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Swim CSS calculator

Use two timed swims to estimate your critical swim speed and plan your pace.

Units
Longer test · 400 m
Shorter test · 200 m

Your CSS pace

—per 100 m

Enter both test times to see your pace.

Training zones

Pace targets based on your CSS and the TritonWear zone reference.

Repeat distance

Calculate CSS to see your zone paces.

ZONE 1

Easy / recovery

Target time · 100 m
—
CSS speed ratio
< 0.91 ×
Heart rate · % max
60–74
RPE · /10
2–3
TritonWear Energy Score
0–77
ZONE 2

Endurance

Target time · 100 m
—
CSS speed ratio
0.92–0.97 ×
Heart rate · % max
74–79
RPE · /10
3–5
TritonWear Energy Score
78–82
ZONE 3

Threshold up to CSS

Target time · 100 m
—
CSS speed ratio
0.97–1.00 ×
Heart rate · % max
80–89
RPE · /10
6–7
TritonWear Energy Score
83–85
ZONE 4

CSS to VO₂ max

Target time · 100 m
—
CSS speed ratio
1.00–1.10 ×
Heart rate · % max
90–95
RPE · /10
8–9
TritonWear Energy Score
86–94
ZONE 5

Sprint / lactate tolerance

Target time · 100 m
—
CSS speed ratio
> 1.10 ×
Heart rate · % max
96–100
RPE · /10
9–10
TritonWear Energy Score
95+

Time = CSS pace ÷ speed ratio × distance / 100, rounded to seconds. The source leaves a gap between 0.91 and 0.92; its ranges are preserved. Heart rate, RPE and Energy Score are reference ranges, not calculated measurements.

Understanding the training zones

ZONE 1

Easy / recovery

Easy, steady swimming for warm-up, cool-down and recovery. Build an aerobic base with short rests.

Effort feels light (RPE 2–3). Prioritize relaxed technique over speed. The displayed time is a lower limit: slower swimming remains within this speed range.

ZONE 2

Endurance

Sustained aerobic work with an emphasis on endurance and technique, with limited rest between repeats.

Effort is light to moderate (RPE 3–5). Focus on steady repeats and consistent technique. Select a repeat distance to see its target time.

ZONE 3

Threshold up to CSS

Hard swimming near critical swim speed, focused on threshold endurance with regular recovery between repeats.

Effort feels hard (RPE 6–7), with speed close to CSS. Keep repeat times consistent and use recovery to maintain technique.

ZONE 4

CSS to VO₂ max

Work above critical swim speed to develop aerobic power and high-intensity output, with extra recovery between repeats.

Effort feels very hard (RPE 8–9). Speed reaches or exceeds CSS, so pace times are shorter. Allow extra recovery to maintain output across repeats.

ZONE 5

Sprint / lactate tolerance

Short, near-maximal efforts focused on speed and lactate tolerance. Fatigue builds quickly, so allow more recovery between repeats.

Effort is near maximal (RPE 9–10). Speed exceeds 1.1 × CSS; the displayed time is an upper limit. Prioritize short-repeat quality. A distance conversion does not predict how long you can sustain this effort.

How it works

Swim two different distances in the same unit, such as 400 and 200, with full recovery between tests. Enter the time for each swim.

CSS pace = (longer time − shorter time) ÷ (longer distance − shorter distance) × 100. Times are in seconds.