What training zones actually do

Zones turn intensity into something you can pace. Heart rate, power, and running pace each get split into ranges that map to different physiological responses. Zone 2 builds aerobic base. Zone 4 raises lactate threshold. Zone 5 pushes VO2max.

Most people training without zones end up in Zone 3 most days. Feels productive, finishes sweaty, builds neither real endurance nor real speed. Coaches call it the grey area for a reason.

How the numbers come out

The calculator picks a model based on what data you give it:

• Heart rate zones use Karvonen (Heart Rate Reserve), which factors in your resting HR rather than just max. More accurate than 220-minus-age.
• Power zones use Coggan's 7-zone model, calculated from FTP. The cycling standard.
• Pace zones come from your 5K time as a threshold proxy. The fallback when you don't have HR or power data.

The fatigue slider drops upper bounds by up to 10%. When you're carrying load, your body produces the same physiological stress at lower outputs. Same effort, smaller numbers.

When to recalculate

Re-test every 6–8 weeks during a build block, or after any benchmark race. Stale zones quietly mis-pace your training: easy days creep up, intervals come up short. A fresh test takes 20 minutes and recalibrates everything.