What is ACWR (acute:chronic workload ratio)?
The ratio of your last 7 days of training load to your 28-day average. Values between 0.8 and 1.3 are the commonly cited sweet spot; above 1.5 injury risk rises steeply.
ACWR compares what you have just done with what you are used to doing. The acute term is the most recent week of training load; the chronic term is the rolling average of the last four weeks. A ratio of 1.0 means this week looks like a typical recent week. Above 1.0 you are doing more than your body has been prepared for; below it you are doing less.
The reason it caught on is that the relationship with injury is not linear. Loads inside roughly 0.8–1.3 sit in a low-risk band. Beyond about 1.5 the hazard climbs sharply — a step change rather than a gradual slope — which gives coaches a concrete number for "too much, too soon".
The low end matters too. Ratios well below 0.8 mean detraining, and a runner who drops volume sharply and then returns to their previous level is walking straight into a spike. Most return-from-injury problems are ACWR problems in disguise.
Two honest caveats. ACWR is a population-level association, not an individual prediction — plenty of runners exceed 1.5 without incident, and plenty get injured at 1.0. And the metric is sensitive to how you measure load in the first place; a load model that under-weights intensity will let a hard week look calm.
Key numbers
| Sweet spot | 0.8–1.3 | low-risk band |
|---|---|---|
| Danger zone | >1.5 | stepped increase in injury hazard |
| Windows | 7-day acute, 28-day chronic |
Work it out for yourself
Training adaptation map — See where a given session puts your ACWR, and what it buys you in return.
Frequently asked questions
What is a safe ACWR?
Roughly 0.8 to 1.3 is the commonly cited low-risk band. Treat it as a guide rather than a rule: it is derived from group data across sports, and individual tolerance varies widely with training history, age and injury background.
How fast can I increase my mileage?
Keeping weekly increases modest enough that the acute:chronic ratio stays under about 1.3 is a more useful guide than the traditional 10% rule, because it accounts for what your recent four weeks actually looked like rather than just last week.
Sources
- Gabbett (2016). The training-injury prevention paradox. British Journal of Sports Medicine.
- Hulin et al. (2016). The acute:chronic workload ratio predicts injury. British Journal of Sports Medicine.
Related explainers
Glossary: ACWR (Acute:Chronic Workload Ratio)