Critical Swim Speed Calculator
Enter a 400 m and 200 m time trial to get your CSS pace, swim training zones, rep times and predicted triathlon swim times.
Your CSS test
In one session, after a good warm-up: a flat-out 400 m, 5–10 minutes easy, then a flat-out 200 m.
- Swim the 400 m first, while you're fresh.
- Push off the wall for both. No dive.
- Use a 25 m or 50 m pool, and the same one each time you re-test.
| Distance | Time | Pace /100m |
|---|---|---|
| 400mYour test | 6:30.0 | 1:37.5 |
| 750mSprint triathlon | 12:20 | 1:38.7 |
| 800mPool / open water | 13:10 | 1:38.8 |
| 1500mOlympic triathlon | 25:30 | 1:42.0 |
| 1900mIronman 70.3 | 32:45 | 1:43.4 |
| 3800mIronman | 1:08:08 | 1:47.6 |
Times are for a continuous pool swim at your best sustainable effort. In open water a wetsuit usually makes you quicker, while sighting, swell and having no walls to push off make you slower, so treat these as a guide to pacing rather than a finish time.
| Zone | Pace /100m |
|---|---|
Warm-up, recovery and technique work. | 1:48.7 and slower |
Steady aerobic swimming. Long-course race pace. | 1:44.2–1:48.7 |
Moderately hard. Middle-distance race pace. | 1:41.0–1:44.2 |
Your CSS. The backbone of threshold sets. | 1:38.5–1:41.0 |
Hard. Short reps that lift your aerobic ceiling. | 1:34.3–1:38.5 |
Near maximal. Speed and power, full recovery. | 1:34.3 and faster |
| Rep | Endurance | Tempo | CSS | VO₂ max |
|---|---|---|---|---|
| 50m | 0:53.2 | 0:51.3 | 0:50.0 | 0:48.2 |
| 100m | 1:46.4 | 1:42.6 | 1:40.0 | 1:36.4 |
| 200m | 3:33 | 3:25 | 3:20 | 3:13 |
| 400m | 7:06 | 6:50 | 6:40 | 6:26 |
10 × 100 m at with 15 s rest. Build towards 20 × 100 m, or 5 × 400 m with 30 s rest.
Your triathlon thresholds
Saved on this deviceHow the CSS calculator works
Critical swim speed uses the critical-speed model: the distance you can swim flat out is a fixed amount you can cover using anaerobic capacity, plus your critical speed multiplied by time. Two time trials give both numbers, so CSS is simply the extra 200 m divided by the extra time the 400 m took.
That model assumes CSS can be held indefinitely, which is only roughly true for about 15 minutes. For longer swims we let sustainable speed tail off gradually with duration, so a 1500 m comes out a little slower than CSS and an Ironman swim several seconds per 100 m slower. The longer you're in the water, the more you slow.
The swim zones
Zones are set as a percentage of your CSS speed, so they scale with your ability. For a swimmer with a CSS of 1:40/100 m they work out close to the familiar offsets: easy swimming 9 seconds or more per 100 m slower than CSS, endurance 4–9 seconds slower, threshold within a second or two of CSS, and VO₂ max work 2–6 seconds faster.
Common questions
What is critical swim speed (CSS)?
CSS is the fastest pace you can sustain without quickly tiring, a swimmer's equivalent of threshold. It's calculated from a 400 m and a 200 m time trial and gives you a pace per 100 m to build your training around.
How do I do a CSS test?
Warm up well, then swim 400 m as fast as you can evenly pace it. Recover with 5 to 10 minutes of easy swimming, then swim 200 m flat out. Push off the wall for both. Enter both times and the calculator does the rest.
How often should I re-test?
Every 6 to 8 weeks works well for most triathletes, or at the start of each training block. Use the same pool each time so the results are comparable.
Can I use CSS for open water races?
Yes, as a pacing guide. A wetsuit usually makes you quicker, but sighting, swell, navigation and having no walls to push off from make you slower, so practise your race pace in open water before race day.