Acute:chronic workload ratio (ACWR) explained, with the evidence for and against

The acute:chronic workload ratio (ACWR) divides your training load from the last 7 days by your average weekly load over the previous 3 to 6 weeks. A value near 1.0 means this week matches what your body is used to. Gabbett (2016) called 0.8 to 1.3 a "sweet spot" and 1.5 or above a "danger zone" for injury, but later studies dispute that any range reliably predicts injury.

ACWR is still a useful way to see sudden jumps in training. This guide shows how to calculate it, what the common ranges mean, how devices use it, and what the critics say.

What is the acute:chronic workload ratio?

The acute:chronic workload ratio is a training-load metric that compares recent load (acute, usually the last 7 days) with longer-term load (chronic, usually the average of the last 3 to 6 weeks). A ratio above 1 means you are doing more than usual. A ratio below 1 means less. The idea is that sudden spikes, not hard training itself, carry the most risk.

Gabbett described the model in a 2016 review in the British Journal of Sports Medicine. His argument was that athletes used to high loads had fewer injuries than athletes on lower loads, and that excessive, rapid increases in load were likely responsible for a large share of non-contact, soft-tissue injuries. The chronic load stands for fitness, and the acute load stands for fatigue.

Term	Typical window	What it represents
Acute load	Last 7 days	Recent fatigue
Chronic load	Rolling average of last 3–6 weeks	Fitness, what your body is prepared for
ACWR	Acute divided by chronic	How this week compares with your usual

Caption: Gabbett TJ, Br J Sports Med 2016.

How do you calculate ACWR?

To calculate ACWR, first choose a load unit, such as minutes times effort rating, distance or a heart-rate-based score. Add up the last 7 days to get acute load. Average the weekly totals of the last 4 weeks to get chronic load. Divide acute by chronic. The result has no unit.

A worked example, using a load unit of minutes times effort (1 to 10):

Week	Weekly load
Week 1	300
Week 2	320
Week 3	310
Week 4	330
Chronic load (average of weeks 1–4)	315
This week (acute load)	450
ACWR	450 ÷ 315 = 1.43

In this example, the athlete jumped about 43% above their usual week. That falls above Gabbett's 0.8 to 1.3 range and close to his 1.5 danger threshold. A smaller increase over two or three weeks would keep the ratio near 1.1 to 1.2 while reaching the same weekly load.

Rolling average or exponentially weighted?

A rolling average gives every day in the window the same weight. An exponentially weighted moving average (EWMA) gives recent days more weight. Williams and colleagues (2017) showed that rolling averages can give the same ratio to athletes with very different load patterns, and proposed EWMA because it reacts more to a spike late in the window.

Method	How it weights days	Strength	Weakness
Rolling average	Every day equal	Simple to compute by hand	Misses when in the window a spike happened
EWMA	Recent days count more	Reacts faster to late spikes	Needs a decay constant, harder by hand

Caption: Williams S et al., Br J Sports Med 2017.

What is a good ACWR range?

The most cited range is 0.8 to 1.3, which Gabbett called the training "sweet spot", with 1.5 or above labeled a "danger zone". Garmin uses 0.8 to 1.4 as optimal. These ranges come mainly from cricket, Australian football and rugby league, and Gabbett himself warned to apply them to individual sports with caution.

ACWR	Gabbett (2016)	Garmin load ratio
Below 0.8	Below the sweet spot	Low
0.8–1.3	Sweet spot	Optimal (0.8–1.4)
1.3–1.5	Above sweet spot	Optimal up to 1.4, then high
1.5 or above	Danger zone	High (1.5–1.9), very high (2.0+)

Caption: Gabbett TJ, Br J Sports Med 2016. Garmin Forerunner 965 manual, as of October 2026.

Gabbett also noted that a ratio that is too low carries its own risk: under-training leaves athletes less prepared for hard sessions or competition.

Is ACWR scientifically valid?

The evidence is mixed. The International Olympic Committee's 2016 consensus on load and injury supported monitoring rapid changes in load, including the ACWR approach. In 2020, Impellizzeri and colleagues concluded there is no evidence supporting ACWR for reducing injury risk, and that the ratio adds noise and statistical artifacts. A 2021 editorial in Frontiers in Physiology called the literature controversial.

The main criticisms:

• No causal evidence: Impellizzeri et al. (2020) argue that no study has properly estimated a causal effect, so changing ACWR to change injury rates is conjecture.
• Ratio math: ratios have known statistical problems, and the acute week is part of the chronic window in many versions (mathematical coupling).
• Arbitrary windows and units: studies use different load measures (heart rate, session effort, GPS distance) and different windows, so results do not transfer easily.
• Mixed results on the sweet spot: the 2021 editorial cites a study in elite soccer players and pentathletes that found associations between load ratios and injury, but no support for a protective 0.8 to 1.3 zone.

A fair reading: ACWR is a reasonable way to notice sudden jumps in training, which coaches have long tried to avoid. It is not a validated injury predictor, and a "safe" number does not make a session safe.

How do Garmin and Apple Watch use ACWR?

Garmin shows a load ratio, defined as acute (short-term) load divided by chronic (long-term) load, with 0.8 to 1.4 labeled optimal. Apple Watch shows Training Load, which compares the last 7 days of workout intensity and duration with the previous 28 days and labels it from well below to well above, without showing a ratio.

Device	What it shows	Windows
Garmin	Load ratio with low, optimal, high, very high	Acute and chronic load, shown after about 2 weeks
Apple Watch	Training Load from well below to well above	7 days vs previous 28 days

Caption: Garmin and Apple documentation, as of October 2026.

Some people search for "cardio load" when they mean this kind of comparison. The idea is the same: today's or this week's cardiovascular work set against your recent history.

How should you use ACWR in practice?

Use ACWR as a speed limit for change, not as a safety guarantee. Increase weekly load gradually, notice when the ratio climbs well above your usual level, and pair it with recovery data. A high ratio together with falling HRV, rising resting heart rate and poor sleep is a stronger reason to ease off than either signal alone.

1. Pick one load unit and keep it. Mixing units breaks the comparison.
2. Watch the trend over weeks. A single high day is less important than a high week.
3. Plan returns carefully. After illness, injury or a holiday, chronic load is lower, so the same session creates a larger ratio.
4. Check recovery. Our recovery score guide explains how HRV, resting heart rate and sleep show how well you are absorbing the load.
5. Know the warning signs. If load stays high and recovery stays low for weeks, read overtraining symptoms.

Daily load itself is covered in what is strain. If you have pain that does not settle with rest, see a doctor or physical therapist instead of training through it.

How Svanu tracks training load

Svanu calculates training load from your daily strain score. Acute load is an exponentially weighted 7-day load, and chronic load is an exponentially weighted 42-day load, so this week is compared with roughly your last six weeks, the upper end of the 3 to 6 week window Gabbett described.

Daily strain itself is a 0 to 100 score from active energy, intensity-weighted workout load and steps in Apple Health. Each day also has a target strain band set from today's recovery and your own recent strain history. Together, recovery, strain and training load show how hard you trained, how well you absorbed it, and whether this week is a jump from your usual.

All scores are computed on the iPhone, and the core app is free. See how the pieces connect on the Svanu home page.