Swimming calculator
Swim set calculator
Write a pool session line by line — reps, distance, stroke, send-off — and get the target time for every repeat, the rest left inside the interval, and what the whole set costs on the pace clock. Every target is priced off your own critical swim speed. Free, instant, no sign-up.
CSS 1:35 per 100 m — every target below is priced from these two trials
Your set
Send-offs are written in seconds because that is what the pace clock counts; the interval is shown beside the field as you type. A line whose send-off is shorter than the swim it has to contain is flagged rather than quietly averaged away.
CSS is measured on freestyle, so backstroke, butterfly, breaststroke and medley lines are stretched by the ratio of world-record pace at 200 m — the same effort at a slower pace. Treat those targets as a first draft and adjust them to your own stroke ratios after a session or two.
Writing one set is easy. The coach decides where it sits in the week and what the other sessions do around it.
What it means to swim a set at CSS
Critical swim speed is the quickest pace you can hold without falling behind on clearing lactate — swimming's threshold, and the anchor almost every distance set should be written around. Swimming "at CSS" does not mean swimming hard; it means holding one specific pace for the whole set, including the first repeat when you are fresh and the last one when you are not. That is the discipline the pace clock exists to enforce, and it is why a set written as "8 × 100 at CSS on 1:40" is a completely different session from "8 × 100 hard".
How this calculator works
Your two time trials go through the same critical-swim-speed model as our CSS calculator — imported here rather than copied, so the two pages can never disagree — to get a threshold pace in seconds per 100 m. Each line is then prescribed as an offset from that pace: CSS −2 for short repeats above threshold, CSS itself for the classic threshold set, CSS +2, +5 and +10 for progressively longer aerobic and recovery work. Multiply the resulting pace by the repeat distance and you have the target time; subtract it from the send-off and you have the rest. Non-freestyle lines are stretched by a stroke factor first, because CSS is measured on freestyle and a breaststroke 100 at the same effort is roughly a quarter slower.
How a send-off interval works
A send-off is the clock time from the start of one repeat to the start of the next, not the rest between them — so "8 × 100 on 1:40" means you leave the wall every 100 seconds whether the previous repeat took 95 seconds or 105. The rest is whatever is left over, which is exactly why the send-off is a training tool rather than a formality: hold the same target and shave five seconds off the interval and you have made the set harder without swimming a single length faster. Threshold sets usually run on five to fifteen seconds of rest; aerobic sets on ten to twenty; anything above thirty seconds is speed work with a different purpose. If you cannot make the send-off, the set is over — swimming through it teaches your body to slow down under fatigue, which is the opposite of the adaptation you came for.
Writing a set that actually works
Start from the main set and build outwards: pick the zone, pick the repeat distance, then choose the send-off that leaves the rest you want. Threshold work wants volume at one pace — 8 × 100, 5 × 200 or 3 × 400 with short rest, somewhere between 1200 and 2500 m of it. Put the warm-up and warm-down in at CSS +10, because a warm-up swum too fast costs you the main set, and add stroke or medley lines at CSS +5 rather than at threshold, since your non-freestyle pace is limited by technique long before it is limited by fitness. Then check the set total: if the clock time is over an hour, something in the middle is longer than it needs to be.