.00, new basis $75.00, total basis $15,000.00, gain -$3,000.00. When the basis is above the price, the holding is at a loss — and the split does not fix it: the $3,000.00 loss is the same before and after.

Reverse — the other way round: the shares owned for 500 post-split shares are 250 (exact 250); the split ratio for a $30.00 post-split price is 4-for-1 (exact 4); the pre-split price for a $15,000.00 total value is $150.00 (exact $150.0000); the cost basis for a $5,000.00 unrealized gain is $70.00 (exact $70.0000); the shares owned for a $20,000.00 total value are 167 (exact 166.6667).

What the defaults assume

The model assumes the split applies to every share you own at the same ratio, the price adjusts by exactly the split factor, and there are no fractional-share cash payments, fees, or taxes on the split itself — a split is not a taxable event in the usual case, because nothing is sold. It assumes the cost basis per share divides by the split factor, so the total basis is unchanged. It leaves out what the market does next: the price after a split is the old price divided by the factor only at the instant of the split — the market moves it from there. With 0 shares owned, everything is $0.00. With a 1-for-1 split, nothing changes: 100 shares at $120.00, basis $80.00, value $12,000.00, gain $4,000.00. With the cost basis equal to the price ($120.00), the gain is $0.00. Rounding convention: totals are computed from unrounded values, so rounded parts may differ by a cent from their rounded sum; the reverses are nearest-value solutions, and the exact unrounded figures (250, 4, $150.0000, $70.0000, 166.6667) are stated wherever the rounded figures appear.

Output · formula · meaning

Who uses this

• The shareholder waking up to a 2-for-1 split: they learn 100 shares at $120.00 become 200 shares at $60.00 — so they know their $12,000.00 did not double, and they check the 1-for-2 reverse branch (50 shares at $240.00) before panicking the other way.
• The investor checking their cost basis after a split: they learn the basis per share falls to $40.00 while the total basis stays $8,000.00 — so they know the tax record is the total, not the per-share number, and they check the 3-for-1 branch ($26.67, exact 26.6667) before entering the wrong basis.
• The investor working out how many shares they need: they learn 250 shares before a 2-for-1 split become 500 after it — so they know what to hold, and they check the 167-share reverse (exact 166.6667) before buying for a $20,000.00 target value.
• The holder of a stock that just reverse-split: they learn a 1-for-10 reverse split turns 100 shares at $120.00 into 10 shares at $1,200.00 — so they know the $12,000.00 is still there, and they check the cost-basis-$150 branch (-$3,000.00) before assuming a reverse split fixed a loss.

Accuracy & limitations

The math is exact for the numbers you enter: new shares = shares owned × a / b; new price = price before × b / a; value before = shares owned × price before; value after = new shares × new price; new basis = cost basis × b / a; total basis = shares owned × cost basis; gain = shares owned × (price before − cost basis). The 4-decimal exacts are shown where a basis does not divide evenly: 3-for-1 gives $26.6667 (shown as $26.67) and 3-for-2 gives $53.3333 (shown as $53.33). What the model cannot do is know what the market does after the split: check the price, because the split only sets the price at the instant of the split — the market moves it from there. It assumes the split applies to every share at the same ratio and leaves out fractional-share cash payments, fees, and taxes. It is a split calculator, not investment advice: for what the shares are worth after tax, see the ESPP calculator; for the tax on a dividend, see the dividend calculator.