How This Calculator Works
This tool uses two complementary formulas. The first estimates your maximum heart rate (HRmax) from age. The second uses your resting heart rate (if provided) to refine the target zones via heart rate reserve.
Tanaka formula (default)
HRmax = 208 − (0.7 × age)
Published in Journal of the American College of Cardiology (Tanaka, Monahan, Seals, 2001). It's significantly more accurate than the older 220 − age rule, especially for adults over 40 where the classic formula systematically overestimates by ~10 bpm.
Karvonen formula (when RHR provided)
Target HR = ((HRmax − HRrest) × intensity) + HRrest
Karvonen anchors zones to your individual heart rate reserve (HRR) rather than a flat percent of max. For trained individuals with low resting HR, this gives more accurate zones than percent-of-max alone.
Why the 4×4 Lives in 85–95% of Max
The Norwegian 4×4 HIIT protocol targets 85–95% of HRmax during work intervals because that's the heart rate band that maximally stresses VO2max — the "central" cardiovascular adaptation (stroke volume, cardiac output) saturates near this intensity. Tjønna et al. (2008) and Helgerud et al. (2007) both anchor the protocol to this zone.
The 60–70% recovery zone keeps blood flow elevated to clear lactate, but is low enough that the next work interval can be hit at full intensity.
How to Measure Max HR Accurately
Formula estimates are within ±10–15 bpm of true max HR for most people. If you want a measured number:
- Graded exercise test (GXT): Most accurate. Performed by a clinical exercise physiologist.
- Maximal field test: After a thorough warm-up, perform a sustained 3–5 minute all-out effort uphill or on a stationary bike. The peak HR seen in the final 30 seconds approximates your max HR.
- Race max: The peak HR seen during the final sprint of a 5K race or shorter is close to true max for trained athletes.
Safety: Don't attempt a maximal field test alone if you have any cardiovascular risk factors. Get medical clearance first.
FAQ
Which max heart rate formula is most accurate?
The Tanaka formula (208 − 0.7 × age) is more accurate than the older 220 − age (Fox) formula, especially for adults over 40. The most accurate method overall is a supervised graded exercise test, but Tanaka is within roughly ±10 bpm for most people.
What is the Karvonen formula?
Karvonen calculates target heart rate using heart rate reserve (HRR): Target = ((Max HR − Resting HR) × intensity) + Resting HR. It accounts for individual fitness level via resting heart rate and is more personalized than percent-of-max for trained athletes.
What heart rate should I hit during the Norwegian 4×4?
85–95% of max heart rate during work intervals, 60–70% during the 3-minute active recoveries. Use this calculator to get your specific target bpm range, then run the Norwegian 4×4 with our interactive timer.
Do I need a heart rate monitor for the 4×4?
Recommended but not required. Without a monitor, use rate of perceived exertion (RPE) — work intervals should feel 8–9 out of 10: breathing hard, unable to hold a conversation, but not all-out sprinting. Recovery intervals should feel 5–6 out of 10.
Why is my measured max HR different from the calculator?
Formulas estimate max HR for the average person at your age. Individual variation is ±10–15 bpm. If you have a measured max HR from a stress test or hard race, use that instead of the formula.