How Much Should You Increase Mileage Per Week?

The 10% rule is the most widely cited guideline in distance running: never increase weekly mileage by more than 10%. It is simple, memorable, and wrong in enough situations to be unreliable as your primary safety mechanism.

At 20km per week, 10% is 2km. That is not even a meaningful run. At 100km per week, 10% is 10km, which is a substantial jump that a well-adapted runner can handle but a returning-from-injury runner probably cannot. The rule treats both situations identically, which is the fundamental problem.

Nielsen et al. (2012) studied 874 novice runners and found that increases above 30% per week significantly elevated injury risk. Below 30%, the relationship between increase rate and injury was much weaker and depended heavily on each runner's individual training history. This finding suggests that the 10% rule is overly conservative for some runners and not conservative enough for others.

The better question is not "how much can I increase?" but "what would this increase do to my ACWR?"

ACWR (Acute-to-Chronic Workload Ratio) compares your last 7 days of training to your rolling 28-day average. If your ACWR stays between 0.8 and 1.3, you are in what Gabbett's research calls the "sweet spot" where training stimulus is high enough to drive adaptation but not high enough to spike injury risk.

This reframes the mileage question entirely. A runner with 8 weeks of consistent 60km training has a high chronic load. Adding 8km (a 13% increase) barely moves their ACWR. The same 8km added to a runner who has been running 25km per week for the last month pushes their ACWR to 1.32, right at the boundary of the danger zone.

The safe increase depends on your denominator. Runners with a higher chronic load, built through weeks of consistent training, can absorb larger absolute increases safely. Runners with a lower chronic load, whether from lower volume or inconsistency, need smaller increases relative to their baseline.

Here is a framework based on your current ACWR, which you can check daily in injury.vision or calculate from your own data:

ACWR below 0.85: You are undertrained relative to your baseline. You can increase volume meaningfully this week. Aim to bring your ACWR up to 0.95 to 1.05, which typically means a 15 to 25% increase depending on how low the ratio is.

ACWR 0.85 to 1.05: You are in the optimal zone. Moderate increases of 5 to 15% are generally well tolerated. This is the range where consistent, progressive building happens.

ACWR 1.05 to 1.2: You are in the upper safe zone. Small increases only, or hold steady. If you have a race or key session coming up, this is not the week to also add mileage.

ACWR 1.2 to 1.3: You are at the boundary. Do not increase. This week should be a maintenance or slight reduction week to bring the ratio back down before building again.

ACWR above 1.3: Reduce this week's volume. An easy week is not wasted training. It is protecting the training you have already done from being undone by an injury.

The What-If Planner in injury.vision lets you input a planned run and see exactly how it would affect your ACWR and risk score before you lace up. Instead of guessing at percentages, you get a specific, personalised answer.

Returning from injury or time off. Your chronic load has dropped, which means your denominator is small and any training feels like a spike. After 1 week off, start at roughly 70% of your pre-break volume. After 2 to 3 weeks, start at 50%. After 4 or more weeks, treat it as a return-to-running protocol starting at 30 to 40% with walk-run intervals.

Building for a race. In the final build phase before a goal race, you may need to push ACWR to 1.2 to 1.3 temporarily to hit peak training volumes. This is acceptable if you have a strong chronic load base from several months of consistent training. The risk is higher, but the training benefit is intentional. Follow it with a taper that brings ACWR back below 1.0.

Returning from a down week. After a planned recovery week (reduced volume), your acute load drops but your chronic load holds relatively steady. The first week back can feel like a spike even though you are just returning to normal. Be cautious: ACWR may read 1.2+ on the first day of your big week back. Distribute the volume across the week rather than frontloading it.

Heat, altitude, or surface changes. Environmental stressors add physiological load that TSS does not capture. Running at altitude, in high heat, or transitioning from road to trail all increase the effective stress per kilometre. Treat these weeks as higher-load than the numbers suggest, and be more conservative with increases.