Stop swimming lengths. Start training to a number.
Two time trials give you a threshold pace, and suddenly every set has a target instead of a feeling. The calculators here are free and show their working — the formula, the study it came from, and what it cannot tell you.
The one test worth doing this week
Critical Swim Speed turns a 400 and a 200 into your aerobic threshold pace and five training zones. It takes forty minutes once, and it changes every session afterwards.
Critical Swim Speed calculator
Two time trials, one subtraction. The result is your aerobic threshold pace and the five training zones built from it.
The common protocol. Both trials are long enough to be paced, so the result is stable week to week. Rest 5–10 minutes between them.
1.000 m/s — the pace you could theoretically hold indefinitely. In practice it lands close to your best 1500 m pace.
| Zone | % of CSS | Pace / 100 m | What it is for |
|---|---|---|---|
| Recovery | 70 % | 2:22.86 | Flushing between hard sets and on easy days. Should feel almost lazy. |
| Endurance 1 | 80 % | 2:05.00 | Aerobic base. The pace you can hold while still thinking about technique. |
| Endurance 2 | 88 % | 1:54.29 | Aerobic capacity. Steady work, breathing hard but controlled. |
| Threshold | 95 % | 1:45.26 | The zone that actually moves CSS. Sustainable but uncomfortable. |
| VO₂max | 105 % | 1:35.24 | Maximum aerobic power. Short, hard, expensive — use sparingly. |
Percentages apply to speed, not to the clock. Recovery at 70 % of CSS is a slower pace than threshold, which is why the seconds go up as the percentage goes down.
What breaks each one, and the drill that fixes it
Technique advice is full of slogans that tell you how it should feel and never what to move. Each stroke here names the fault first.
Freestyle
The fastest, most efficient stroke — and the one worth fixing first.
Breaststroke
Timing over power — glide long and whip fast.
Butterfly
Rhythm and undulation — power that comes from the core.
Backstroke
Stay long, rotate, and let the water hold you up.
Numbers you can check
Every tool prints the formula and links the source. A number you cannot verify is a number you should not train against.
Critical Swim Speed
Two trials, five training zones, and the study behind the formula.
Course converter
Both methods side by side, plus the wall count that explains the gap.
SWOLF
Your score, your distance per stroke, and why it only compares to itself.
Workout generator
Your pace and the time you have become a full session, distances included.
Interval calculator
A pace and a rest become the “on 1:40” you actually leave on.
Swim pace
Pace per 100, speed in m/s, and what the clock should read at each split.
World Aquatics points
Official 2026 base times: score a race, or find the time a target needs.
Time standards by age
Every USA Swimming B to AAAA cut, and how far a time is from the next one.
Triathlon swim
Pool pace to race split, plus the wetsuit and time limits from both rulebooks.
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