Swim pace
Enter your 400 m and 200 m times: we work out your critical swim speed (CSS), your 100 m threshold pace and the training zones around it.
For example 6:00
For example 2:45
Critical swim speed (CSS)
1:38 / 100 m
1.03 metres per second
| Zone | Pace | |
|---|---|---|
| Easy | 1:50 / 100 m | Warm up, cool down, drills |
| Aerobic | 1:44 / 100 m | The body of long sets |
| Threshold (CSS) | 1:38 / 100 m | Main threshold set, 100-400 m repeats |
| Speed | 1:34 / 100 m | Short fast repeats |
Distances at CSS pace
| Distance | Time |
|---|---|
| 25 m | 0:24 |
| 50 m | 0:49 |
| 100 m | 1:38 |
| 200 m | 3:15 |
| 400 m | 6:30 |
| 1500 m | 24:23 |
| 1900 m | 30:53 |
| 3800 m | 1:01:45 |
Critical swim speed comes from the gap between your 400 m and 200 m times: CSS = 200 / (400 time - 200 time). The result is metres per second and is the practical equivalent of threshold pace in swimming. The zones shift around CSS in seconds per 100 m: easy about 12 seconds slower, speed work 4 seconds faster. The test asks for two rested, maximal swims; pool length and turns affect the outcome.
These are general ranges and assume normal conditions. Trail, heat, elevation gain, wind and technical ground all make the real time longer.
Frequently asked questions
(10)
What is CSS (critical swim speed)?
CSS is the fastest pace you can hold for a long time, the practical equivalent of threshold pace in swimming. It is calculated from the gap between your 400 m and 200 m times and used as the reference pace for training sets.
How do I run the test?
After a warm up, swim 400 m as hard as you can, recover with 10-15 minutes of easy swimming, then swim 200 m as hard as you can. Do both in the same session and the same pool; going out too hard on the 400 makes your CSS look slower than it is.
What pace should I target in a triathlon?
Most athletes swim an open water triathlon 2-5 seconds per 100 m slower than CSS: close to CSS over short distances, slightly slower over long ones. Chop, a wetsuit and a crowded start make the number look different from the pool, so check effort as well as pace.
Why am I slower in open water than in the pool?
No push off, no turns, harder to swim straight and there is chop. Losing 5-15 seconds per 100 m at the same effort is normal; a wetsuit gives some of it back by lifting your legs.
How often should I retest CSS?
Every 6-8 weeks is enough. Technique changes such as breathing side or stroke length move the number faster than fitness does, so retesting after a technique block makes sense.
Does pool length change the result?
Yes. A 25 m pool means more turns and more push offs; the same swimmer is typically 2-4 seconds per 100 m slower in a 50 m pool. Always test in the same pool, or what looks like progress may just be the venue.
How long will the 1500 m triathlon swim take me?
Multiply your CSS pace by 15: 1:45/100 m gives 26:15 and 2:00/100 m gives 30:00. Add one to three minutes in open water, because there are no turns or push offs, swimming straight is harder and the start is crowded.
What pace should I hold for 25 and 50 metre repeats?
Short repeats are usually swum 3-6 seconds per 100 m faster than CSS, so with a 1:40 CSS that is about 24 seconds for 25 m and 49 seconds for 50 m. As the rest interval shrinks the target should move back towards CSS; on 15 seconds rest, CSS pace is already hard enough.
How do SWOLF and stroke count relate to pace?
SWOLF adds the time for a length to the number of strokes taken, and lower is better. Holding the same pace with fewer strokes is a genuine efficiency gain, but over-gliding costs pace. Track SWOLF and your 100 m pace together during technique work.
How much faster is a wetsuit?
For most swimmers 3-8 seconds per 100 m. The gain comes from lifting the legs and cutting drag, so swimmers with sinking legs gain the most. Train in the suit at least twice before racing: the shoulder restriction can be a surprise.