What Is ACWR? The Runner's Guide to Injury Prevention's Most Useful Number

ACWR stands for Acute-to-Chronic Workload Ratio. It compares how much you have trained recently to how much you normally train.

Acute load = your training stress over the last 7 days. Chronic load = your average training stress over the last 28 days. ACWR = acute ÷ chronic.

If you normally run 40km per week and you ran 40km this week, your ACWR is 1.0. You are training at your baseline. If you ran 52km this week, your ACWR is 1.3. You are doing 30% more than your body is used to. If you ran 28km this week, your ACWR is 0.7. You are doing less.

The concept was developed by sports scientist Tim Gabbett and published in his landmark 2016 paper. His research across multiple sports found a clear relationship: when ACWR goes above 1.5, injury risk increases 2-4 times. Between 0.8 and 1.3, injury risk is lowest. This range is called the sweet spot.

The 10% rule says never increase mileage by more than 10% per week. It is simple and memorable, but it fails in important situations.

At 20km per week, 10% is just 2km — barely meaningful. At 100km per week, 10% is 10km — a big jump that an adapted runner can handle but a returning-from-injury runner probably cannot. The rule treats both situations identically.

ACWR fixes this because it accounts for your individual training history. A 10km increase for a runner with a high chronic load (built over months of consistent training) barely moves their ACWR. The same 10km increase for a runner coming back from 3 weeks off could spike their ACWR above 1.5.

The key insight: your safe training increase depends on your baseline, not a fixed percentage. Runners with a higher chronic load can absorb larger increases. Runners with a lower chronic load — from time off, inconsistency, or low volume — need to be more conservative.

For runners, ACWR works best when training load accounts for both distance and intensity. Running 50km at easy pace is very different from running 50km with 15km of tempo work. Raw mileage misses this distinction.

Training Stress Score (TSS) solves this by weighting each run based on duration and intensity relative to your threshold pace. A hard 10km tempo generates more TSS than an easy 15km jog. TSS captures the actual physiological stress of each session.

There are two ways to calculate the ratio:

Simple rolling average: Add up TSS for the last 7 days, divide by the average weekly TSS over the last 28 days. Straightforward but treats every day equally.

Exponentially Weighted Moving Average (EWMA): Gives more weight to recent days. If you did a big session 2 days ago, EWMA picks this up faster than the simple method. This is the preferred method in current research (Menaspà, 2017) and what injury.vision uses.

Both methods produce a single number. The interpretation is the same regardless of method.

Below 0.8 — Undertrained. You are doing significantly less than your body is used to. This is not necessarily dangerous in the short term, but sustained periods below 0.8 mean your chronic load is dropping, which makes you more vulnerable to spikes when you ramp back up. A recovery week might put you at 0.7 temporarily — that is fine. Several weeks at 0.6 means you are detraining.

0.8 to 1.3 — The sweet spot. This is where you want to be most of the time. Training stimulus is high enough to improve fitness but not so far above your baseline that tissue damage outpaces repair. The exact target depends on your goals — building fitness means working the upper end (1.1-1.3), maintaining means staying around 1.0, recovering means the lower end (0.8-0.9).

1.3 to 1.5 — Caution zone. You are doing more than your body is accustomed to. This does not guarantee injury, but the risk is elevated. If you are in this zone, it should be temporary and intentional (a hard race week, a planned overreach) — not an accident.

Above 1.5 — Danger zone. Gabbett's research found that athletes in this range were 2-4 times more likely to get injured in the following week. For runners, where 80% of injuries are overuse injuries, this relationship is even more significant. If your ACWR is above 1.5, reduce this week's training immediately.

Check before you plan. Before adding a hard session or increasing mileage, check your current ACWR. If you are already at 1.2, this is not the week to add a new tempo session or jump up 10km.

Build the denominator. The safest way to train harder is to build a higher chronic load through weeks of consistent training. A runner with a chronic load of 400 TSS per week can absorb a 120 TSS long run easily. A runner with a chronic load of 150 TSS cannot. Consistency is protection.

Respect time off. After any break — injury, illness, holiday — your chronic load drops while your perceived fitness stays high. Even a normal first week back can spike your ACWR dangerously. After 1 week off, start at 70% of your pre-break volume. After 2-3 weeks off, start at 50%.

Use a tool. You can calculate ACWR manually with a spreadsheet, but it is tedious and easy to get wrong. injury.vision computes your ACWR automatically from Strava or Garmin data after every run and flags when planned sessions would push you into the caution or danger zone. The What-If Planner lets you test a planned run before you do it.