Every training system needs a way to measure how much work you’re doing. Some use miles. Some use heart rate zones. Some use proprietary “training effect” scores that nobody can explain.

Pacewright uses a formula you can calculate in your head.

The Formula

Training Load = Duration (minutes) × RPE (1-10)

That’s it. A 45-minute easy run at RPE 4 produces a load of 180. A 30-minute tempo at RPE 7 produces a load of 210. The tempo was shorter but harder — and the training load reflects that.

This approach — called session RPE — was validated by Foster and colleagues and is now one of the most widely used training load metrics in sports science. It correlates strongly with heart rate-based methods but doesn’t require any equipment. You need a clock and an honest answer to “how hard was that?”

Why Not Heart Rate? Why Not Pace?

Heart rate is affected by caffeine, heat, dehydration, stress, sleep, and medication. The same effort produces different heart rates on different days, making it an unreliable load measure unless you control for everything else.

Pace measures speed, not effort. Running 8:30/mile on a cool flat road and running 8:30/mile uphill in 90°F heat are radically different efforts — but pace calls them equal.

RPE captures what you actually experienced. It automatically accounts for hills, heat, fatigue, illness, stress, and every other variable that affects how hard a run feels. That’s why Pacewright uses it as the foundation of everything.

How Load Becomes Safety

A single session’s load is useful. But the real power is in tracking load over time using an exponentially weighted moving average (EWMA):

  • Acute load (7-day EWMA): Your recent fatigue — how hard you’ve been training this week
  • Chronic load (28-day EWMA): Your fitness baseline — what your body is adapted to

The ratio between these two numbers is your ACWR (acute-to-chronic workload ratio). When acute load spikes relative to chronic load — meaning you suddenly did a lot more than your body is used to — injury risk climbs sharply. That’s the guardrail system behind every workout Pacewright prescribes.

The ACWR article covers the zones and thresholds in detail. The key point here: all of it flows from Duration × RPE. The simple formula powers the entire safety system.

What About Rest Days?

Rest days contribute a load of zero — and that zero matters. The EWMA includes rest days in its calculation, which naturally lowers your acute load and gives your body credit for recovery. Taking a rest day isn’t just “not training.” It’s actively bringing your load ratio back toward the safe zone.

This is why Pacewright mandates minimum rest days. They’re not wasted time — they’re load management.