Treasury Stock Method in Interviews: Diluted Shares Without the Panic

"How do you calculate diluted shares?" is a classic IB technical. The treasury stock method (TSM) is the usual answer for in-the-money options and warrants. Interviewers want the formula, a clean example, and the awareness that convertibles and RSUs are not all treated the same way.

The short answer

"For diluted share count with options, I use the treasury stock method. I assume in-the-money options are exercised, the company receives the strike proceeds, and it uses those proceeds to repurchase shares at the current market price. Net new shares equal options exercised minus shares bought back. Out-of-the-money options are ignored for basic diluted EPS style dilution."

Formula

Net Dilutive Shares = Options Outstanding

(Options x Strike Price / Current Share Price)
Only for options where Strike < Current Share Price (in the money).
Diluted Shares = Basic Shares + Net Dilutive Shares (+ other dilutive securities)

Equivalently: proceeds = options x strike; shares repurchased = proceeds / price; net dilution = options - shares repurchased. The repurchase price is the current market price, never the strike. That single swap (using strike as the buyback price) is a common fail.

Worked example

Basic shares                     100.0m
Options outstanding               10.0m
Weighted average strike           $10
Current share price               $20

Proceeds = 10m x $10 = $100m Shares repurchased = $100m / $20 = 5.0m Net dilutive shares = 10.0m - 5.0m = 5.0m Diluted shares = 100.0m + 5.0m = 105.0m

If the price were $8 (below $10 strike), TSM dilution from these options is zero for this interview convention. If price rises to $25, proceeds stay $100m, buyback falls to 4.0m, and net dilution rises to 6.0m. Higher prices mean more dilution under TSM because the same strike cash buys fewer shares.

Why "treasury stock"?

The method pretends the firm buys its own shares in the market with the strike cash, like a treasury repurchase. It is an accounting / EPS convention, not a prediction of what management will do tomorrow. Banks still use it in screens because it is standard, auditable, and matches how many models and 10-K diluted share footnotes are framed.

What TSM does not cover alone

If the question says "fully diluted," do not stop at options. Ask whether converts, warrants, and RSUs are in the data pack, then apply the right method for each bucket.

M&A and offer price follow-up

In a takeout, diluted share count affects equity value implied by a per-share offer. Banks also care about treasury method vs deal-specific treatment (cashless exercise, option cancellation for intrinsic value). In interviews, start with TSM, then say deal docs may settle options for cash equal to intrinsic value, which is economically similar to net dilution.

Offer equity value = offer price x diluted shares (with the deal's option settlement convention). Getting the share count wrong by using basic shares alone can understate the check the buyer writes by tens or hundreds of millions on a large option overhang.

Common mistakes

1. Diluting OTM options
2. Forgetting to subtract repurchased shares (counting gross options)
3. Using strike as the repurchase price
4. Mixing basic shares from one date with option counts from another
5. Ignoring converts when the question said "fully diluted"
6. Using a stale share price when the case gives a clear current price

Mechanics checklist

Verbal 30 seconds

"Diluted shares start from basic shares. For in-the-money options I apply the treasury stock method: assume exercise, use proceeds to buy back shares at the market price, and add only the net shares. Out-of-the-money options drop out. Convertibles use if-converted, not TSM. That diluted count feeds equity value per share, offer equity value, and diluted EPS."

Practice drill

Change price to $25 and $12 in the example above. Recompute net dilution both times out loud in under one minute. Then connect to Enterprise Value vs Equity Value: diluted shares sit on the equity side of per-share math. Also skim Cash Free Debt Free so you do not confuse share-count dilution with purchase-price adjustments.

Related

Football Field Walkthrough, Control Premium, and M&A Accretion Dilution all lean on a clean diluted share count when the story is per-share.