Caffeine content varies widely. Here's what actually affects it.
Brewing Method: Longer contact time = more caffeine. Cold brew and French press extract more than espresso per ounce.
Roast Level: Light roasts have slightly MORE caffeine than dark roasts (about 7%). Dark roasting reduces bean mass slightly.
Coffee Type: Robusta has 2x the caffeine of Arabica (2.2% vs 1.2%).
Serving Size: 12oz coffee has more total caffeine than 1oz espresso, even though espresso is more concentrated.
FDA says 400mg/day is safe for most adults (about 4 cups drip coffee).
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