What temperature rating do I need for a 3-season sleeping bag?

Most reviewers and gear testers point to a 20°F to 30°F comfort rating as the sweet spot for spring, summer and fall use across the lower 48. If a manufacturer only publishes a 'lower limit' or 'extreme' number, treat it as roughly 10–15°F warmer than the temperature you'll actually be comfortable in, and consider a liner for shoulder-season nights below 35°F.