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Your chronic training load (CTL, your "fitness"), your acute training load (ATL, your "fatigue") and the gap between them (TSB) are the most common way to anticipate how you'll show up on race day. Tell me where you stand today, how much TSS you plan to do per week and how many weeks you have left, and this calculator projects how all three evolve, week by week, up to race day.
If you don't know your CTL and ATL, most watches and platforms connected to a power meter or heart rate monitor calculate them for you from your training history. If you don't have them, leave the defaults — they represent a moderate baseline — and focus on how the TSB changes rather than the exact starting figure.
Each row is the state at the end of that week, after spreading the weekly TSS evenly across its seven days. A week's TSB is the form you start the next week with, not the form at the midpoint of that week.
| Week | TSS | CTL | ATL | TSB |
|---|
The last row already has that week's reduction applied. If you race midway through a taper week, your real TSB that day will fall between the previous row's and this one's.
Three numbers that boil down your recent training history into a single load-per-day figure. TrainingPeaks popularised them from Bannister's impulse-response model, later adapted by physiologist Andrew Coggan to TSS (Training Stress Score) — the same metric behind this site's session fuelling plan calculator.
The underlying idea is simple: the same workout makes you fitter in the long run and more tired in the short run, and what decides whether you're ready to race is the gap between the two, not either one on its own.
CTL (Chronic Training Load): your fitness. A daily-TSS average weighted with a 42-day time constant: last week's training barely counts, the last six weeks count a lot. It rises and falls slowly, the way real fitness should.
ATL (Acute Training Load): your fatigue. The same kind of average, but with a 7-day time constant. It reacts fast: one hard week spikes it, and a few easy days bring it back down almost as fast.
TSB (Training Stress Balance): CTL minus ATL. Positive means your recent fatigue is lower than your underlying fitness — you're fresh; negative means accumulated fatigue outweighs your fitness — you're loaded.
Every day, CTL and ATL update with the same formula — only the time constant changes:
CTL(today) = CTL(yesterday) + (TSS(today) − CTL(yesterday)) / 42
ATL(today) = ATL(yesterday) + (TSS(today) − ATL(yesterday)) / 7
This calculator spreads the weekly TSS you enter evenly across its seven days — it doesn't distinguish a week with two very hard sessions from one with six easy sessions of the same total — and applies that formula day by day up to your target week. It's the same simplification any planner makes when working in weekly blocks rather than session by session.
The final taper is the main lever for arriving fresh: since ATL reacts in days and CTL in weeks, cutting TSS in the last week pulls ATL down much faster than CTL, and TSB climbs. That's exactly the mechanism any taper week before a race is chasing.
Try different taper percentages with your own CTL, ATL and weekly TSS numbers to see how much you need to cut the load, and how far out, to land in the form range you're after. A common reference point — not a fixed rule — for race day is a TSB between +5 and +25.
TSS is a combined measure of external and internal load, but it doesn't distinguish the type of stimulus: 400 TSS of short, very hard intervals doesn't fatigue you the same way as 400 TSS of a long easy ride, even though the model treats them as equal. It also doesn't account for sleep, stress outside training, or illness.
Spreading the weekly TSS evenly across seven days is a deliberate simplification: in reality you'll concentrate the load into two or three sessions and rest the rest, which makes your real ATL swing more than this week-by-week projection shows. And the model assumes you start exactly at the CTL and ATL you enter — if those numbers are a rough guess, the result inherits that margin of error.
If you train with a power meter or heart rate monitor and upload your sessions to TrainingPeaks, Intervals.icu, Garmin Connect or similar, those platforms calculate them for you from your training history and display them directly. If you don't keep that kind of log, use the default values as a reasonable starting point and pay attention to how the TSB changes rather than the exact starting figure.
As a common reference point, between +5 and +25 is often described as the zone where most good results happen: fresh, without having lost fitness. Above +25 you may be too rested if you've sat there for several weeks; below −10, loaded. It's not a fixed rule — some athletes race well with a neutral or even slightly negative TSB, especially over very long distances.
Because the 42-day time constant is designed to do exactly that: CTL represents an underlying adaptation, and real adaptation doesn't show up in a week. If you spike weekly TSS suddenly, what moves fast is ATL — fatigue — and TSB drops hard even though CTL barely shifts. That's precisely the signal that you're in a loading block, not yet cashing in the fitness.
It depends on how much ATL you're carrying and how many days of margin you have before racing. Cuts of 40-60% in the final week are common for middle- and long-distance races; for shorter events, with less accumulated fatigue, a gentler 20-30% cut can be enough. Try several percentages in the calculator with your real load and see what final TSB each one gives you.
It works for any discipline where you can express effort as TSS or an equivalent load unit (rTSS for running, sTSS for swimming, a generic TRIMP). In triathlon, the usual approach is to add up the TSS from all three disciplines into a single daily load, which is exactly what this calculator expects.
The model keeps piling on the same weekly TSS right up to the end, so ATL stays high and race-day TSB usually ends up negative or, at best, neutral. It's a useful way to see how much fatigue you'd be carrying "if you did nothing special" before the race, and to compare it against different taper amounts.
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