Selling Puts 101: Understand Cash-Secured Puts Before You Sell
Selling a put sounds like free money: you collect premium first, and as long as the S&P 500 stays above your strike, the money is yours. But it is not a free lunch — on the day the market falls below the strike, you pay the difference. This page works out the cash-secured math first, then shows how to practice it in the simulator.
How selling a put makes money
Selling a put means collecting premium first: with the S&P 500 at 7550, selling one 7400 put at, say, 1.50 collects $150 (1.50 × 100), minus a $0.65 fee to open.
If the settlement price ends above 7400 at expiry, the put is worth nothing and you keep the premium minus round-trip fees: 150 − 1.30 = $148.70.
Breakeven = strike − premium + round-trip fee per share: 7400 − 1.50 + 0.013 = 7398.51. Settle above it and you profit; settle below it and you lose.
Time is the seller's friend: a 0DTE has only today left, so its premium shrinks every hour — even if the S&P 500 does not move, the put gets cheaper, and that decay is what the seller earns.
What "cash-secured" means
Cash-secured is the old rule for selling puts: sell one put, keep strike × 100 in cash on hand — a 7400 put needs $740,000, so the money is already there if you are assigned.
SPX has no shares to take: it is a cash-settled index option — at expiry it pays out on the S&P 500 official close. Settle at 7350 and the seller of a 7400 put pays (7400 − 7350) × 100 = $5,000, plus fees.
So for SPX, "cash-secured" means holding the cash to pay that loss, not to buy shares: one 7400 put can lose up to $740,000 in theory (if the settlement price fell to zero).
Selling puts without the cash set aside is selling naked: the premium looks like free money until a down day like Nov 5, 2026 — when the S&P 500 fell about 4.35% in one session — and the account cannot cover the loss. That is the position that blows up first.
How to practice it in the simulator
First, the honest part: in this simulator, a plain short put cannot be opened — selling locks 20% of the strike as margin, so a 7400 put locks $148,000 while the virtual account starts with $10,000, and the order is rejected with "insufficient margin".
That is not a bug: the simulator's 20% is a simplified rule, not a real broker margin. A real cash-secured put needs the full strike × 100 set aside ($740,000 for a 7400 put) — even more than the simulator locks.
To practice the same idea — selling a put below the market to collect premium — use a bull put spread instead: sell the 7400 put and buy the 7350 put. In a live test, opening 1 contract locked $4,910 of margin, collected $90 of net premium, cost $5 to open, and caps the outcome at $80 maximum profit and $4,920 maximum loss.
How to practice: pick a pair of strikes on the chain, work out the breakeven (sold strike − net credit + fees per share: 7400 − 0.90 + 0.10 = 7399.20), write your exit plan before you enter; no new positions after 3:30 p.m. ET, and sort out any open position before the 3:55 p.m. hint — do not leave it to the close.
Four mistakes beginners make most
Looking only at the premium, not at how far the strike sits from the market: the closer a put is to the spot, the fatter its premium — and the likelier it ends up in the money. Ask "how far down before I lose" before "how much do I collect".
Forgetting the fees: a short put costs $0.65 to open and $0.65 to close. A deep out-of-the-money put at 0.05 collects only $5, and the $1.30 round-trip fee eats a quarter of it.
Selling with no exit plan: on a 0DTE day, a 1% move in the S&P 500 is about 75 points and a premium can double in a moment — write down how much loss makes you leave and by when, before you enter, not at the close.
Mistaking the simulator's 20% margin for real cash-secured money: the simulator lets a 7400 put "sell" with $148,000 locked, but a real cash-secured put needs $740,000 — five times as much. Never size a real account by the simulator's rule.
This simulator uses real CBOE option chains delayed about 15 minutes, starts with $10,000 in virtual funds, charges $0.65 per contract per side in fees, and shows P&L net of fees; short-sale margin is a simplified simulated rule at 20% of the strike, not a real broker margin. The simulator is for practicing orders, discipline, and review. It is not investment advice and promises no returns.