Five zones, one fitness number
The zones are derived from a single race result, so they move together when fitness changes. That is the point: if your 10K improves by a minute, every zone shifts, and you do not have to re-test five separate paces. Re-run the calculator after a race or a time trial rather than after every workout.
The 80/20 split
Roughly 80% of weekly volume should be easy and 20% quality. The zone table shows a suggested share for each: E takes 75–80%, M up to 15% for marathon training, T 5–8%, and I and R no more than 8% combined. These are guides, not laws, but the ratio is one of the most consistent findings in endurance coaching.
Why zones beat fixed paces
A pace that was threshold in March may be marathon effort in June. Zones tied to current fitness prevent the slow drift where every run becomes moderately hard — the pattern that produces plateaued performances and chronic fatigue.
Frequently Asked Questions
How often should I re-test?
Every 6–10 weeks, or after a race. Fitness changes slowly enough that weekly re-testing adds noise rather than information.
What if I do not have a race result?
Run a 5K time trial on a measured loop, all out, and use that. It is uncomfortable but it is the cheapest accurate input.
Are heart rate zones the same?
No. These are pace zones derived from race performance. Heart rate zones use percentages of maximum heart rate and are affected by heat, caffeine and fatigue.
Related tools
Privacy & your data
SwimRunPace calculates everything in your browser. Distances, times and body weights are never uploaded — there is no account, no server-side record and no training history stored between visits.