Running Injury Risk Calculator
Check whether your weekly mileage is ramping up too fast using the 10% rule and the acute:chronic workload ratio (ACWR).
How it works
ACWR = this-week load ÷ 4-week average load · 10% rule: next week ≤ last week × 1.10
Most running overuse injuries trace back to doing too much, too soon rather than to any single run. The acute:chronic workload ratio compares this week (acute load) with the rolling 4-week average (chronic load); ratios of roughly 0.8–1.3 sit in the "sweet spot" with the lowest injury rates, while ratios above 1.5 sharply raise risk. The classic 10% rule is a simpler cousin: cap each week within 110% of the last. Use both as guardrails, not guarantees.
Worked example
You averaged 20 mi/week over the past month and ran 20 mi last week, but plan 24 mi this week. ACWR = 24 ÷ 20 = 1.20 (still the sweet spot), yet the week-over-week jump is (24−20)/20 = +20%, which exceeds the 10% cap of 22 mi. Trimming to 22 mi keeps both metrics in bounds.
Sources: Gabbett, Br J Sports Med (acute:chronic workload ratio)
Frequently asked questions
What ACWR is considered safe?
Research on training load suggests an acute:chronic workload ratio between about 0.8 and 1.3 carries the lowest injury risk, 1.3–1.5 is a caution band, and above 1.5 is associated with a marked jump in soft-tissue injuries. It is a population-level guide, not a personal guarantee.
Is the 10% rule outdated?
The 10% rule is a rough heuristic, not a law. It is a good sanity check for beginners, but the workload ratio captures your recent training history better. When the two disagree, favour the more conservative number, especially if you are returning from a break or injury.
What else drives running injuries?
Beyond mileage, sudden increases in intensity or vertical (hills), inadequate sleep, worn-out shoes, and poor recovery all contribute. Load management lowers risk but does not replace strength work, rest days and listening to early niggles.
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Educational estimate only, not medical advice. Recovery, hydration, heat and cold tolerance vary widely between individuals — stop any protocol that causes pain, dizziness or distress and consult a qualified professional.