The talk test is a simple, no-equipment way to judge how hard you are exercising by how easily you can speak. The U.S. Centers for Disease Control and Prevention (CDC) gives the rule of thumb: during moderate-intensity aerobic activity, such as brisk walking, you can talk but not sing; during vigorous-intensity activity, such as running, you cannot say more than a few words without pausing for a breath.
It measures relative intensity, so the same pace can feel moderate to a fit person and vigorous to someone less fit. The CDC lists it alongside a 0 to 10 effort scale, where moderate effort is about a 5 or 6 and vigorous effort starts at about a 7 or 8. Research reviews have found the talk test a valid and reliable guide for steady aerobic exercise, though it is less practical for high-intensity interval training.
Source: CDC, “How to Measure Physical Activity Intensity”.
Image: Joggers on Morro Strand State Beach, photo by Mike Baird, CC BY 2.0, via Wikimedia Commons.