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Swim CSS Calculator
Critical swim speed pace
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Reference range
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Full calculation and sourcesOpen Full calculation and sources to audit the formula, assumptions, fixtures, and interpretation limits.
Versioned calculationFormula v1.0.0
Cross-checked
Supporting outputs expose unit and protocol mistakes.
Actionable
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Step 1Swim a well-paced maximal 400.
400 in 6:40 and 200 in 3:10Step 2Recover consistently, then swim a maximal 200.
400 in 6:40 and 200 in 3:10Step 3Enter both times from the same course length.
400 in 6:40 and 200 in 3:10Interpretation
CSS is a field estimate and does not directly measure lactate threshold.
Push-offs, turns, pool length, drafting, pacing, and rest interval change the result.
The 1,500 time is an arithmetic projection rather than a prediction guarantee.
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Public verification recordFormula v1.0.0
- Review scope
- Clock Composition, Time-order Validation, CSS Equation, Course Label, Pace References, And Projection.
- Review team
- CalculatorGeek Expert Review Team
- Verified
- 2026-09-27
- Next source review
- 2027-03-27
- Automated fixtures
- 1 cases
Review method
Recompute CSS seconds per 100 = (400 time in seconds - 200 time in seconds) / 2. independently; verify unit normalization, protocol selection, internal consistency, and invalid boundaries.
Known limitations
- CSS is a field estimate and does not directly measure lactate threshold.
- Push-offs, turns, pool length, drafting, pacing, and rest interval change the result.
- The 1,500 time is an arithmetic projection rather than a prediction guarantee.
Primary sources
Latest update
2026-09-27 · Added complete endurance-suite calculator with verified model, source ledger, premium result, and unified contextual navigation.
Published calculation checks
| Case | Inputs | Expected result |
|---|---|---|
| 400 in 6:40 and 200 in 3:10 | course=meters, time_400_minutes=6 min, time_400_seconds=40 sec, time_200_minutes=3 min, time_200_seconds=10 sec | css_seconds_per_100=105, predicted_1500_seconds=1575 |
On this page
Direct answer
CSS pace per 100 equals the 400 time minus the 200 time, divided by two.
Use two well-paced maximal tests in the same course length and repeat the same protocol when monitoring change.
Convert other swim distances with the Swim Pace Calculator and plan cross-discipline work with the Brick Workout Calculator.
How to use the calculator
- Swim a well-paced maximal 400.
- Recover consistently, then swim a maximal 200.
- Enter both times from the same course length.
- Use repeat tests, not one isolated number.
Inputs, outputs, and result meaning
Inputs
- Pool course: Use the same course for both tests.
- 400 time - minutes: Whole minutes from the 400 test.
- 400 time - seconds: Remaining seconds from the 400 test.
- 200 time - minutes: Whole minutes from the 200 test.
- 200 time - seconds: Remaining seconds from the 200 test.
Outputs
- CSS pace: A displayed result from the stated method and the entered assumptions.
- CSS speed: A displayed result from the stated method and the entered assumptions.
- Arithmetic 1,500 projection: A displayed result from the stated method and the entered assumptions.
- Tempo reference: A displayed result from the stated method and the entered assumptions.
- Aerobic reference: A displayed result from the stated method and the entered assumptions.
Formula and methodology
Method: CSS seconds per 100 = (400 time in seconds - 200 time in seconds) / 2.
The two-point critical-speed equation uses the difference in distance and time. The 400/200 form reduces to the visible division by two.
Worked examples
6:40 and 3:10
400 - 190 = 210 seconds; 210 / 2 = 105 seconds per 100, or 1:45 per 100.
Interpretation and limitations
- CSS is a field estimate and does not directly measure lactate threshold.
- Push-offs, turns, pool length, drafting, pacing, and rest interval change the result.
- The 1,500 time is an arithmetic projection rather than a prediction guarantee.
Frequently asked questions
Should the 400 be first?
Use one consistent protocol; the common test performs the 400 before the 200.
Can I mix yards and meters?
No. Both test swims must use the selected course.
Why can CSS be slower than both test paces?
It estimates the slope between the two efforts, representing a longer-duration pace.
How should I compare results over time?
Repeat the same protocol, units, equipment setup, and environmental conditions, then review the supporting outputs rather than only the headline number.
Sources and transparency
CalculatorGeek independently recomputed every published fixture on 2026-09-27. Sources establish the equation, protocol, or interpretation boundary; they do not turn an estimate into a laboratory measurement or personalized advice.