Effective cost basis on a cash-secured put
Last updated: May 30, 2026 · Cash-Secured Puts series
When I talk to traders who are new to cash-secured puts, the conversation almost always goes the same way: they tell me the strike, I ask about the effective cost basis, they pause. They've been thinking about the wrong number.
The strike is what someone has the right to sell you shares at. The effective cost basis is what you actually pay per share if you're assigned, after netting out the premium you collected. They're different by exactly the premium amount, and that difference is the entire economic point of selling a CSP instead of just placing a limit order.
The simple formula
Effective cost basis = Strike − Premium received per share
That's it. The strike is the gross obligation; the premium reduces it because you collected cash for taking on that obligation.
SPY at $400. You sell a $390 put expiring in 30 days for $3 premium ($300 per contract).
Strike: $390
Premium: $3
Effective cost basis: $390 − $3 = $387
If SPY drops below $390 by expiration and you're assigned, you buy 100 shares at $390 ($39,000 out, $300 already received). Net cost: $38,700, or $387 per share.
Notice what the effective cost basis is doing: it's converting the trade from “commit to buy at $390” into “buy at $387 if needed.” That's a 3.25% discount versus today's market price, locked in regardless of where SPY actually ends up.
Why this number matters
Three practical implications:
1. It's the right comparison to fundamentals. When you're thinking about whether the trade is worth doing, you should compare your effective cost basis (not the strike) against the underlying's value. “Would I buy SPY at $390?” is the wrong question. “Would I buy SPY at $387?” is the right one.
2. It's the number you track for the wheel strategy. If you're running the wheel and get assigned, your effective cost basis becomes your reference price for selling covered calls. You want CC strikes above your effective cost basis to ensure any “called away” outcome is profitable.
3. It's the breakeven on the trade if you're assigned. If SPY closes exactly at $387 on the day after assignment, you've broken even. Below $387, you have a paper loss; above, a paper gain.
How to use it to choose between strikes
Here's how I compare two SPY CSPs side by side:
$390 put, $3 premium → effective cost basis $387 (3.3% discount to current). Higher assignment probability (~30% at this delta).
$375 put, $1 premium → effective cost basis $374 (6.5% discount). Lower assignment probability (~15%).
Option A pays you more but commits you to a smaller discount. Option B pays you less but commits you to a bigger discount with much lower probability of assignment.
The question isn't which has higher ROC. It's: at which price would you genuinely want to be a SPY buyer? If you'd buy at $387 but not at $374, take Option A and accept the higher assignment probability. If you'd only want SPY at a 6%+ discount, take Option B and live with the smaller premium.
What the effective cost basis doesn't capture
The effective cost basis is a useful single number but it's not the complete picture. It doesn't include:
- Opportunity cost on the cash. If your $39,000 was earning 5% in a money-market fund, you forfeit some of that interest while it's locked behind the put. The CSP's true incremental return is its premium minus the money-market interest you'd have otherwise earned.
- Tax friction. Premium in a taxable account is ordinary income. The effective cost basis assumes you're tracking the trade on a pre-tax basis.
- Downside if you're assigned and the stock keeps falling. If you're assigned at an effective $387 and SPY goes to $360, your effective cost is $387 but you're holding shares worth $360. The cost basis was just the entry price, not a floor.
Use the effective cost basis as your decision-making anchor for the trade. Use the wheel tracker (or a spreadsheet) to track the after-tax, after-interest reality across the full cycle.
FAQ
Is the effective cost basis my breakeven if assigned?
Yes, on a per-share basis. If the stock closes at exactly your effective cost basis when you sell or evaluate the position, you've broken even on the CSP trade as a whole.
Does the effective cost basis change if I roll the put?
Yes. Rolling means closing the current short put (at a cost) and opening a new one (at a credit). The net cash flow updates your effective cost basis: new effective cost = original strike − total net premium across all rolls.
Why isn't the strike the actual purchase price if I'm assigned?
It is, mechanically — your account is debited the strike × 100 per contract. But the premium you collected at the start (and any subsequent rolls) reduces your net economic cost. The 'effective' cost basis is the true per-share price accounting for all cash flows.
How is the effective cost basis treated for taxes if I'm assigned?
In most US brokerages, the assigned shares' tax cost basis is the strike price (not the effective cost basis). The premium is reported as a separate short-term gain. Some brokers offer 'adjusted cost basis' reporting; check your 1099-B. This is the territory where talking to a CPA pays for itself.
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