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How we calculate your Zone 2 heart rate

The calculator uses six sources for different parts of the estimate. Karvonen defines the heart rate reserve method, and Tanaka supplies the default HRmax formula. Uth-Sorensen and Pandolf support the optional pace estimate. Looney and Simpson inform the guidance for adjusting pace under load.

By Joel KellyPage updated: August 23, 2026

Summary

Zone 2 = 60-70% of heart rate reserve via the Karvonen method (Karvonen 1957, PMID 13470504). HRmax defaults to Tanaka's 208 - 0.7 × age regression (Tanaka 2001, PMID 11153730). The 220-age formula remains available as an option.

When you add body weight and ruck weight, the tool estimates VO2max from your HRmax/HRrest ratio (Uth 2004, PMID 14624296), converts that to a metabolic rate at Zone 2 intensity, and works backward through the Pandolf equation (PMID 908672) to find the walking pace that should produce that effort. Use the heart rate reading to adjust that starting pace.

1. Karvonen target HR (the primary output)

The Karvonen method anchors the calculator. The equation is simple:

Target HR = ((HRmax - HRrest) × intensity) + HRrest

For Zone 2 we use intensity = 0.60 to 0.70 (60-70% of heart rate reserve). The midpoint (0.65) is the value the calculator suggests as a daily target.

2. Why Tanaka 2001 instead of 220-age

The 220-age formula has no peer-reviewed primary source - Fox, Naughton, and Haskell published it as an observational fit in 1971, not as a research-derived equation. It systematically overestimates HRmax in adults over 40 by 5-10 BPM.

Tanaka et al. (JACC 2001;37(1):153-156, PMID 11153730) derived HRmax = 208 - 0.7 × age from a meta-analysis of 351 studies (492 groups, 18,712 subjects), then cross-validated it on 514 healthy lab subjects. The formula has a standard deviation of about 10 BPM at any given age, which is why the calculator reports a band and discloses the uncertainty.

Both formulas are available in the UI. Tanaka is the default. Fox remains available for comparison with watches that still use 220-age.

3. How the pace estimate is calculated

Heart rate is the primary target. When you enter body weight and ruck weight, the tool also estimates a flat-pavement pace that may correspond to the heart rate band.

The calculation has four steps:

  1. Estimate VO2max from HR ratio using Uth-Sorensen (PMID 14624296): VO2max ≈ 15.3 × (HRmax / HRrest). The formula has limits (it overestimates in trained athletes and underestimates in beta-blocker users). It provides an estimate when no lab measurement is available.
  2. Convert %HRR to %VO2 reserve. Heart rate reserve fraction approximates VO2 reserve fraction across most of the working range, so 60-70% HRR ≈ 60-70% of the gap between resting VO2 (3.5 mL/kg/min) and VO2max.
  3. Convert VO2 to metabolic rate in watts using the standard 1 L O2 ≈ 5 kcal and 1 kcal/min = 69.78 W conversions.
  4. Solve Pandolf for V. At grade = 0 and terrain coefficient eta = 1.0 (paved), the equation reduces to a quadratic in V that yields the walking speed in m/s. We convert to min/mile or min/km for display.

These conversions add uncertainty. The result shows a pace range and the VO2max estimate used. Adjust the pace using your measured heart rate.

4. Why heart rate comes before pace under load

Research on loaded walking supports using heart rate to adjust pace.

Looney et al. 2021 (PMID 33652153) studied 15 soldiers carrying 0/22/44/66% body mass with the MOLLE 4000 backpack. The study reported that relative work intensity under heavy loads may be better described as a percentage of HRmax than as a percentage of VO2max.

Simpson et al. 2010 (PMID 20962921) studied 18 British Parachute and SAS soldiers carrying a fixed 20 kg pack. Raising speed from 6.4 to 7.4 km/h increased mean heart rate by 20 BPM. A pace change can move heart rate outside the selected band, so the tool treats pace as an estimate.

Where the estimate can miss

  • Beta-blockers and AFib change the HR response. Pregnancy and recent cardiac events can also change it. Use RPE ("conversational pace") and check with a clinician.
  • Marathon runners and high-VO2max athletes commonly exceed Tanaka HRmax by 5-10 BPM. The reported band is a starting point, not a hard cap.
  • Resting HR drops 5-10 BPM in the first 3 months of regular cardio training. Recalculate every 4-6 weeks for accuracy.
  • The pace back-solve assumes flat pavement. Hills and soft surfaces shift the pace that produces the target HR. Heat and altitude do too, as can dehydration or short sleep.
  • The calculator stops its optional pace estimate above 25% of body weight. The cited military studies included heavier loads, but they do not validate this combined estimate for higher-load training or sessions over two hours.

How to use the band

Take the middle of the HR band as your default target for slow, conversational rucks. If your watch shows HR climbing toward the top of the band, slow your pace rather than push through. If it sits below the bottom of the band and you feel like you could talk easily, you may pick up the pace a little if that fits the day's goal. Do not add weight simply to raise your heart rate.

For race pace, interval work, or GORUCK event prep, this is the wrong tool. Those sit in higher zones and need different targets.

Keeping this page current

We update this page when the calculator changes or when a cited source is corrected or replaced. The date above reflects the latest page change.

Sources

  • Karvonen, 1957

    The effects of training on heart rate; a longitudinal study

    Karvonen MJ, Kentala E, Mustala O. Annales Medicinae Experimentalis et Biologiae Fenniae 35(3):307-315. PMID: 13470504.

    Origin of the heart rate reserve (HRR) method used in the calculator. Target HR = ((HRmax - HRrest) * intensity) + HRrest.

  • Tanaka et al., 2001

    Age-predicted maximal heart rate revisited

    Tanaka H, Monahan KD, Seals DR. Journal of the American College of Cardiology 37(1):153-156. PMID: 11153730. DOI: 10.1016/S0735-1097(00)01054-8.

    The regression equation HRmax = 208 - 0.7 * age, derived from a meta-analysis of 351 studies (492 groups, 18,712 subjects) and cross-validated in 514 lab subjects. Default HRmax formula in this tool.

  • Uth et al., 2004

    Estimation of VO2max from the ratio between HRmax and HRrest - the Heart Rate Ratio Method

    Uth N, Sorensen H, Overgaard K, Pedersen PK. European Journal of Applied Physiology 91(1-2):111-115. PMID: 14624296. DOI: 10.1007/s00421-003-0988-y.

    VO2max approx 15.3 * (HRmax / HRrest), in mL/kg/min. Used to back-solve the Pandolf equation when body weight and ruck weight are entered.

  • Pandolf et al., 1977

    Predicting energy expenditure with loads while standing or walking very slowly

    Pandolf KB, Givoni B, Goldman RF. Journal of Applied Physiology 43(4):577-581. PMID: 908672. DOI: 10.1152/jappl.1977.43.4.577.

    The load-carriage equation we invert to estimate the walking pace that produces the target metabolic rate at the entered body weight and ruck weight.

  • Simpson et al., 2010

    Effect of load mass on heart rate and oxygen uptake during walking with backpack loads

    Simpson KM, Munro BJ, Steele JR. Applied Physiology, Nutrition, and Metabolism 35(5):650-656. PMID: 20962921. DOI: 10.1139/H10-053.

    Measured a 20 BPM mean HR increase when British Parachute and SAS soldiers raised speed from 6.4 to 7.4 km/h while carrying a fixed 20 kg backpack. This result informs the calculator's guidance to adjust pace using measured heart rate.

  • Looney et al., 2021

    Cardiorespiratory responses to heavy military load carriage over complex terrain

    Looney DP, Hennessy ER, Potter AW, Pryor JL, Roberts BM, Friedl KE. Applied Ergonomics. PMID: 33652153. DOI: 10.1016/j.apergo.2021.103401.

    Found that under heavy loads (0/22/44/66% body mass with the MOLLE 4000), relative work intensity is better described relative to HRmax than VO2max - validating HR-driven pacing for loaded walking.

Run the calculator

Enter your age, resting heart rate, and (optionally) body weight and ruck weight to see your Zone 2 band and the corresponding walking pace estimate.

Open the Zone 2 calculator