Critical Swim Speed Calculator
Estimate critical swim speed from matched 200 m and 400 m tests, with pace per 100 metres and a modelled repeat-distance comparison.
Pool performance tests workspace
Comparable 200 m and 400 m pool tests
Your pool critical-speed estimate
00:02:00 critical pace per 100 m, hh:mm:ss
Two-point 200 m / 400 m model
Full result table
| Repeat metres | Constant-CSS seconds | hh:mm:ss |
|---|---|---|
| 25 | 30 | 00:00:30 |
| 50 | 60 | 00:01:00 |
| 100 | 120 | 00:02:00 |
| 200 | 240 | 00:04:00 |
| 400 | 480 | 00:08:00 |
| 800 | 960 | 00:16:00 |
| 1,500 | 1,800 | 00:30:00 |
Two-point estimate with no independent fit-quality statistic. Tests should use the same stroke, pool length and timing convention. Average 200 m speed must exceed 400 m speed and the fitted intercept must be positive. Repeat times are arithmetic scenarios, not safe training prescriptions.
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How to use Critical Swim Speed Calculator
- Enter completed 200-metre and 400-metre test times in seconds.
- The slope between the two time-distance points gives critical speed, while the intercept is a fitted distance capacity.
- Pace per 100 metres is a unit expression of the fitted speed.
Worked example
At 210 s for 200 m and 450 s for 400 m, CSS = 200/240 = 0.833333 m/s, equivalent to 120 seconds per 100 m.
Formula, accuracy & limitations
Critical speed = (400 − 200)/(t400 − t200) m/s. CSS pace per 100 m = (t400 − t200)/2 seconds. Distance intercept D′ = 200 − CS × t200. Constant-CSS repeat time = repeat metres / CS.
Two-point estimate with no independent fit-quality statistic. Tests should use the same stroke, pool length and timing convention. Average 200 m speed must exceed 400 m speed and the fitted intercept must be positive. Repeat times are arithmetic scenarios, not safe training prescriptions.
These outputs use the stated model and your supplied inputs. A mathematically consistent model can still differ from personal measurements. Scenario curves and formula comparisons are not statistical confidence intervals.
Source and method context ↗Use two comparable timed swims
Enter completed 200-metre and 400-metre test times in seconds. Keep stroke, pool length, turns and timing practice consistent. A 200-yard swim is not 200 metres, and a flying start should not be mixed with a different start convention without understanding the effect.
Inspect the fitted relationship
The slope between the two time-distance points gives critical speed, while the intercept is a fitted distance capacity. With only two points there is no independent residual check; the line fits by construction. The calculator therefore reports no invented R² confidence claim.
Read pace and repeat scenarios separately
Pace per 100 metres is a unit expression of the fitted speed. The selected repeat time and distance table simply extend that constant speed. They do not include rest periods or guarantee that a swimmer can safely sustain the estimate for any chosen distance.
Know what the two-point model cannot verify
The default tests yield a 240-second difference over an extra 200 metres, giving 0.833333 m/s. The fitted intercept is 25 metres. Those two points determine that line exactly, even if one effort was not representative, so a perfect-looking line is not independent evidence of a good test. The repeat-distance table uses constant critical speed and does not spend or recover the intercept as though it were a physical tank. Additional testing and consistent timing remain important outside the worksheet.
Frequently asked questions
Why does a 200 m test need faster average speed?
The positive-capacity two-point model assumes average speed decreases with duration. Reversed or inconsistent efforts do not support that relationship.
Can I use yards?
The two tests are fixed at 200 m and 400 m. Yard tests need a correctly defined alternative and cannot be relabelled as metres.
Why is there no fit-quality percentage?
Two points always define a line. A meaningful internal residual check requires more independent test points.
Does the repeat time include recovery?
Only constant-speed swimming time is shown. Rest and interval scheduling are outside this tool.
