It depends on the material you're buying.
Metal rings (tungsten, titanium, stainless steel, zirconium, etc.): Size up. Metal rings cannot be resized, so it's better to have a slightly looser fit than one you can't get off. A touch of extra room also accounts for finger swelling throughout the day.
Silicone rings: Size down. Silicone has natural stretch and will loosen slightly with wear over time. Starting snug means it'll settle into the right fit.
Still not sure? The Ring Sizing Kit ($9.95, refundable on any ring purchase) lets you try physical ring gauges on your finger so you can feel the difference between adjacent sizes before committing.
And remember — even if you get it slightly wrong, we offer free 90-day exchanges. No stress.