You know your critical swim speed. This is what to do with it: pick the repetition distance, how many you want, which zone and how you rest, and you get the target time for every repetition and the send-off times on the pace clock, one by one. The ones you write on your hand before you get in.
If you do not know your CSS, work it out first from 200 and 400 m tests in the pace and CSS calculator. The fixed send-off is what almost every squad uses: you go when the clock hits the number, however long you took, and whoever swims slower rests less.
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The column that matters is the send-off: it is the pace clock time at which you push off the wall, counting from zero on the first repetition. The arrival column is when you should touch if you are on target pace.
| Repetition | Goes at | Touches at | Rests |
|---|
If you start arriving late halfway through the set, do not speed up the next one: that is the sign that the zone or the rest was wrong. Note how far you held it and drop a step next time.
Critical swim speed is the pace you can hold for a long time without accumulating fatigue at an accelerating rate. It is an excellent reference precisely because it is not a maximum time: it is a pace, and from a pace you can build any set by adding or subtracting seconds per 100 metres.
That offset in seconds is all the technique there is here. Swimming five seconds per 100 slower than your CSS is aerobic work; four seconds faster is VO₂ max. And because it is an offset rather than a percentage, the figures work the same for someone on 1:20 and someone on 2:10 per 100.
Z1, CSS + 15 seconds. Loosening off, technique, cool down. It should be hard to believe you are training.
Z2, CSS + 9 seconds. The day to day volume. Long repetitions — 400, 800, 1000 — with little rest. This is where the base is built and where most of your metres should be.
Z3, CSS + 5 seconds. Long distance race pace: roughly what a half or full Ironman swim is done at. A good zone for 200s and 400s on short rest.
Z4, at CSS. The zone that raises your sustainable ceiling, and the most profitable one if you do not swim much. Repetitions of 100 to 400 with 10 to 20 seconds rest. This is where the pace clock earns its keep.
Z5, CSS − 4 seconds. VO₂ max. Repetitions of 50 to 200 with generous rest, because the point is to hold the quality, not to accumulate fatigue.
Z6, CSS − 10 seconds or more. Pure speed and technique at speed. Repetitions of 25 and 50 with long rest. If you finish the set unable to hold your technique together, it was too much.
With fixed rest, you rest the same number of seconds after every repetition, however long you took. It is the comfortable option, and it is also what lets a set quietly fall apart: if you slow down, the rest waits for you all the same and the average intensity drops.
With a fixed send-off, you go when the clock hits the number, and the rest is whatever is left. That is the important difference: if you slow down, you rest less. The set regulates itself and forces you to pace. It is how every squad trains, and it is what this calculator proposes by default.
Send-off times are rounded to multiples of five, ten or fifteen seconds for a practical reason: the pace clock has a hand that goes round once a minute, and going "on the 40" is something you can read at a glance through fogged goggles. Going on 1:47 is not.
It depends on what you are training, and the rule is simple: the closer to CSS or above it, the more rest you need to hold the quality.
| Zone | Typical rest | Distances that fit |
|---|---|---|
| Z2 aerobic | 10 – 15 s | 200 to 1000 m |
| Z3 tempo | 15 – 20 s | 200 to 400 m |
| Z4 threshold | 10 – 20 s | 100 to 400 m |
| Z5 VO₂ max | 30 – 60 s | 50 to 200 m |
| Z6 speed | 45 – 90 s | 25 to 50 m |
Below five seconds there is no rest to speak of: that is swimming straight through with a pause to look at the clock. And above half the repetition time, in aerobic zones, the set stops being a set.
Going out too fast on the first one. The only pace that counts is the last one. If the first comes in five seconds under target, you did not do the set you meant to do.
Picking the zone by how you feel that day. That is what the fine tuning is for: if you turn up tired, add a second or two and do the whole set, rather than starting in the right zone and falling off halfway.
Using last year's CSS. If you have improved, every set you do is running slow and you do not know it. Repeat the tests every six to eight weeks.
Putting in too many repetitions. A quality set you hold all the way through is worth more than one twice as long where the last four fall apart. Start short and add one repetition a week.
It is the interval at which you start each repetition, counted from the start of the previous one. In 10×100 on 1:50 you go at 0:00, 1:50, 3:40 and so on, however long each one takes you. The rest is whatever is left over: swim the 100 in 1:35 and you rest 15 seconds; swim it in 1:45 and you rest 5.
Add or subtract the zone's seconds per 100 to your CSS and multiply by the repetition distance. With a CSS of 1:40 per 100, a Z4 set at CSS is 1:40 for a 100 and 3:20 for a 200; in Z2, with nine seconds more, it is 1:49 and 3:38.
Fixed send-off for almost everything, because it regulates itself: if you slow down you rest less, and that forces you to pace properly. Fixed rest makes sense in very hard speed sets, where what you want is to arrive recovered at each repetition and you do not care how long the whole set takes.
It depends on your total volume, but as a guide: between 1200 and 2500 metres of main set in an hour session is a reasonable range for a triathlete. Above 4000 metres of set you are into club swimmer volume, and it had better be at low intensity.
Because the pace clock on the wall has a hand that goes round once a minute, and round numbers can be read at a glance. Going "on the 30" or "on the 45" is instant even through fogged goggles; going on 1:47 makes you think and look twice, and in the middle of a hard set that does not happen.
Drop a step: add five or ten seconds to the send-off, or put a second or two into the fine tuning. There is no merit in doing a set where you touch the wall and leave again without breathing; the stimulus you were after is already gone and you are training something else.
Every six to eight weeks, or whenever the threshold sets start feeling easy. The 200 and 400 metre tests are done in a single session and go into the pace and CSS calculator.
If this one was useful, these three will be too.
CalculatorWork out your critical swim speed (CSS) from 200, 400 and 1000 m tests, your training zones, your per-100 paces and your open water time with a wetsuit.
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CalculatorWork out your SWOLF, your distance per stroke and your stroke rate, and find out whether a lower SWOLF actually made you faster or just slower and prettier.
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CalculatorEnter a 1 km, 5 km or 12 minute (Cooper) test and get your VO2max, your vVO2max, your training paces by zone and predicted times from 5 km to marathon.
Open →This calculator is an informational estimate and does not replace a coach or a laboratory test. Nothing you type ever leaves your browser.