Do a 200, 400 or 1000 m test and this calculator gives you your critical swim speed (CSS), your six training zones with the exact pace per 100 m, predicted times for any distance and your real open water time with wetsuit, chop, current and drafting.
Tick the ones you have done and enter the time as mm:ss. One is enough; with two or three the result is considerably more accurate.
Careful with the "out and back" result: even though the current cancels out over the distance, you always lose time, because you spend longer fighting it than benefiting from it.
Target pace per 100 m in each zone, calculated from your CSS.
| Zone | /100 m | /50 m | What it is for |
|---|
Predictions in a 25 m and a 50 m pool, and in open water with the settings you chose above. The pace column corresponds to the 25 m pool.
| Distance | 25 m pool | 50 m pool | Pace /100 m | Open water |
|---|
Triathlon distances (750, 1,900 and 3,800 m) are highlighted. Predictions assume continuous, well paced swimming; going out too fast costs more than the model can foresee.
Each bar is the effect of that factor on the time accumulated so far, so the sum of all of them gives exactly the final difference against the 50 m pool.
Concrete sessions with the target times already worked out for you. "Send off" is the interval from the start of one repetition to the start of the next.
| Set | Zone | Target per repetition | Send off | Purpose |
|---|
CSS —critical swim speed— is the fastest speed you can hold in a sustained way without lactate running away from you. It is the swimming equivalent of functional threshold in cycling, and it is the reference on which all the zones are built.
It comes from comparing two tests over different distances: the difference in metres divided by the difference in times.
With the classic 400 and 200 m tests, a swimmer doing 7:00 and 3:20 has a CSS of (400−200)/(420−200) = 0.909 m/s, that is 1:50 per 100 m. If you tick all three tests, the calculator fits the line by least squares instead of using only two points, which is more robust against one bad swim.
To estimate any other distance, a power law is fitted between distance and time:
The exponent k is your fatigue index: how much your pace degrades as the distance grows. A very durable swimmer sits around 1.02-1.04; an explosive one who fades as soon as they pass 400 m goes above 1.08. With two or more tests the exponent is calculated from your data; with only one it is taken from the profile you choose, which is why doing at least two is worth it.
The difference is not the water, it is the walls. Every turn includes a push-off that gives you free speed, and in a 25 m pool you do twice as many turns as in a 50 m one. That is why the same swimmer is about 2 % faster short course, and in open water —where there is not a single wall— loses around 3.5 % before chop or sighting is even considered.
| Factor | Typical effect | Why |
|---|---|---|
| Wetsuit | −3 % to −8 % | Lifts your legs and reduces drag. The worse your position in the water, the more it helps |
| Drafting on feet | −6 % | Swimming in someone's wake reduces drag; on the hip is slightly less effective but easier to hold |
| No walls | +3.5 % | No turns and no push-offs, and speed has to be built from scratch continuously |
| Sighting | +1 % to +6 % | Lifting your head sinks your hips and brakes you; and if you zigzag, you swim extra metres |
| Chop | +3 % to +13 % | Breaks your breathing rhythm and your stroke, and forces constant correction |
| Cold water | +1 % to +4 % | Less shoulder mobility and disrupted breathing in the first minutes |
| Level | CSS /100 m | 400 m | 1,900 m |
|---|---|---|---|
| Beginner | 2:30 – 2:10 | 9:30 – 8:10 | 48:00 – 42:00 |
| Age group starter | 2:10 – 1:50 | 8:10 – 6:55 | 42:00 – 35:30 |
| Solid age grouper | 1:50 – 1:35 | 6:55 – 5:55 | 35:30 – 30:30 |
| Competitive | 1:35 – 1:20 | 5:55 – 5:00 | 30:30 – 25:30 |
| Club swimmer | 1:20 – 1:05 | 5:00 – 4:05 | 25:30 – 21:00 |
Warm up for 10-15 minutes with some drills and a couple of build swims. Swim the 400 m flat out, pushing off from in the water and with no equipment. Recover easily for 8 to 10 minutes, then swim the 200 m flat out as well. Do both in the same session and in the same pool: if you do each test on a different day, the result stops being comparable.
It gives you a ballpark, but accuracy drops noticeably. With one test the calculator has to assume your fatigue index from the profile you pick, and that is where the biggest error lives. With two tests the index comes from your own numbers and the result improves a lot. If you are only going to do one, make it the 400 m.
The three usual causes are the start (the first 200 m in a pack, with contact and a sky-high heart rate, are always chaos), real sighting —almost everyone swims more metres than the buoy line— and simply not being used to swimming without the black line on the bottom. Raise the sighting penalty to "little experience" if that sounds like you.
Every 6 to 8 weeks. That is long enough for training to produce a measurable change, and frequent enough that your zones do not go stale. Always repeat it under the same conditions: same pool, same time of day, same warm-up.
No, and this is the part that surprises people most: the worse you swim, the more it helps. A swimmer whose legs sink can gain 8 % or more, while a technically strong one with a good horizontal position gains barely 3 %, because the wetsuit is fixing something they already did well. Set the slider to match your case.
The mathematical model is valid for any stroke —the distance-time relationship behaves the same way— but the level reference tables and the open water factors are built for freestyle. If you swim breaststroke, use the paces and the zones but ignore the level comparison.
If this one was useful, these three will be too.
CalculatorPick the distance, the repetitions, the zone and the rest, and get the target time for every repetition and the send-off times you write on your hand.
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CalculatorWork out your SWOLF, your distance per stroke and your stroke rate, and find out whether a lower SWOLF actually made you faster or just slower and prettier.
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CalculatorEnter a 1 km, 5 km or 12 minute (Cooper) test and get your VO2max, your vVO2max, your training paces by zone and predicted times from 5 km to marathon.
Open →This calculator is an informational estimate and does not replace a coach or a laboratory test. Nothing you type ever leaves your browser.