What is a swim interval calculator?

A swim interval calculator turns a pace anchor (usually CSS) into ready-to-swim sets: rep distance, rep count, target time per rep, and send-off. You get exact pool-clock numbers for every zone.

How is CSS pace calculated?

CSS is the slope between your 200 m and 400 m time trial: (400 − 200) / (T400 − T200) m/s, converted to seconds per 100 m. It approximates sustainable threshold pace — the speed you can hold for roughly 20 to 30 minutes.

What are swim training zones?

Anchored to CSS: easy (CSS + 20 s/100), aerobic (CSS + 10), threshold (= CSS), VO₂max (CSS − 5), and sprint (CSS − 15 or faster). Each zone targets a distinct training stimulus: recovery, endurance, lactate threshold, aerobic ceiling, and top-end speed.

How do I pick my CSS if I haven't time-trialed?

Estimate: take a recent 1500 m time and divide by 15, or use a race pace you can hold for ~20 minutes. Accuracy jumps once you do a real 200 + 400 time trial — reserve 15 minutes for the test once a month.

Does this work for yard pools?

Yes. Switch to yards; the calculator reports target paces per 100 yd and maps rep distances to conventional yard sets (400 m maps to 500 yd). Math stays in metric to keep CSS accurate.

Free Swim Interval Calculator — CSS Pace Sets & Send-Offs | AquaPlan

Free swim interval calculator. Enter your CSS pace or a 200 m + 400 m time trial and get ready-to-swim sets with target times and send-offs for easy, aerobic, threshold, VO₂max, and sprint zones.

Free swim interval calculator. Enter your CSS pace (seconds per 100 m) or a 200 m + 400 m time trial and the page derives five training zones with target paces and ready-to-swim sets. Five swim zones, anchored to CSS. Easy: CSS + 20 s/100, long rest, recovery pace. Aerobic: CSS + 10, short rest, endurance base. Threshold: CSS, short rest, lactate threshold work.

VO₂max: CSS − 5, equal work:rest, aerobic ceiling. Sprint: CSS − 15 or faster, full recovery, top-end speed. What is a send-off in swimming? Send-off is total time from one rep's start to the next — rep time plus rest. A set like 8 × 100 on 1:45 means leave every 1:45 regardless of rep time. The calculator rounds send-offs up to the nearest 5 s to match whiteboard convention.

What is CSS? Critical Swim Speed is the slope between a 200 m and 400 m time trial: (400 − 200) / (T400 − T200) m/s, converted to seconds per 100 m. It approximates the fastest pace you can hold for 20 to 30 minutes. Example sets at CSS 1:30 / 100 m. Threshold: 5 × 400 on 6:20, 8 × 200 on 3:15, 12 × 100 on 1:45. Aerobic: 6 × 200 on 3:25, 10 × 100 on 1:55.

VO₂max: 8 × 100 on 2:50, 12 × 50 on 1:25. Sprint: 10 × 50 on 2:05, 16 × 25 on 1:00. Yard pools. Switch the unit to yards; target pace shows per 100 yd. Rep distances map to conventional yard sets: 400 m becomes 500 yd. The underlying math stays metric so CSS is accurate. Related tools: Swim Pace Calculator for conversions and splits, SWOLF Calculator for stroke efficiency, Free Swim Workout Builder to wrap the set into a full session and export to PDF.