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A stock’s quoted price is adjusted downward after a bonus issue because the same company value is represented by more shares. In an A:B bonus, investors receive A additional shares for every B shares already held. The mechanical adjustment factor is (A+B) ÷ B, so the illustrative ex-bonus price is the pre-bonus price divided by that factor. The lower per-share quote is paired with additional shares; by itself, it does not mean the shareholder has lost an equivalent amount of value.
What a bonus issue changes
A bonus issue gives eligible shareholders additional shares without a separate payment for each bonus share. The company capitalizes eligible reserves and converts them into issued shares. Because the number of shares rises while the company’s aggregate equity value is assumed unchanged at the instant of the accounting adjustment, each share represents a smaller fraction of that value.
That is why exchanges and market-data systems show a mechanically adjusted reference price around the ex-bonus date. The adjustment is an arithmetic normalization, not a prediction that buyers and sellers will continue to trade at exactly that number.
Reading the ratio correctly
The convention is A bonus shares for every B existing shares. A 1:1 bonus means one new share for each existing share. A 2:1 bonus means two new shares for each existing share. The ratio does not mean that the company is worth twice or three times as much; it describes how many pieces the existing equity is divided into.
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- With a 1:1 issue, each original share becomes two shares in total.
- With a 2:1 issue, each original share becomes three shares in total.
- With any A:B issue, B old-share units become A+B total-share units after the entitlement is applied.
The exact entitlement, treatment of holdings that do not divide evenly, and dates for a particular company come from that company’s exchange and shareholder notices. Do not assume that a fractional entitlement will be credited as a fractional share or paid in cash unless the notice says so.
The adjustment-factor formula
SEBI’s February 2025 investor presentation, Corporate Action – Dividends, Bonus, Splits etc., states the educational formula:
Adjustment factor = (A + B) ÷ B
The illustrative adjusted price is:
Adjusted price = pre-bonus price ÷ adjustment factor
This calculation keeps the aggregate value constant for the instant of the corporate-action adjustment. It is a ratio calculation, not a guaranteed traded price.
| Bonus ratio (A:B) | Total shares represented by each B old shares | Adjustment factor | Illustration if the pre-bonus price is ₹120 |
|---|---|---|---|
| 1:1 | 2 | 2 | ₹120 ÷ 2 = ₹60 |
| 2:1 | 3 | 3 | ₹120 ÷ 3 = ₹40 |
| 3:2 | 5 | 2.5 | ₹120 ÷ 2.5 = ₹48 |
| 3:1 | 4 | 4 | ₹120 ÷ 4 = ₹30 |
The table uses simple arithmetic to show the method. Your company’s reference price will depend on its own pre-adjustment price and declared ratio, and the first traded price can differ as soon as the market opens.
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SEBI’s ₹15-to-₹5 worked example
SEBI’s February 2025 presentation gives a 2:1 bonus example with a pre-bonus market price of ₹15. The adjustment factor is:
- Identify A and B: A = 2 bonus shares and B = 1 existing share.
- Calculate the factor: (2 + 1) ÷ 1 = 3.
- Divide the pre-bonus price by that factor: ₹15 ÷ 3 = ₹5.
SEBI labels ₹5 as a post-bonus market-price illustration. It is not evidence that every 2:1 bonus stock will trade at ₹5, nor is it a return statistic.
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Why the lower quote does not automatically mean a loss
Consider an investor holding 100 shares at an illustrative pre-bonus price of ₹15. Before the adjustment, the holding is worth ₹1,500. Under a 2:1 bonus, the investor receives 200 additional shares, making 300 shares in total. At the mechanically adjusted price of ₹5, the holding is still illustrated at ₹1,500:
| Shares | Price per share | Illustrative holding value | |
|---|---|---|---|
| Before adjustment | 100 | ₹15 | ₹1,500 |
| After a 2:1 bonus, at the formula price | 300 | ₹5 | ₹1,500 |
The per-share number fell by two-thirds, but the share count tripled. Those changes offset one another in the mechanical illustration. Your actual account value can be different because the market may reprice the stock above or below ₹5 once trading reflects supply, demand and new information.
A price chart can also look as though a large one-day loss occurred if the data provider has not applied the corporate-action adjustment consistently. When comparing historical returns, check whether the chart is adjusted for the bonus and whether the quoted price is the exchange’s ex-bonus reference or a live traded price.
Mechanical adjustment versus the actual market price
Mechanical adjustment
The mechanical number follows only the announced ratio and the relevant pre-adjustment reference price. It answers: “What price would represent the same aggregate value after the share count changes?” It is useful for reconciling an account, interpreting a chart and checking whether the ratio arithmetic has been applied correctly.
Actual trading price
The market price is set by orders after the stock begins trading on an ex-bonus basis. It may be higher or lower than the formula illustration because investors reassess earnings, growth prospects, risk, liquidity and broader market conditions. A move beyond the formula amount is not an error in the bonus calculation; it is a market movement layered on top of the corporate action.
Therefore, do not describe the formula as “the price the stock must trade at.” The precise wording is mechanically adjusted price, illustrative adjusted price or reference price, depending on the exchange’s terminology.
How to calculate an ex-bonus price yourself
- Read the ratio convention. Write the announced ratio as A:B, where A is the number of bonus shares and B is the number of existing shares.
- Find the correct pre-adjustment price. Use the reference price specified by the exchange or the price immediately before the ex-bonus adjustment. Do not substitute a price from an unrelated date.
- Compute the adjustment factor. Add A and B, then divide by B: (A+B) ÷ B.
- Divide the pre-bonus price by the factor. The result is the illustrative adjusted price, usually rounded according to the market’s price-tick rules.
- Check the live quote separately. The first traded price may differ because trading conditions determine the market price.
For example, suppose a stock’s relevant pre-adjustment price is ₹84 and the bonus is 1:1. The factor is (1+1) ÷ 1 = 2, giving an illustrative adjusted price of ₹42. If the bonus is 2:1 instead, the factor is 3 and the illustration is ₹28. The different results come from the ratio, not from a different valuation method.
What happens to your shareholding
For a holding that exactly matches the ratio’s denominator, multiply the old share count by (A+B) ÷ B to obtain the post-bonus total. With 250 shares and a 1:1 bonus, 250 new shares are added and the total becomes 500. With 100 shares and a 2:1 bonus, 200 new shares are added and the total becomes 300.
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For holdings that are not an exact multiple of B, use the company’s entitlement and allotment instructions. The simple formula explains the proportional economics, but the operational treatment of odd lots, fractional entitlements and any sale of fractions is company-specific.
Dates and account display: what to verify
A bonus announcement can include several dates, including a record date, an ex-date and an allotment or credit date. They answer different operational questions: who is entitled, when the security begins trading without the entitlement, and when the additional shares appear in the investor’s account.
Do not infer those dates from the ratio or from a generic calendar. Check the current notice from the company, the relevant exchange and your broker or depository. A temporary mismatch between the adjusted market price and the visible bonus-share balance can be an administrative timing issue rather than a calculation error; contact the intermediary if the notice’s stated credit window has passed.
India-specific regulatory points
The cited SEBI investor presentation says a bonus issue is made from eligible reserves and is not issued in lieu of a dividend. Those statements describe the Indian regulatory context. They should not be generalized automatically to another country’s company-law, accounting or tax rules.
SEBI’s older bonus-issue guidance also surfaced in the source material. Because rules and circulars can change, verify the current SEBI and exchange requirements before relying on any statement for compliance, a corporate-action decision or legal advice. This article does not establish a company’s eligibility, approval process or tax treatment.
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Bonus adjustments in derivatives
A derivative contract on the underlying share is not handled simply by multiplying the number of shares in a cash-market account. SEBI’s 2023 material on exchange-traded derivatives describes a separate corporate-action adjustment methodology. For a bonus, the adjustment factor is used to revise contract specifications so that the contract is not unintentionally enriched or diluted by the share-count change.
- Strike price: the new strike is arrived at by dividing the old strike price by the adjustment factor.
- Market lot or multiplier: the contract’s quantity or multiplier can be adjusted under the applicable exchange procedure.
- Position: the number of contracts or the contract position can be revised so the economic exposure remains aligned with the pre-action contract.
For a purely illustrative 2:1 bonus, the factor is 3. A ₹150 strike would therefore be divided by 3 to produce a ₹50 adjusted strike under the stated methodology, subject to the exchange’s rounding and contract rules. This example concerns a derivatives contract; it does not change the arithmetic for shares held in a demat account.
Always read the exchange adjustment circular for the specific contract. Expiry, lot-size, rounding and position rules can vary, and a retail investor should not assume that an underlying-share adjustment automatically describes an option or futures position.
Common mistakes and how to avoid them
- Using the wrong ratio direction: Treating 1:2 as one bonus share for every two existing shares is different from 2:1. Rewrite the ratio in words before calculating.
- Subtracting a percentage instead of dividing by the factor: A 2:1 bonus requires division by 3, not an arbitrary 200% or 66.67% adjustment.
- Calling the illustration a guaranteed price: The formula sets a reference expectation; trading determines the actual quote.
- Judging wealth from the quote alone: Compare the post-bonus price with the new share count and any cash or corporate-action entries.
- Confusing shares with derivatives: Futures and options may have revised strikes, lots and positions under separate rules.
- Assuming every date or rule is universal: Record dates, credit timing and legal requirements must come from the current company, exchange and jurisdictional notices.
- Ignoring data-provider adjustments: Check whether a chart is split- or bonus-adjusted before interpreting a historical price drop.
A practical verification checklist
- Confirm the announced A:B ratio in the company’s official notice.
- Confirm the record date and ex-date from the current exchange notice.
- Record the exchange’s relevant pre-adjustment reference price.
- Calculate (A+B) ÷ B and divide the reference price by that factor.
- Multiply your old holding by the announced entitlement to estimate the expected new share count, subject to the allotment terms.
- Compare the live quote with the illustration, recognizing that the difference is market movement rather than necessarily a failed adjustment.
- If the account or derivative position does not match the notice after the stated processing period, ask the broker, depository participant or exchange support desk for a corporate-action reconciliation.
What the adjustment tells you—and what it does not
The adjustment tells you how the per-share representation changes when the share count increases. It helps explain why a 1:1 bonus generally halves the reference price and why a 2:1 bonus generally reduces it to one-third, before normal market trading effects.
It does not tell you whether the company’s business is improving, whether the stock is cheap, what return you will earn, or where the market will settle. Those questions require analysis beyond the corporate-action arithmetic.
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