Training Load Calculator

Why the 10% rule isn't enough

The 10% rule — "don't increase weekly mileage by more than 10% from one week to the next" — is the most widely repeated piece of running advice in print. It's also the most criticized in modern sports-science literature, for two reasons.

It ignores your training history.A 10% jump from 5 to 5.5 miles is a different ask than a 10% jump from 50 to 55. More importantly, the rule treats every week the same — it doesn't care whether you've been consistent or whether you just took two weeks off. After a layoff, your tissues de-adapt, and a 10% bump from your last week can still be a 40% jump from your real recent average. That mismatch is where most comeback injuries happen.

It ignores cumulative load. Four straight weeks of +10% means a 46% increase over a month with no recovery. The tissue stress builds week over week even though no single jump looks aggressive. The 10% rule has nothing to say about this — it only looks at the previous week.

What ACWR does instead

The Acute:Chronic Workload Ratio compares your acute load(next week's planned volume) to your chronic load (the rolling average of the previous four weeks). The math is simple:

ACWR = next week ÷ avg(last 4 weeks)

The thresholds come from research in running, soccer, rugby, and Australian rules football, where injury databases were big enough to find clear patterns:

• < 0.8 — Undertraining.You're losing fitness, or this is a planned deload week.
• 0.8 to 1.3 — Sweet spot. Productive training with the lowest observed injury rates.
• 1.3 to 1.5 — Caution.You're pushing faster than the safe range. Hold for a week or trim back slightly.
• > 1.5 — High injury risk. Studies have repeatedly linked this zone to a sharp rise in soft-tissue and bone-stress injuries in the following 1–2 weeks.

Because chronic load is a 4-week average, ACWR automatically adjusts for missed weeks. If you took a vacation, your safe ceiling for next week comes down with you — exactly what your body needs.

How the progression plan works

Below the ACWR readout, the calculator builds a 12-week mileage progression toward your goal. Three rules drive the plan:

• Experience-adjusted weekly increase.Beginners cap at 7% per week, intermediates at 10%, and advanced runners at 12%. The 10% rule lives on as a default for the middle bucket — it's a reasonable ceiling for most runners.
• Deload every fourth week. Every 4th week drops 25% from the previous build week. This is non-negotiable in well-designed plans and is the single biggest difference between a plan that works for 12 weeks and one that breaks down at week 5.
• Build resumes from the pre-deload week. After a deload, the next build week grows from the prior build mileage, not from the deload itself. This is what keeps the trajectory moving up instead of stalling out.

How to use this calculator

1. Enter the last 4 weeks of weekly mileage. Leave any missed week blank.
2. Enter what you plan to run next week. Watch the ACWR and risk band update as you change it.
3. Enter your goal weekly mileage (peak training volume) to see the full 12-week plan.
4. Pick the experience level that best matches you — it controls how aggressively the plan ramps.
5. Use the "Copy link" button to share your numbers with a coach or training partner.

Common situations

Coming back from a break

Don't resume at your previous peak. Enter the actual mileage you've done in the last 4 weeks (which may be 0 for some of them). The chronic average will be low, and the calculator will keep your safe ceiling sensible. Plan on 4–6 weeks to rebuild to your prior level — frustrating but far cheaper than another injury.

Building toward a marathon block

Use the goal field to enter your peak training week (often 50–70 miles for amateurs, more for advanced runners). The calculator will show whether 12 weeks is enough runway given your current mileage and experience. If "goal reached" falls past week 12, you need either a longer build or a more conservative peak.

Staying healthy at maintenance

Even at maintenance volume, take the deload week every fourth week. ACWR < 0.8 on a deload is correct and expected — that line on the chart is what allows the next three build weeks to stay productive and uninjured.

Limitations and what this doesn't replace

ACWR is one signal, not the whole picture. It doesn't know about intensity (a 30-mile week of all intervals is harder than 30 easy miles), terrain, sleep, illness, life stress, or strength training. Treat the readout as a sanity check on your plan, not as a prescription.

If you're managing a current injury or just coming off one, work with a physical therapist or coach. The calculator is designed for healthy runners planning their week — not for return- to-run protocols.

Related tools

• VDOT calculator — set the paces to run inside each weekly mileage target.
• Pace calculator — pace, time, and distance arithmetic for any single run.
• Race time predictor — make sure your goal mileage matches a realistic race outcome.
• Heart rate zone calculator — intensity control on easy days so volume stays sustainable.