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12-3-30 and Incline Walking: Is It Actually Zone 2?

Short answer: 12-3-30 (12 percent incline, 3 miles per hour, 30 minutes) is Zone 2 for some people, below Zone 2 for fit people, and above it for people who are just starting. The workout is fixed; your heart is not. The only way to know is to check it against your own Zone 2 range, then adjust the incline until you sit inside it.

What 12-3-30 is

Lauren Giraldo posted the routine on YouTube in November 2019, and it took off on TikTok a year later. Set the treadmill to a 12 percent incline and 3 mph, and walk for 30 minutes. It is simple, it is low impact, and it feels hard enough to count. That is why it spread, and why so many people now ask whether it is Zone 2.

What the research measured

Heart rate. In an ACE-funded study at Western Colorado University, reported by the American Council on Exercise in March 2025, participants averaged 124 bpm during 12-3-30, about 67 percent of their maximum heart rate, at 5.5 METs and about 221 kcal per session. The study was done at altitude, which pushes heart rate up a little compared with sea level.

Here is the interesting part. 67 percent of maximum sits inside Zone 2 on a percent-of-max scale (60 to 70 percent), the default on many watches. But the same heart rate was only about 47 percent of heart rate reserve, which sits below Zone 2 on a reserve scale. Same people, same walk, two different answers, depending on which formula your watch uses.

Fuel. A 2025 study in the International Journal of Exercise Science (Wong and colleagues) compared 12-3-30 with running at a self-selected pace. Running burned more per minute, 13.08 vs 10.23 kcal, but a larger share of the energy in 12-3-30 came from fat, 40.6 vs 33.1 percent. That fits the idea of an easy aerobic effort. The study did not report heart rate, so it cannot tell you whether a given person was in Zone 2.

The math. The standard ACSM walking equation predicts about 8.3 METs for 3 mph at 12 percent grade. Measured values come in lower when people hold the handrails or walk with a shorter stride, one reason two people on identical settings can have very different heart rates.

Who it is Zone 2 for

• Just starting or returning after a break: 12 percent at 3 mph may push you into Zone 3 within ten minutes. Start at 5 to 8 percent and build.
• Moderately fit: this is where 12-3-30 is most often real Zone 2.
• Fit runners and cyclists: it may sit below Zone 2. Add incline (up to 15 percent on most treadmills) or speed until your heart rate reaches your range.

How to dial incline walking into Zone 2

1. Know your range first. A range built from your own maximum and resting heart rate beats an age chart. Our free Zone 2 heart rate by age calculator is a starting point.
2. Start at 3 mph and a moderate incline. Give it 5 to 10 minutes. Heart rate keeps rising for a while after you change the settings.
3. Adjust 1 to 2 percent at a time. Too high: drop the incline. Too low: raise it. Small moves, then wait a couple of minutes.
4. Hands off the rails. Holding on lowers the work and your heart rate. If you need the rails, the incline is too steep.
5. Use the talk test as a cross-check. In Zone 2 you can speak in full sentences but would rather not.
6. Go longer than 30 minutes when you can. Zone 2 coaches commonly suggest 45 to 60 minute sessions, three or four times a week. 12-3-30 is a fine start, and 45 minutes at the right incline is better.

Why incline walking works so well for Zone 2

Running often makes Zone 2 hard to hold: many people drift into Zone 3 the moment they jog. Walking uphill lets you raise the work without the impact or the urge to speed up, and the treadmill keeps it steady. It is one reason incline walking ranks near the top of our list of the best Zone 2 exercises.

Sources: American Council on Exercise, "The 12-3-30 Workout: ACE-Supported Research Puts a Viral Workout to the Test" (March 2025); Wong and colleagues, International Journal of Exercise Science 18(6):56-64 (2025); ACSM's Guidelines for Exercise Testing and Prescription, metabolic equations for walking. A training guide, not medical advice.