🏃 Personal Fitness Assistant

Unified Fitness Calculator

Running · Cycling · Walking · Hiking · Treadmill — pace, calories, heart rate zones, VO₂, fat burned, and performance predictions. One tool, every metric.

ACSM FormulasKarvonen HR ZonesRiegel PredictionsMET (Ainsworth 2011)Natural Language Input
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Educational estimates only. This calculator provides fitness estimates for informational purposes. It is not medical advice. Actual results vary by fitness level, health condition, terrain, weather, and device accuracy. Consult a physician before starting any exercise program.

Calculate Your Workout

Units:
h
mm
ss

Measure first thing in the morning

Assumptions & Limitations

  • →Calorie estimates use MET × body weight × hours. Accuracy ±10–20% vs. metabolic lab testing (indirect calorimetry).
  • →Heart rate uses Fox formula: Max HR = 220 − age. Tanaka formula (208 − 0.7 × age) is slightly more accurate for adults over 40.
  • →Estimated average HR uses Karvonen method with sex-adjusted VO₂max. Individual fitness level is the largest source of error.
  • →VO₂ consumption = MET × 3.5 ml/kg/min — oxygen cost, not VO₂max. Actual VO₂max requires laboratory treadmill testing.
  • →Fat/carb split is based on exercise intensity zones. Real substrate use depends on glycogen levels, diet, and training state.
  • →Riegel race predictions assume consistent aerobic conditioning. Predictions lose accuracy beyond 3× the input distance.
  • →Hiking MET uses a flat terrain base (5.3 METs) plus incline adjustment. Altitude, terrain roughness, and pack weight are not modelled.
  • →Treadmill running uses the same ACSM equation as outdoor running. No wind resistance on a treadmill means ~1% grade is needed to match outdoor effort.

Understanding Your Fitness Metrics

MET — Metabolic Equivalent of Task

1 MET is the rate of energy consumption sitting quietly: ~3.5 ml O₂/kg/min or ~1 kcal/kg/h. Running at 10 km/h ≈ 10 METs — 10× resting metabolism. The Ainsworth Compendium (2011) catalogues 821 activities with validated MET values used in research and clinical practice worldwide.

Heart Rate Zones (Karvonen Method)

Training zones are percentages of Heart Rate Reserve (HRR = Max HR − Resting HR). Zone 2 (60–70% HRR) is the aerobic base zone — most elite athletes spend 80% of training here. Zone 4 (80–90%) builds lactate threshold. Zone 5 (90–100%) develops VO₂max and neuromuscular power.

VO₂ and Aerobic Capacity

VO₂max is the maximum oxygen uptake — the gold standard for cardiovascular fitness. Untrained adults: 30–40 ml/kg/min. Recreational athletes: 45–55. Elite marathoners: 70–85. Every 1-MET increase in exercise capacity is associated with ~13% reduction in cardiovascular mortality (Myers et al., NEJM 2002).

Fat vs. Carbohydrate Oxidation

Your body always burns a mixture of fat and carbohydrates. At low intensity (<60% VO₂max), fat provides ~50–60% of energy — ideal for long-duration, low-intensity exercise. At high intensity (>80% VO₂max), carbohydrates dominate. The "fat-burning zone" is real but not superior for total fat loss — total calorie deficit matters most.

Pace, Speed, and Conversion

Pace (min/km or min/mile) is the time per unit distance: pace = 60/speed(km/h). A 10 km/h pace = 6:00/km = 9:39/mile. Converting units: 1 km = 0.621 miles; 1 mph = 1.609 km/h. Riegel's formula relates race performances: t₂ = t₁ × (d₂/d₁)^1.06 — the exponent reflects the ~6% additional fatigue cost per doubling of distance.

Cycling Power and Efficiency

Cycling power (watts) is the rate of mechanical work output. It depends on speed, rolling resistance (Crr ≈ 0.005 for road tires), and aerodynamic drag (CdA ≈ 0.25–0.35 m² for a road cyclist). Power = (Crr × m × g + 0.5 × ρ × CdA × v²) × v. Functional Threshold Power (FTP) — 1-hour sustainable power — is typically 3.5–5.5 W/kg for trained cyclists.

Frequently Asked Questions

References & Sources

  1. Ainsworth BE et al. 2011 Compendium of Physical Activities. Med Sci Sports Exerc. 2011;43(8):1575–81.
  2. Karvonen MJ et al. The effects of training on heart rate. Ann Med Exp Biol Fenn. 1957;35(3):307–15.
  3. Riegel PS. Athletic Records and Human Endurance. American Scientist. 1981;69(3):285–90.
  4. ACSM Guidelines for Exercise Testing and Prescription, 11th Ed. 2022. American College of Sports Medicine.
  5. Uth N et al. Estimation of VO₂max from the ratio heart rate max to resting heart rate. Eur J Appl Physiol. 2004;91(1):111–15.
  6. Fox SM 3rd et al. Physical activity and the prevention of coronary heart disease. Ann Clin Res. 1971;3(6):404–32.

Specialized Calculators

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Running

Pace, splits, VO₂max

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Cycling

Speed, power, cadence

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Walking

Distance, steps, MET

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Heart Rate Zones

Karvonen 5 zones

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Calories Burned

80+ activity MET table

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Pace Calculator

Riegel predictions

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Steps Calculator

Steps to distance

A unified workout calculator for running, cycling, walking, hiking, treadmill, and general cardio. Enter distance, time, body weight, and a few personal details to estimate pace, calories, MET, VO₂ consumption, heart-rate zones, fat-vs-carb energy split, and equivalent race times — all in one tool.

How it works

Each activity uses a metabolic equivalent (MET) value from the 2011 Ainsworth Compendium. Calories burned are then computed as MET × body-weight (kg) × duration (hours). Heart-rate zones use the Karvonen heart-rate-reserve method, which combines age and resting heart rate for individualised target zones.

For running and cycling, the assistant also estimates VO₂ consumption using the ACSM walking and running metabolic equations. These are population-derived equations that estimate the oxygen cost of locomotion at common speeds and grades.

Race-time predictions use Peter Riegel's 1981 endurance formula: t₂ = t₁ × (d₂ / d₁)¹·⁰⁶. Riegel's exponent (1.06) assumes an evenly trained aerobic athlete; predictions become less reliable far from the input distance.

Formulas

  • Speed: speed = distance ÷ time — Average speed during the activity.
  • Pace: pace = time ÷ distance — Time taken per unit of distance (min/km or min/mile).
  • Calories (MET): kcal = MET × weight(kg) × hours — Ainsworth 2011 Compendium. 1 kcal ≈ 4.184 kJ.
  • Max heart rate: HRmax ≈ 220 − age (or 208 − 0.7 × age, Tanaka) — Population estimate; individuals vary by ~±10 bpm.
  • Karvonen target HR: THR = (HRmax − HRrest) × intensity% + HRrest — Heart-rate reserve method.
  • VO₂ consumption: VO₂ ≈ MET × 3.5 ml·kg⁻¹·min⁻¹ — Resting MET ≈ 3.5 ml O₂/kg/min.
  • Riegel race prediction: t₂ = t₁ × (d₂ / d₁)^1.06 — Endurance event time scaling.

Worked examples

Recreational 5 km run

Inputs: Activity: running. Distance: 5 km. Time: 25 min. Weight: 70 kg. Age: 30. Sex: male.

  1. Speed = 5 ÷ (25/60) = 12 km/h
  2. Pace = 25 ÷ 5 = 5:00 min/km
  3. Running at 12 km/h ≈ MET 11.5 → kcal = 11.5 × 70 × (25/60) ≈ 335 kcal
  4. HRmax ≈ 220 − 30 = 190 bpm

Result: 12 km/h, 5:00/km, ≈ 335 kcal (≈ 1 402 kJ), HRmax ≈ 190 bpm.

Indoor cycling session

Inputs: Activity: cycling. Distance: 24 km. Time: 60 min. Weight: 80 kg. Age: 35. Sex: female.

  1. Speed = 24 km/h, Pace = 2:30 min/km
  2. Cycling at 24 km/h ≈ MET 9.8 → kcal = 9.8 × 80 × 1 ≈ 784 kcal
  3. HRmax ≈ 220 − 35 = 185 bpm

Result: 24 km/h, ≈ 784 kcal (≈ 3 280 kJ), HRmax ≈ 185 bpm.

Use cases

  • Plan training intensity using personalised heart-rate zones
  • Estimate energy expenditure for nutrition planning
  • Predict realistic finishing times for an upcoming race
  • Compare the relative effort of different activities
  • Track weekly aerobic load across mixed sports

Common mistakes to avoid

  • Using HRmax = 220 − age as a precise number (it has ±10–12 bpm error)
  • Comparing kcal totals across devices that use different MET tables
  • Ignoring incline and wind, which can change cycling kcal by 30–50 %
  • Trusting Riegel race predictions for ultra-distances (it overpredicts > 30 km)
  • Mixing units (km with min/mile pace) without converting first

Limitations

  • MET values are population averages and ignore individual metabolism, fitness level, and biomechanics
  • No GPS-based elevation profile is used — only optional incline percentage
  • VO₂ output is the activity's oxygen cost, not your VO₂max
  • Riegel formula degrades beyond ~3× the input distance
  • Heart-rate output is an estimate, not a measurement

References

  1. Ainsworth BE et al. 2011 Compendium of Physical Activities. Med Sci Sports Exerc. 2011;43(8):1575–1581.
  2. American College of Sports Medicine. ACSM's Guidelines for Exercise Testing and Prescription, 11th ed., 2021.
  3. Karvonen MJ, Kentala E, Mustala O. The effects of training on heart rate. Ann Med Exp Biol Fenn. 1957;35(3):307–315.
  4. Tanaka H, Monahan KD, Seals DR. Age-predicted maximal heart rate revisited. J Am Coll Cardiol. 2001;37(1):153–156.
  5. Riegel PS. Athletic records and human endurance. American Scientist. 1981;69(3):285–290.

Frequently asked questions

How accurate are the calorie estimates?

Calorie estimates are typically within ±10–20 % of indirect-calorimetry measurements. They are excellent for goal-setting but not for clinical use.

What is MET?

1 MET is the energy used at rest, ≈ 3.5 ml O₂ per kg per minute, or ≈ 1 kcal per kg per hour. Walking at 5 km/h is ~3.5 METs; running at 10 km/h is ~10 METs.

How does the Karvonen formula work?

Target HR = (HRmax − HRrest) × intensity% + HRrest. It uses heart-rate reserve, which adapts the zone to your resting heart rate.

What is the Riegel race-prediction formula?

Riegel's 1981 formula scales finishing time with distance: t₂ = t₁ × (d₂/d₁)^1.06. It assumes consistent aerobic fitness and even pacing.

Can I trust the VO₂ output as my VO₂max?

No. The figure shown is the oxygen cost of the workout you entered, not your maximal aerobic capacity. A true VO₂max test requires lab equipment.

Does the calculator handle imperial units?

Yes — distance in km or miles, weight in kg or lb, height in cm or feet/inches. Outputs include both km/h and mph plus min/km and min/mile pace.

Does it work for treadmill incline?

Yes. For running and walking on a treadmill, you can enter incline as a percentage and the ACSM equation will adjust the oxygen cost.

Why is fat-vs-carb energy split estimated?

Approximate substrate use is inferred from intensity (% of HR reserve). Lower intensities use a higher proportion of fat; higher intensities use more carbohydrate.

Can children use it?

Equations were validated mainly on healthy adults aged 18–65. Estimates for children, pregnant individuals, or athletes with chronic conditions can be inaccurate.

Is my data sent to a server?

Inputs stay in your browser. Optional AI Insight requests are anonymised and proxied through the server with no permanent storage of personal data.

How often is the data reviewed?

The formulas, MET tables, and references are reviewed against current ACSM guidance at least once per year.

Does it replace medical advice?

No. The tool is educational. If you have heart, joint, or metabolic conditions, consult a qualified healthcare professional before changing your training.

Educational use only. Educational use only. The figures shown are population-level estimates from published exercise-physiology equations and may differ from your true values by 10–25%. Nothing on this page diagnoses, treats, prevents, or cures any condition. Consult a qualified healthcare professional before changing diet, exercise, or medication, especially if you have a heart condition, are pregnant, or take prescription drugs.

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