Every options pricing model, from Black-Scholes on, takes the same six inputs and returns a fair value. You don't need the math to trade well, but knowing what feeds the price tells you what can move it — and what can quietly work against you.
The six inputs
1. Underlying price. Where SPY is trading. The most obvious driver — how it flows into the option is delta.
2. Strike price. The fixed level your option is measured against. It sets how far in- or out-of-the-money you are.
3. Time to expiration. More time = more premium, because more can happen. As time runs out, that value bleeds away — theta.
4. Implied volatility. The market's expected movement. Higher IV means a richer premium; a drop deflates it (IV crush).
5. Interest rates. A small carry effect — matters for long-dated options, negligible intraday.
6. Dividends. Expected payouts before expiration nudge call and put values; for a same-day SPY trade, essentially a rounding error.
Which ones matter for a 0DTE scalp
On a same-day SPY option, the price is dominated by the underlying, the strike, time, and IV. Rates and dividends are noise on that timescale. Practically: the stock moving your way helps (via delta), the clock always hurts (theta), and a shift in expected volatility can help or hurt (IV). That's the trio a scalper actually manages.
Four inputs move your 0DTE trade: price, strike, time, and IV. The other two are rounding errors before the bell rings again.
Why this is worth knowing
When your option does something that surprises you — green stock, red option; a fill that jumps — it's almost always one of these six inputs, not a glitch. Understanding them is how you stop being surprised. From there, the game is entering where the underlying has room to move faster than time decays it. See the expected move for how far a session is realistically likely to travel.