Swim CSS Calculator
Estimate your Critical Swim Speed (CSS) pace from two time trials.
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Test 1
Longer time trial (e.g. 400m) — must be the longer distance.
Time
Test 2
Shorter time trial (e.g. 200m) — must be the shorter distance.
Time
Results
CSS Pace
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Predicted 100m
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Predicted 200m
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Predicted 400m
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Enter both time trials. Test 1 must be the longer distance and slower time.
How CSS Works
Critical Swim Speed (CSS) is the theoretical swimming speed that can be maintained continuously without exhaustion. It's calculated using two time trials of different distances.
The formula: CSS = (D1 − D2) / (T1 − T2), where D is distance and T is time.
CSS is useful for setting training paces and predicting race times. Perform both time trials on the same day with adequate rest between them.
FAQ
- What is Critical Swim Speed (CSS)?
- Critical Swim Speed (CSS) is the theoretical swimming speed that can be maintained continuously without exhaustion. It is useful for setting training paces and predicting race times.
- How is Critical Swim Speed calculated?
- CSS is calculated from two time trials of different distances using the formula CSS = (D1 − D2) / (T1 − T2), where D is distance and T is time. The result is typically expressed as pace per 100m or 100y.
- Which time trial should be longer?
- Test 1 must be the longer distance with the slower time (for example 400m then 200m). Perform both trials on the same day with adequate rest between them.
- What predictions does the CSS calculator provide?
- Besides CSS pace, the calculator estimates predicted times for 100, 200, and 400 meters or yards based on your CSS.
Updated July 2026
