TL;DRThe standardized quantity that one futures contract represents. Knowing the contract size tells you exactly how much exposure you're taking on and how much each tick is worth in dollars.
Contract size is the standardized quantity of the underlying asset that one futures contract represents. For crude oil (CL), one contract is 1,000 barrels. For the E-mini S&P 500 (ES), one contract is $50 multiplied by the index level. For corn (ZC), one contract is 5,000 bushels.
This standardization is what makes futures tradeable on an exchange. Every participant knows exactly what they're trading without negotiation.
Contract size directly determines how much money each price movement is worth. If CL is at $72.00 and the contract size is 1,000 barrels, one contract represents $72,000 in crude oil. A $1.00 move is worth $1,000.
For ES at 5,200 with a $50 multiplier, one contract controls $260,000. A 1-point move is worth $50.
For gold (GC) at $2,400 with a 100 troy ounce multiplier, one contract controls $240,000. A $1.00 move is worth $100.
Always calculate the notional value (price times contract size) before trading a new product. This tells you the total exposure you're taking on.
Many popular products come in multiple sizes. The E-mini S&P 500 (ES) is $50 per point. The Micro E-mini (MES) is $5 per point, exactly 1/10th the size. 10 MES contracts equal 1 ES contract in exposure.
Crude oil has the full-size CL (1,000 barrels) and the Micro crude MCL (100 barrels). Gold has GC (100 oz) and MGC (10 oz).
Smaller contract sizes let traders with smaller accounts participate with proper position sizing. If your risk calculation says you should trade 0.3 ES contracts, you can trade 3 MES contracts instead.
Contract size comparison
You want to compare the exposure of different products in your portfolio.
1 ES at 5,200: $50 x 5,200 = $260,000. 1 CL at $72: $72 x 1,000 = $72,000. 1 GC at $2,400: $2,400 x 100 = $240,000. 1 ZC (corn) at $4.50: $4.50 x 5,000 = $22,500. One ES contract has 3.6x the exposure of one CL contract.
Using micros for proper sizing
Your account is $10,000. You want to risk 1% ($100) per trade on the S&P 500 with a 10-point stop.
ES: 10 points x $50 = $500 per contract. That's 5% of your account, way too much. MES: 10 points x $5 = $50 per contract. You can trade 2 MES contracts ($100 risk), which is exactly 1% of your account.
Not knowing the contract size before placing a trade
Every product has a different contract size. Trading 1 CL thinking it's similar to 1 ES is a $72,000 vs $260,000 exposure difference. Check the specs first.
Confusing contract size with margin requirement
Margin is the deposit, not the exposure. You might post $7,000 margin for CL, but you're controlling $72,000 in crude oil. Contract size tells you the real exposure.
Treating 1 contract the same across all products
1 contract of ES is very different from 1 contract of corn. Always think in dollar exposure and dollar risk, not in number of contracts.