CTL / ATL / TSB Calculator

The Chronic Training Load (CTL), Acute Training Load (ATL), and Training Stress Balance (TSB) model — often called the Fitness-Fatigue Model or the Performance Management Chart (PMC) — is the gold standard framework used by endurance coaches and athletes worldwide to quantify training, manage fatigue, and time peak performance for race day.

CTL (Fitness) represents the long-term average of your daily training stress over roughly 42 days. Think of it as your aerobic base or fitness level. The higher your CTL, the more training your body has absorbed over the past six weeks. CTL changes slowly because it tracks a 42-day rolling average, which means it takes weeks of consistent work to raise it meaningfully — and weeks of rest to let it fall.

ATL (Fatigue) reflects your short-term training load over approximately 7 days. It responds quickly to what you have done in the past week. A hard training block drives ATL up rapidly; a few easy days bring it back down within days. ATL is your body's most recent stress signal, and it is the key driver of how tired you feel right now.

TSB (Form) is simply CTL minus ATL. When your fitness exceeds your current fatigue, TSB is positive and you feel good — fresh legs, sharp mind, ready to race. When ATL exceeds CTL, TSB goes negative, meaning fatigue is suppressing your ability to express your fitness. The art of periodization is building CTL high enough while managing ATL so that TSB lands in the optimal positive window on race day.

This calculator implements the standard exponentially weighted moving-average (EWMA) model used by platforms like TrainingPeaks, Strava, and Garmin to power their Performance Management Charts. The inputs are straightforward: your current CTL and ATL values, today's Training Stress Score (TSS), and the number of days until your target event. The calculator then shows your updated metrics and projects your race-day form assuming a standard taper at 50 TSS per day.

The Fitness-Fatigue Model uses an exponentially weighted moving average (EWMA) with different time constants for CTL and ATL. Each day, your current values are updated based on that day's Training Stress Score (TSS). The formulas are designed so that CTL decays slowly (42-day time constant) while ATL decays quickly (7-day time constant), capturing the intuition that fitness builds and fades gradually while fatigue accumulates and dissipates rapidly.

After entering your current CTL, current ATL, and today's TSS, the calculator computes your updated metrics. The TSB (Training Stress Balance), also called Form, is then found by subtracting ATL from CTL. A positive TSB means you are fresher than your fitness baseline — the ideal state for racing. A large negative TSB means accumulated fatigue is masking your true capabilities.

For the race-day taper projection, the calculator iterates the same update formulas for each day until your event, assuming a fixed 50 TSS per day during the taper period. This models a light, maintenance-level load that allows ATL (fatigue) to drop quickly toward a positive TSB while preserving most of your CTL (fitness).

Your TSB value tells you exactly where you stand in the training-to-racing continuum. The calculator displays one of five status labels based on your current TSB, each corresponding to a different physiological state and appropriate training response.

The optimal race-day TSB window is +15 to +25. Below +10 and you may still carry too much fatigue to race at your best. Above +25 and the taper has gone too long — your fitness has started to erode faster than fatigue is dropping. The goal of a well-designed taper is to land in that sweet spot where CTL remains high while ATL has fallen enough to produce a meaningfully positive TSB.

During base and build training phases, spending time in the Fatigued or Neutral zones is perfectly appropriate and expected. Elite endurance athletes regularly push their TSB to −30 or lower during intense training blocks, then rely on a structured taper to restore form before their key race. The Fitness-Fatigue Model gives you the numbers to manage this cycle deliberately rather than by feel alone.

One of the most powerful features of the CTL/ATL/TSB calculator is the race-day projection. By entering the number of days until your target event, the calculator simulates a standard taper at 50 TSS per day and shows you the projected CTL, ATL, and TSB you can expect on race morning.

The 50 TSS/day taper assumption models a light aerobic load — enough to maintain neuromuscular activation and aerobic pathways without adding meaningful fatigue. Because ATL has a 7-day time constant, it drops very quickly during a taper: in just 7 days of 50 TSS/day, ATL will already be more than halfway to its new equilibrium. CTL, by contrast, barely moves in 7 days thanks to its 42-day time constant. This asymmetry is exactly what makes tapering work — fatigue melts away while fitness stays nearly intact.

Use the projection to answer key periodization questions: How long should my taper be? Will a 10-day taper get me to a +15 TSB, or do I need 14 days? What if I add one more big training day before cutting back? You can experiment with your inputs and watch the projected race-day TSB change in real time.

As a general rule for endurance sports, a 7–14 day taper is appropriate for most athletes targeting a TSB of +10 to +25 on race day. Shorter events like 5 km and sprint triathlons often call for shorter tapers (4–7 days), while Ironman-distance events or marathon races may benefit from a 2–3 week taper period. The calculator lets you dial in your specific numbers rather than guessing from generic recommendations.

The weekly ramp rate is how much your CTL changes per week based on your planned training load. The calculator derives this from your weekly TSS plan: it averages your seven daily TSS values, then applies the same EWMA formula to estimate how much CTL will shift per week at that training load.

The formula is: weeklyRamp = (avgDailyTSS − CTL) / 42 × 7. If your average daily TSS equals your current CTL, your ramp rate is zero — you are maintaining. If your average daily TSS is higher than CTL, ramp rate is positive (building fitness). If lower, it is negative (losing fitness or recovering).

The safe ramp rate range is 3–7 CTL points per week. Going above 7 CTL points per week dramatically increases injury and overtraining risk, particularly for runners. Bone stress injuries, tendon overuse problems, and cardiovascular overreaching all become significantly more likely when training volume spikes too fast. The calculator flags your weekly ramp rate in red if it exceeds 7 CTL/week so you can adjust before your body forces you to.

The daily TSS targets panel provides three quick reference values based on your current CTL. To maintain fitness, target TSS equal to your CTL each day. To build fitness by approximately 10%, target CTL × 1.1 per day. On recovery days, aim for CTL × 0.5 — enough to keep the body moving without adding meaningful stress. These guidelines are starting points; actual TSS will vary by workout type, intensity, and individual response.

Tracking your weekly TSS across all seven days in the planner lets you spot imbalances before they become problems. A week heavily loaded on the weekend with nothing in between creates spiky ATL patterns that increase injury risk. Distributing load more evenly — even if the total TSS is the same — generally produces a smoother, more manageable fatigue curve and better long-term adaptation.

Using the CTL ATL TSB calculator is straightforward. Start by entering your current CTL (Fitness) and current ATL (Fatigue) values. If you use a platform like TrainingPeaks, Garmin Connect, or Today's Plan, these numbers appear directly on your Performance Management Chart. If you are starting fresh, a CTL of 40–60 is typical for a recreational endurance athlete; competitive age-groupers often sit in the 70–90 range; elite athletes may carry 120 or higher.

Next, enter today's TSS for your planned workout. TSS is calculated by your GPS device or training platform based on duration and intensity relative to your threshold. A one-hour ride at threshold is 100 TSS. A 90-minute easy jog might be 60 TSS. A two-hour hard group ride could be 160 TSS or more.

Enter the days until your target event to see the race-day projection. Then fill in your weekly TSS plan for all seven days of the week — this drives the ramp rate calculation and weekly total display.

Read your results in the output panels. The large TSB number and status label tell you your current form at a glance. The "After Today's Workout" panel shows how this session will move your metrics. The race-day projection shows where you will land if you taper at 50 TSS/day from today. And the daily TSS targets give you concrete numbers to target in your upcoming training days.