Swim Pace Calculator

The theoretical fastest pace you can sustain without accumulating fatigue — swimming's equivalent of functional threshold power in cycling or threshold pace in running. It comes from two maximal time trials, and it approximates the velocity at which lactate production and clearance are balanced.

CSS = (400 − 200) ÷ (T400 − T200) in metres per second, where the times are your all-out trials in seconds. In a metric pool there is a shortcut for the pace: CSS per 100 m is simply (T400 − T200) ÷ 2, which you can do on the pool deck.

Because both swims contain an anaerobic contribution, and a single time cannot separate it from your aerobic capability. Taking the slope between two distances subtracts that contribution out, which is exactly what makes CSS a threshold rather than an average.

Your CSS comes out too fast, and every training pace derived from it will be too hard. The model reads a small gap between the two paces as evidence of enormous endurance. This calculator flags a suspiciously narrow spread for that reason.

Wakayoshi and colleagues reported a correlation of r = 0.949 with the velocity at onset of blood lactate accumulation, and r = 0.864 with 400 m performance. Practically, it lands within about 3 to 5% of laboratory-determined threshold velocity for trained swimmers.

Twenty to forty minutes for most swimmers, despite the theory describing an indefinitely sustainable speed. Treat it as a hard but controlled effort, not a pace you could hold all afternoon.

Expressed as offsets in seconds per 100 from CSS: recovery about +20, aerobic base about +10, threshold at CSS itself, VO2 max around −6, and speed work around −12. Published ranges vary a little, so treat the offsets as bands.

Whichever you train in, and then always the same one. A 25 m pool gives three turns per 100 m against one in a 50 m pool, and a strong push-off is faster than swimming, so short course times run 1 to 3 seconds per 100 quicker for identical fitness.

It converts the distances, not the times. A 400 yd trial covers 365.76 m and a 200 yd trial 182.88 m, since a yard is exactly 0.9144 m. Converting the times instead is a common error that produces a subtly wrong CSS.

Every four to eight weeks in a training block, or whenever your threshold sets start feeling clearly easy. Consistent CSS work usually improves the figure by 1 to 3 seconds per 100 over eight to twelve weeks.

As a rough guide only. Open water adds sighting, chop, wetsuit buoyancy, drafting and no turns, and each of those shifts your effective pace. CSS is a pool-derived threshold, useful for structuring training rather than predicting an open-water time.

The arithmetic works for any stroke, but the validation and all the published zone offsets are freestyle. If you test butterfly, the resulting number is a butterfly threshold and is not comparable with anything else.

The seconds taken for one length plus the number of strokes taken in it — 18 seconds and 16 strokes gives a SWOLF of 34. It captures efficiency, which pace alone cannot: two swimmers at identical pace can be many strokes apart, and the more efficient one will hold that pace far longer.

Conceptually yes, physiologically close. Both describe the highest intensity at which lactate is cleared as fast as it is produced. The numbers are not transferable between sports, and a swimmer's threshold heart rate is typically around 10 beats lower than their running threshold because of the horizontal position and water immersion.

Ten to fifteen minutes of easy swimming is the standard protocol. Much less and the 200 is compromised by the 400; much more and you cool down. What matters most is doing it the same way each time so the tests are comparable.

Yes — the slope method works between any two distances, and 300/100 is a common alternative for less experienced swimmers. Keep the protocol identical between retests, because changing the test distances changes the number.

It should be. Your 400 pace includes an anaerobic contribution you cannot sustain indefinitely, so threshold sits below it. If CSS came out faster than your 400 pace, the 200 trial was not maximal.

Threshold sets are the core: 8 × 100 at CSS with 15 seconds rest, or 5 × 200 slightly above it with 20 seconds. Keep roughly 70% of total volume in the easy zones, 20% at threshold, and 10% faster.

Not directly. It is a training-intensity anchor, not a race predictor. It correlates well with 400 m performance, but race times depend on starts, turns, pacing and, in open water, on conditions that CSS knows nothing about.

The test becomes much less reliable. Fatigue, sleep and water temperature all shift performance between days, and the model attributes any of that difference to physiology. Swim both on the same day, in the same session, with the prescribed recovery between them.