What is heart rate reserve (HRR)?

Heart rate reserve is the difference between your maximum heart rate and your resting heart rate. It represents the full range of heart-rate capacity available for exercise. A higher reserve generally indicates better cardiovascular fitness because a well-trained heart beats more slowly at rest while retaining the same (or higher) ceiling.

What is the Karvonen formula?

The Karvonen formula calculates a target training heart rate as: Target HR = HRrest + (intensity% x HRR), where HRR = HRmax - HRrest. Because it anchors intensity to both ends of your personal HR range rather than just the maximum, it is considered more accurate than simple percentage-of-max methods — especially for beginners with a high resting HR or athletes with a very low resting HR.

How does HRR differ from percentage of max HR?

Simple percent-of-max prescribes intensity relative to HRmax only. Karvonen uses the reserve — the gap between rest and max — so two people with the same HRmax but different resting HRs will train at different absolute BPM targets at the same stated intensity. A runner with HRmax 190 and HRrest 50 has a reserve of 140; at 70% Karvonen that is 50 + (0.70 x 140) = 148 bpm. Another runner with HRmax 190 but HRrest 75 would target 75 + (0.70 x 115) = 156 bpm — a meaningful difference.

What is a good resting heart rate?

For most adults, 60-80 bpm is normal. Endurance-trained athletes typically measure 40-60 bpm. Values below 40 bpm in non-athletes or above 100 bpm consistently at rest (tachycardia) warrant a medical review. To get an accurate figure, measure before getting up in the morning, on three separate days, and average the readings.

Which max-HR formula should I use?

Tanaka (208 - 0.7 x age) is the most widely validated for the general adult population. Fox (220 - age) is the oldest and simplest but over-estimates max HR in older adults. Gulati (206 - 0.88 x age) was derived specifically from a female cohort and is recommended for women. If you have measured your true HRmax in a field test or lab, choose Custom — real measurements always beat predictions.

Is the calorie estimate accurate?

The calorie figures use the Keytel et al. (2005) equation, which estimates energy expenditure from heart rate, age, weight and sex. Like all HR-based calorie models it carries a margin of error of roughly plus or minus 15-20%. It is most accurate at moderate intensities (50-70% HRR) and less reliable at very low or near-maximal efforts. Use it as a directional guide rather than an exact count.

What is the Heart Rate Reserve Calculator?

Free heart rate reserve (HRR) calculator. Enter age, resting HR and weight to get Karvonen target heart-rate zones for every intensity level, a calorie-burn estimate and a custom intensity lookup — all in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Heart Rate Reserve Calculator

Name: Heart Rate Reserve Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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Heart rate reserve (HRR) is the single most important number for precision-guided endurance training. Unlike blunt percentage-of-max formulas, the Karvonen HRR method pins your training zones to your personal physiology — accounting for both how high your heart can go and how low it drops at rest. The result is a set of target heart-rate zones that are genuinely personalised rather than estimated from age alone.

This calculator implements the Karvonen (1957) formula in full: enter your age, resting heart rate and weight, choose the max-HR prediction that fits you best, and instantly see your HRR, a target BPM for every standard intensity level from 30% to 90%, a calorie-burn guide for each zone, and a custom intensity lookup for any percentage your coach prescribes.

How it works

The three numbers that drive every result are:

Max HR (HRmax) — estimated from your age and formula choice, or entered directly if you have measured it.
Resting HR (HRrest) — your heart rate at complete rest, ideally measured first thing in the morning before rising.
Heart Rate Reserve (HRR) = HRmax − HRrest — the usable range.

The Karvonen target at any intensity fraction f is:

Target HR = HRrest + f × (HRmax − HRrest)

So at 70% intensity for someone with HRmax 185 bpm and HRrest 55 bpm:

Target = 55 + 0.70 × (185 − 55) = 55 + 91 = 146 bpm

Compare this to simple % of max: 70% × 185 = 130 bpm — a 16-bpm gap. For a well-trained athlete with a low resting HR that difference is even larger, which is precisely why Karvonen is considered the gold standard for serious training prescription.

Worked example

Profile: 35-year-old male runner, resting HR 52 bpm, body weight 72 kg. Using the Tanaka formula: HRmax = 208 − (0.7 × 35) = 183.5 → 184 bpm (rounded).

HRR = 184 − 52 = 132 bpm

Intensity	% of HRR	Calculation	Target HR
Light	40%	52 + 0.40 × 132	105 bpm
Moderate	50%	52 + 0.50 × 132	118 bpm
Tempo	70%	52 + 0.70 × 132	144 bpm
Threshold	80%	52 + 0.80 × 132	158 bpm
Maximum	90%	52 + 0.90 × 132	171 bpm

A training plan calling for “Zone 2 at 50-60% HRR” means holding 118–131 bpm for this athlete — information a simple percent-of-max table would have placed nearly 15 bpm too low.

Formula note

The HRmax formulas used by this calculator are:

Fox & Haskell (1971): 220 − age — the textbook classic; tends to over-predict for older adults and under-predict for younger ones.
Tanaka, Monahan & Seals (2001): 208 − 0.7 × age — meta-analysed from more than 18,000 subjects; preferred for mixed adult populations.
Gulati et al. (2010): 206 − 0.88 × age — derived from an asymptomatic female cohort; recommended for women.
Custom: enter your own field-measured or lab-tested value.

Calorie estimates use the Keytel et al. (2005) HR-based energy-expenditure equations. All calculations run locally in your browser — no data is uploaded.