MAF vs. Karvonen vs. Percent of Max: Which Zone 2 Method Should You Trust?

Run your numbers through a Zone 2 calculator and you’ll notice something annoying: the three methods don’t agree. MAF gives you one ceiling, percent of max gives you another, and Karvonen lands somewhere else entirely. For a 45-year-old, the spread can be 10 to 15 beats. That’s the difference between a genuinely easy run and a run that quietly wrecks your week.

So which one is right? Short answer: none of them, and all of them. Longer answer below.

Why formulas disagree in the first place

Every one of these methods is a population estimate. It takes something easy to measure, like your age, and predicts something hard to measure, like the intensity where your body stops running mostly on fat and aerobic metabolism. Population estimates are right on average and wrong for individuals. You are an individual.

That’s not a reason to skip the math. It’s a reason to treat the number as a starting hypothesis instead of a commandment. The formulas get you into the neighborhood. Your own body tells you which house.

MAF: the strict parent

The MAF method (Maximum Aerobic Function, from Phil Maffetone) is dead simple: 180 minus your age, then adjust for health and training history. Coming back from injury or illness, subtract more. Training consistently for two-plus years without problems, add 5.

MAF’s defining trait is that it’s conservative. For most runners it produces the lowest ceiling of the three, and the adjustments push it lower for exactly the people most likely to overdo it. That’s not a flaw. That’s the design. Maffetone built it for chronically overtrained endurance athletes, and its job is to force a real aerobic base on people whose egos won’t allow one.

Who it fits: runners stuck in the injury-restart cycle, anyone rebuilding after time off, and anyone who suspects every “easy” run they’ve done for the last five years was secretly medium. If that’s you, MAF’s strictness is the medicine.

Percent of max: the default setting

Take your max heart rate, multiply by 0.60 to 0.70, and that’s Zone 2. It’s what most watches do out of the box, which is why it’s the number most runners have seen.

The problem is upstream: most people don’t know their actual max. The watch guesses it with 220 minus age, a formula so rough that researchers have been trying to retire it for decades. A better estimate is Tanaka’s 208 minus 0.7 times age, which is what a good calculator uses. But even Tanaka has a standard deviation around 10 beats. If your true max is 10 beats above the estimate, every zone your watch shows you is shifted, and your “Zone 2” might be Zone 3 in disguise.

Who it fits: runners who know their real max from a recent race finish or a proper field test. If your max is measured, percent of max is clean and simple. If your max is a formula’s guess, you’ve stacked one estimate on another.

Karvonen: the personalized one

Karvonen uses heart rate reserve: the gap between your resting heart rate and your max. Zone 2 is a percentage of that reserve, added back on top of resting.

The insight is that resting heart rate carries real information about your fitness. Two 45-year-olds with the same estimated max but resting rates of 48 and 72 are not the same athlete, and Karvonen is the only method of the three that notices. It personalizes the estimate with a number you can actually measure accurately, for free, by checking your watch before you get out of bed.

The catch: garbage in, garbage out, twice. Karvonen needs both a true resting rate (morning, rested, not the day after a hard workout or a rough night) and a decent max estimate. Get the inputs right and it’s arguably the best formula of the three. Get them sloppy and it’s confidently wrong.

Who it fits: runners with a few weeks of consistent morning resting-rate data and either a tested max or at least an honest Tanaka estimate.

How to actually pick

Here’s the protocol I use with athletes:

Run all three. The calculator does it in one shot. Look at the spread.

Start at the low end of the range. Usually that’s MAF. Training too easy for a few weeks costs you almost nothing. Training too hard for a few weeks costs you the base you were trying to build, and sometimes a tendon.

Verify with the talk test. At your chosen ceiling, you should hold full conversational sentences. Comfortable talking at that heart rate and feeling recovered the next day? The number works. Breathing gets ragged below your ceiling? Your ceiling is too high, regardless of what any formula said.

Watch pace at heart rate over eight weeks. If the same ceiling starts producing faster paces, the zone is doing its job and you can trust it. That trend line is worth more than all three formulas combined.

The formulas are the map. The talk test and the trend line are the terrain. When they disagree, believe the terrain.

One last thing: don’t switch methods every two weeks chasing the “right” number. Pick one, hold it for two months, and let consistency do what precision can’t. The runner who trains 60 days at a slightly imperfect ceiling beats the runner who spent those 60 days optimizing.