Formulas and sources
Each formula this calculator uses, where it comes from, and what it leaves out. "How it works" shows the same formula with your own numbers.
Pace
Pace mode
- Pace
- time per kilometer or per mile
- T
- the time you ran
- D
- the distance, in km or miles
Times and paces are shown to the nearest second. Switching between km and miles converts with 1 mile = 1.609344 km exactly.
Source: Standard formula; no single publisher. The mile is NIST's exact factor [1].
Finish time
Time mode
- Time
- the finish time
- Pace
- the pace you enter
- D
- the race distance
Source: Standard formula; no single publisher.
Distance
Distance mode
- D
- the distance covered
- T
- the time you enter
- Pace
- the pace you enter
Source: Standard formula; no single publisher.
Race splits
Pace mode
The splits are 1 km, 5 km, 10 km, the half marathon (21.0975 km) and the marathon (42.195 km). A pace per mile is turned into a pace per km first.
Source: The marathon is 42.195 km in World Athletics’ road race rules [2]; the half marathon is half of it.
Estimated maximum heart rate
With an age entered
For ages 10 to 100, rounded to a whole beat.
Source: A widely used estimate. Robergs and Landwehr (2002) found it was not derived from original research, with a typical error of 7 to 11 beats a minute [3]. Tanaka and colleagues (2001) found that it underestimates the maximum in older adults [4].
Heart-rate zones
With an age entered
- Zone 1: Very light
- 50% to 60% of Max HR
- Zone 2: Light
- 60% to 70%
- Zone 3: Moderate
- 70% to 80%
- Zone 4: Hard
- 80% to 90%
- Zone 5: Maximum
- 90% to 100%
Each bound is rounded to a whole beat, so neighboring zones share an edge. The zones split an estimate; they are not a measured test.
Source: Based on the five zones Garmin [5] and Polar [6] define for their own watches, at the same 10% bands; the names are Polar’s. They are those makers’ conventions, not a medical classification.
Sources
- National Institute of Standards and Technology. NIST Guide to the SI, Appendix B.8: Factors for units listed alphabetically, mile. Special Publication 811, 2008 edition.
- World Athletics. Book C, C1.1 & C2.1: Competition Rules & Technical Rules, Technical Rule 55.1, Road Races. Edition of July 1, 2026.
- Robergs RA, Landwehr R. The surprising history of the "HRmax=220-age" equation. Journal of Exercise Physiology online, 2002; 5(2): 1-10.
- Tanaka H, Monahan KD, Seals DR. Age-predicted maximal heart rate revisited. Journal of the American College of Cardiology, 2001; 37(1): 153-156.
- Garmin. Forerunner 265 Series Watch Owner's Manual, Heart Rate Zone Calculations. Version 9, June 2025.
- Polar. Heart Rate Zones. Undated guide, read October 5, 2026.
What this calculator does not cover
- It assumes one even pace over the whole distance, with no allowance for hills, wind, heat or fatigue. Race splits only scale the pace you entered.
- The maximum heart rate is an estimate from age alone, and the zones are bands of that estimate.
- It does not estimate your heart rate from your pace.
- It reports pace, not speed.
Sources checked on .