Same chart, a tenth of the dollars
YM and MYM follow the same Dow Jones Industrial Average, so they move the same number of points in a session: whatever the market does, MYM gains or loses a tenth of the dollars of YM. The volatility in points is the same; only the size of your position changes. Both trade on CME Globex, with the same hours.
Which one for a 50K prop account?
Take a 50K evaluation with a $2,000 max loss, a common size, and a 40-point stop. On YM, that stop costs $200 per contract: 10 losing trades in a row reach the max loss. On MYM, it costs $20: 100 losing trades. MYM also lets you size in steps, 2, 3 or 5 micros, instead of all or nothing.
Start on MYM. Move to YM when your plan risks at least $200 per trade with that stop, the cost of 10 micros, and your journal shows you follow your rules at that size. Commissions are charged per contract: check what 10 MYM cost you against 1 YM. The position size calculator gives the number of contracts for your stop and your risk.
Contract limits at prop firms
Firms also cap your size: on a 50K evaluation, the firms Sweep tracks allow 3 to 10 full-size contracts depending on the account, and each firm sets how micros count toward that limit. The exact limit of each account is on its firm’s rules page.
Full specs: YM and MYM. Trading hours: CME Globex: Sunday to Friday, 6:00 p.m. to 5:00 p.m. New York time, with a daily one-hour break from 5:00 to 6:00 p.m. See futures market hours in your time zone.