Swim pace and CSS calculator
Swim a 400 and a 200 time trial and this works out your critical swim speed (CSS), the pace per 100 you could hold for about half an hour, then your easy, steady, CSS and fast training paces and rough times for the triathlon swims. A 400 in 6:40 and a 200 in 3:10, for example, give a CSS of 1:45 per 100. It assumes both swims were honest, evenly paced efforts in the same session.
How it works
CSS per 100 = (400 time − 200 time) ÷ 2. The 400 takes longer than two 200s because you cannot hold your 200 speed for twice as long; the difference is what the extra 200 costs you at a pace you could keep going, and halving it gives the pace per 100. Our triathlon plans then set every swim from it, in seconds either side: easy is CSS plus 10 to 20 seconds, steady plus 5 to 10, CSS itself for threshold sets, and fast 3 to 6 seconds quicker for short repeats.
It assumes two honest, evenly paced swims with full rest between them. A 400 started too fast, or a 200 swum tired, gives a CSS that is too slow or too fast. The race-swim times are at CSS plus 5 seconds per 100 for a steady continuous swim; open water, a wetsuit and a crowded start move them either way, so treat them as a rough guide.
What each pace is for: easy for warm-ups, drills and recovery swims; steady for longer aerobic sets; CSS for the main sets that raise your threshold, such as 8 to 10 × 100 with 10 to 15 seconds rest; fast for short repeats that sharpen speed. The glossary has the short version, and the triathlon finish time calculator takes your race pace into a whole race.
CSS chart: training paces and swim times
Per 100 m (the same per 100 yd, though the race distances are in metres). The 1500 m and 3800 m times are at CSS plus 5 seconds per 100.
| CSS /100 | Easy /100 | Steady /100 | 400 at CSS | 1500 m swim | 3800 m swim |
|---|---|---|---|---|---|
| 1:20 | 1:30–1:40 | 1:25–1:30 | 5:20 | 21:15 | 53:50 |
| 1:30 | 1:40–1:50 | 1:35–1:40 | 6:00 | 23:45 | 1:00:10 |
| 1:40 | 1:50–2:00 | 1:45–1:50 | 6:40 | 26:15 | 1:06:30 |
| 1:50 | 2:00–2:10 | 1:55–2:00 | 7:20 | 28:45 | 1:12:50 |
| 2:00 | 2:10–2:20 | 2:05–2:10 | 8:00 | 31:15 | 1:19:10 |
| 2:10 | 2:20–2:30 | 2:15–2:20 | 8:40 | 33:45 | 1:25:30 |
| 2:20 | 2:30–2:40 | 2:25–2:30 | 9:20 | 36:15 | 1:31:50 |
| 2:30 | 2:40–2:50 | 2:35–2:40 | 10:00 | 38:45 | 1:38:10 |
| 2:40 | 2:50–3:00 | 2:45–2:50 | 10:40 | 41:15 | 1:44:30 |
Questions
- What is CSS in swimming?
- Critical swim speed is the pace per 100 you could hold for a long, hard swim of roughly half an hour, about the swimming version of a runner’s threshold pace. You work it out from two time trials rather than swimming that long flat out, and it gives every swim set a target you can check on the pace clock.
- How do I do a CSS test?
- In one pool session: warm up well, swim 400 as fast as you can hold evenly, then rest fully (five to ten minutes of easy swimming) and swim 200 all out. Push off the walls as you would in a normal set, not with long glides. A 400 in 6:40 and a 200 in 3:10 give a CSS of 1:45 per 100.
- How often should I retest my CSS?
- Every six to eight weeks while you are training regularly, or when your sets at CSS start to feel easy. Test in the same pool, at the same point in your week, so the numbers compare.
- Why is my open-water pace slower than in the pool?
- No walls to push off every 25 or 50, no black line to follow, and sighting, waves and other swimmers all cost time. A wetsuit gives some of it back. That is why the race times here are at CSS plus 5 seconds per 100: a rough guide, not a promise, and the first open-water swims of a season are usually the slowest.
Build a triathlon plan with these swim paces
Put your CSS into the plan builder and every swim session comes with its pace per 100. A plan in miles swims in yards and one in kilometres in metres, so pick the units that match your pool.