How to use a sports watch well
A sports watch is only as useful as the numbers you give it. Heart rate zones drive your training intensity, battery life decides how often you must charge, and race length decides whether the watch can record the whole event. This page explains each calculation so that you can set up a new watch quickly and compare models on facts instead of marketing.
Maximum heart rate
The old formula 220 minus age is easy to remember but can be off by 10 beats or more for an individual. The Tanaka formula, 208 minus 0.7 times age, fits large studies better, especially for people over 40. Neither beats a measured value from a hard, well-warmed-up effort or a supervised test, so if you have one, enter it directly.
Zones with the heart rate reserve method
The Karvonen method uses the difference between your maximum and resting heart rate, called the heart rate reserve. Because it includes your resting pulse, it adapts as you get fitter. Each zone boundary equals resting heart rate plus a percentage of the reserve.
Zone boundary = resting HR + % × reserve
| Zone | % of reserve | Purpose |
|---|---|---|
| 1 Recovery | 50–60% | Warm-up, cool-down, easy movement |
| 2 Endurance | 60–70% | Base building, long easy sessions |
| 3 Tempo | 70–80% | Steady, “comfortably hard” effort |
| 4 Threshold | 80–90% | Race pace for 10 km to half marathon |
| 5 Maximum | 90–100% | Short intervals and sprints |
Most people spend the majority of their weekly training in zones 1 and 2 and add a smaller amount of hard work in zones 4 and 5. These figures are general guides and not medical advice; anyone with a heart condition should ask a doctor before training hard.
GPS battery planner
Sports watches publish two battery numbers: days in smartwatch mode and hours in GPS mode. Your real charge interval combines them. Each day drains a fraction of the battery for ordinary wear, and every hour of GPS training adds the extra drain of GPS over normal wear.
Extra per GPS hour = 1 ÷ GPS hours − (daily drain ÷ 24)
Days per charge = 7 ÷ (7 × daily drain + weekly GPS hours × extra per hour)
Will it last your race?
Finish time is distance multiplied by pace. A watch should have a margin of at least 20–25% on top of your expected time, because a cold morning, strong notifications, music playback or a bright screen can lower real endurance, and because slow finishes happen. Ultra runners should look for dedicated “ultra” or “expedition” GPS modes, which trade accuracy for much longer runtime.
Battery needed = finish time × 1.25
Buying advice
- Prefer multi-band GPS if you run in cities or forests; it improves accuracy at the cost of battery.
- An optical wrist sensor is fine for steady running; use a chest strap for intervals and cycling.
- Check water resistance of at least 5 ATM if you swim.
- Long-term: a replaceable strap and software updates matter as much as the launch specs.