Why the numbers are estimates

Runsei's zones, paces, load and scores are computed from your own data, and they're honest estimates — they can be off, and they shift from day to day with heat, sleep, fatigue, altitude and more. Use them as a guide, not an exact measurement. Where a value rests on thin data, Runsei says so (and often suggests a field test to sharpen it) rather than presenting a guess as fact.