Here's a fact that changes everything for serious options traders:

When you trade SPX options, 60% of your gains are taxed as long-term capital gains (20% rate) and 40% are taxed as short-term capital gains (37% rate)—regardless of how long you held the position.

You could close a profitable SPX call trade 30 seconds after opening it, and 60% of the profit is still taxed at the long-term rate.

This is so tax-efficient that it alone can justify trading SPX instead of SPY—even though SPX has lower volume and larger bid-ask spreads.

This guide explains why SPX gets this treatment, how it works, and how to calculate your tax bill.

Why SPX Gets Special Tax Treatment

The IRS classifies SPX options as Section 1256 contracts under IRC Section 1256(b)(1).

Section 1256 includes:

Broad-based stock indices (SPX, RUT, MID)
Foreign currency contracts
Commodity futures
Regulated futures contracts

SPX qualifies because it's a broad-based index—the top 500 stocks, not a single company.

SPY, by contrast, is a single ETF. Even though it holds 500 stocks, the IRS treats SPY options as regular equity options under normal capital gains rules.

Result:

SPX options: 60% long-term / 40% short-term (max 20% + 37% = combined ~26% effective rate)
SPY options: 100% short-term (37% rate for holding < 1 year)

The 60/40 Tax Split

Here's how it works in practice:

SPX Call Profit Calculation:

Item	Amount
Trade entry	Sold 1 SPX 6,000 call for $2.50
Trade exit	Bought back for $0.50
Holding time	3 days (definitely short-term)
Gross profit	$2.50 - $0.50 = $2.00 × 100 = $200

Tax Calculation
60% treated as LTCG	$200 × 60% = $120 taxed at 20% = $24
40% treated as STCG	$200 × 40% = $80 taxed at 37% = $29.60
Total tax owed	$24 + $29.60 = $53.60
Effective rate	$53.60 / $200 = 26.8%

Compare this to SPY:

SPY Call Profit (same trade, 3-day holding period):

Item	Amount
Gross profit	$200
Tax: 100% STCG	$200 × 37% = $74
Effective rate	$74 / $200 = 37%

SPX saves you: $74 - $53.60 = $20.40 on this single $200 trade.

Scale this: If you do 100 such trades per year, SPX saves you $2,040 in taxes vs. SPY.

Comparing SPX vs SPY Tax Impact (Real Example)

Let's say you make $10,000 total profit trading call spreads over Q4.

SPY (normal short-term capital gains):

Profit: $10,000
Tax rate: 37% (short-term)
Tax owed: $3,700
Net profit: $6,300
Effective rate: 37%

SPX (60/40 split):

Profit: $10,000
60% at 20% (LTCG rate): $6,000 × 20% = $1,200
40% at 37% (STCG rate): $4,000 × 37% = $1,480
Total tax: $2,680
Net profit: $7,320
Effective rate: 26.8%

Difference: SPX lets you keep $1,020 more on $10,000 profit.

If you're a full-time options trader, this difference compounds throughout the year.

How the 60/40 Split is Applied on Form 6781

When you trade Section 1256 contracts, you file Form 6781 (Gains and Losses From Section 1256 Contracts and Straddles).

Form 6781 has two sections:

Part I: Section 1256 Contracts

You list each closed contract:

Opening date
Closing date
Proceeds (closing price)
Cost basis (opening price)
Gain/loss

The IRS then automatically applies the 60/40 split:

60% of net gain = Long-term capital gain (Schedule D, long-term section)
40% of net gain = Short-term capital gain (Schedule D, short-term section)

Example Form 6781 entry:

Trade #1:
Open date: 10/01/2025
Close date: 10/04/2025
Proceeds: $250
Cost: $50
Gain: $200

IRS applies 60/40 split automatically:
LTCG: $200 × 60% = $120
STCG: $200 × 40% = $80

Part II: Straddles

If you have straddles (long call + long put, or short call + short put), Part II requires you to show:

The straddle components
The gains/losses
How you handled the straddle rules

Most retail traders skip straddles, so you'll leave Part II blank.

Mark-to-Market Rules for Day Traders

If you're a Section 1256 trader (not the same as a PDT trader), the IRS treats all Section 1256 positions as if they're closed on Dec 31 each year, even if you don't actually close them.

This is called mark-to-market accounting.

What this means:

Dec 31 closes year
Your unrealized gains/losses on SPX, RUT, MID positions are taxed as if closed
Jan 1 opens new year with basis reset
You can elect to mark-to-market if you're a serious trader

Example:

You hold a profitable SPX call spread through year-end.

Dec 31, 2025 market value: Position worth $1,500 profit.

Without mark-to-market election:

Position stays open
You don't pay taxes until you close it in 2026
When closed, taxes are calculated from original entry

With mark-to-market election:

Dec 31: IRS assumes you closed for $1,500 gain
You pay taxes on $1,500 profit (60% LTCG, 40% STCG) in 2025
Jan 1: Position is "reset" for new year
When you actually close in 2026, you only pay taxes on gains after Jan 1

Most retail traders don't elect mark-to-market. It locks in taxes earlier. But if you're running a trading business, it can simplify accounting.

SPX vs SPY: The Complete Trade-Off

Factor	SPX	SPY
Tax treatment	60% LTCG / 40% STCG	100% STCG (if held < 1 year)
Effective tax rate	~26.8% (on short holds)	~37% (on short holds)
Bid-ask spread	Wider (0.05 - 0.10)	Tighter (0.01 - 0.02)
Volume	Lower (mid-market)	Higher (very liquid)
Implied volatility (IV)	Similar to SPX spot IV	Usually slightly lower IV
Theta decay	Same daily decay %	Same daily decay %
Option size	100 shares / contract (same)	100 shares / contract (same)
Best for	Spread traders, long holds	Day traders, scalpers

When SPX Makes Sense (Tax-Wise)

SPX is better for:

Holding periods of 1-7 days
- You save ~10% in taxes vs SPY
- Spreads are wider but tax savings offset it
- Example: 2-day profitable trade saves $200+ on $10k profit
Call selling / covered calls
- Theta decay accelerates in final 3-7 days
- SPX spreads are widest in OTM territory (where most spreads are sold)
- Tax advantage is huge if you're a consistent seller
Quarterly rotation strategies
- Open positions rotate through quarters
- Mark-to-market at year-end becomes valuable
- SPX lets you harvest gains with lower rate each quarter
Position sizing by tax efficiency
- If you trade 50 SPY spreads, you could trade 50 SPX spreads
- Same capital, but SPX saves $1,000+ per year in taxes
- The tax savings alone can fund additional trades

When SPY Makes Sense (Practical Reasons)

SPY is better for:

Scalping (holds < 5 minutes)
- Bid-ask spread on SPY is so tight that the tax difference doesn't matter
- The $0.01 tighter spread on 10 contracts = $10 saved
- Tax difference on $50 profit: ~$5
- They're wash; use SPY for easier fills
High-volume trading (50+ contracts per day)
- SPX bid-ask spreads are too wide
- You'll leave $200-500 per day in slippage
- Tax savings don't offset slippage costs
Non-directional / hedge positions
- Protective puts (insurance)
- Collars (protection structures)
- For hedges, you usually hold longer than a day
- SPX tax advantage applies here
Earnings-week trading
- IV crush is unpredictable on both
- SPY fills are more predictable (tighter spreads)
- Use SPY for earnings week, SPX for normal weeks

Calculating Your Tax on SPX Positions

Step 1: Gather data from your broker

Export all closed SPX positions for the year:

Entry date, exit date
Entry price, exit price
Quantity
Commissions

Step 2: Calculate P&L for each position

P&L = (Exit - Entry) × Quantity × 100 - Commissions

Step 3: Sum total P&L

Add up all profits and losses. If you have losses, they reduce gains dollar-for-dollar.

Step 4: Apply 60/40 split

LTCG (60%) = Total P&L × 60% × 20% federal rate
STCG (40%) = Total P&L × 40% × 37% federal rate

Add state taxes (varies by state; CA = 9.3% LTCG + STCG).

Step 5: File Form 6781

List all closed positions. The IRS calculates the 60/40 split automatically. You just report total gain/loss.

Form 6781 Filing Tips

Keep it simple: If you have < 20 closed positions, list each separately. If > 20, you can aggregate by month.
Be consistent: If you list SPX calls one way, list all SPX the same way.
Attach to Schedule D: Form 6781 feeds directly into Schedule D (Capital Gains/Losses).
E-file: Use tax software that supports Form 6781. Most major packages (TurboTax, H&R Block) include it.
Hire a CPA if: You have > 100 closed positions. The complexity isn't worth DIY.

Advanced: Straddle Rules

A straddle is a long call + long put (or short call + short put) on the same underlying, strike, and expiration.

Section 1256 straddles have special rules:

Losses in a losing straddle leg are deferred to next year
The holding period for each leg might differ
Gains and losses net before the 60/40 split applies

Example:

You open a straddle:

Long $6,000 SPX call for $2.50
Long $6,000 SPX put for $2.00
Cost: $4.50 total ($450 per contract)

In 3 days, SPX moves to $6,050:

Call profit: $2.00 → $2.50 → $3.50 = +$3.00 ($300)
Put loss: $2.00 → $0.50 = -$1.50 ($150)
Net: +$150

On Form 6781 Part II, you'd report:

Call gain: $300 (60% LTCG, 40% STCG)
Put loss: -$150 (offsets gains)
Net straddle gain: $150

The 60/40 split applies to the net, not each leg.

For most retail traders: You'll rarely use straddles, so you can ignore these rules.

Common Section 1256 Questions

Q: If I trade SPX spreads (short call + long call), is this a straddle?

A: No. Spreads use different strikes. Form 6781 Part II (straddles) only applies to same-strike positions. Spreads are reported as individual positions on Part I.

Q: Can I use SPX losses to offset SPY gains?

A: Yes. All capital gains/losses (SPX and SPY) net together on Schedule D. SPX and SPY losses combine to reduce your total gains.

Q: Do RUT options get the same 60/40 treatment as SPX?

A: Yes. RUT (Russell 2000) is also a broad-based index. 60/40 applies to RUT, MID, and any other Section 1256 contract.

Q: What if I hold an SPX position for 2 years?

A: Still 60/40. The 60% is taxed as LTCG regardless of holding period. The 40% is taxed as STCG regardless of holding period. No change.

Q: If I trade SPX professionally, can I use Section 1256 mark-to-market?

A: Yes, but you must make an election on your tax return. Consult a CPA before doing this; it has implications for all your investments.

Bottom Line: When to Trade SPX vs SPY

Use SPX when:

You're holding positions 1-21 days (where the tax advantage compounds)
You're systematically selling call spreads (weekly or monthly)
Tax efficiency matters more than tightest fills
You're willing to trade slightly wider bid-ask spreads

Use SPY when:

You're scalping (sub-5 minute holds)
You're doing high-frequency trading (50+ contracts/day)
You need the tightest bid-ask spreads
You want maximum liquidity and volume

For most retail traders: SPX for spreads and defined-risk positions (where you hold 2-7 days). SPY for scalps and day trades.

Tax savings example (practical):

Trade 100 profitable SPX spreads per year
Average profit: $200 per spread
Total: $20,000 profit
Tax difference SPX vs SPY: $20,000 × 10.2% = $2,040 annual tax savings

That's real money. Reinvest it into more strategies or improve your personal finances.

Master the tax side of options completely:

Complete Options Tax Guide – Full framework
Options Tax Calculator – Estimate your bill
Form 1099-B Walkthrough – Reading your 1099-B
Covered Call Tax Rules Explained – Covered call specific taxes
Understanding Wash Sale Rules – Avoid this trap

SPX Options Tax Treatment: The 60/40 Rule Explained