How do you calculate CSS?

Swim a 400 m and a 200 m time trial (with full recovery between). CSS pace per 100 m = (400 m time − 200 m time) ÷ 2. For example, 6:40 and 3:10 give (400 − 190) ÷ 2 = 105 s, or 1:45 per 100 m.

What is CSS used for?

CSS approximates your threshold swimming pace — roughly the pace you could hold for about 30 minutes. It is used to set interval targets and to predict race pace.

How do I convert pace per 100 m to per 100 yd?

100 yards is 91.44 m, so multiply pace per 100 m by 0.9144. 1:45/100 m is about 1:36/100 yd.

Is open-water pace the same as pool pace?

Usually slower: no push-offs, sighting and waves add time, although a wetsuit and drafting can help. The splits here use your pool CSS pace plus the slowdown you choose.

CSS Swim Calculator — Critical Swim Speed, Pace per 100m & Triathlon Splits

The CSS formula

CSS pace per 100 m = (T400 − T200) ÷ 2
CSS speed (m/s) = 200 ÷ (T400 − T200)

Example: 7:00 (420 s) and 3:20 (200 s) → (420 − 200) ÷ 2 = 110 s = 1:50 per 100 m. CSS approximates the fastest pace you could hold for about 30 minutes — your swimming threshold.

CSS training zones

Session	Target pace	Example
Endurance / easy	CSS + 8–15 s/100 m	Continuous 1,000–2,000 m
Threshold (CSS)	CSS ± 2 s/100 m	10 × 100 m on 10–15 s rest
Fast	CSS − 3–6 s/100 m	8 × 50 m on 20 s rest

Zone offsets are common coaching practice; adjust to how the sets feel.

Pace per 100 m and race swim times

/100 m	/100 yd	Sprint (750 m)	Olympic (1500 m)	Half (70.3) (1900 m)	Full (Ironman 140.6) (3800 m)
1:15	1:09	9:22	18:45	23:45	47:30
1:25	1:18	10:38	21:15	26:55	53:50
1:35	1:27	11:52	23:45	30:05	1:00:10
1:45	1:36	13:08	26:15	33:15	1:06:30
1:55	1:45	14:22	28:45	36:25	1:12:50
2:05	1:54	15:38	31:15	39:35	1:19:10
2:15	2:03	16:52	33:45	42:45	1:25:30
2:25	2:13	18:08	36:15	45:55	1:31:50
2:35	2:22	19:22	38:45	49:05	1:38:10
2:45	2:31	20:38	41:15	52:15	1:44:30

Plan the whole race with the triathlon calculator; the run-side equivalent of CSS is the critical speed calculator.

Last reviewed: September 2026 · Health disclaimer

CSS Swim Calculator & Swim Pace Calculator

Your test times

The CSS formula

CSS training zones

Pace per 100 m and race swim times

CSS Swim Pace questions

Your test times

The CSS formula

CSS training zones

Pace per 100 m and race swim times

CSS Swim Pace questions

How do you calculate CSS?

What is CSS used for?

How do I convert pace per 100 m to per 100 yd?

Is open-water pace the same as pool pace?

Related running calculators

Triathlon Calculator

Critical Speed Calculator (with D′)

Race Fueling Calculator