How to use the swim critical speed calculator
- Choose whether your time trials were 400 and 200 meters or 400 and 200 yards.
- Enter your 400 time and 200 time as minutes and seconds, such as 6:10 and 2:55. Each time should come from its own best effort, swum fully rested.
- Enter the pool length in the same unit to see times per length.
- Read the critical swim speed as a pace per 100 and the table of paces around it.
Formula
speed = (400 - 200) / (time for 400 - time for 200)
pace per 100 = 100 / speed
time for one length = pace per 100 x pool length / 100
Critical swim speed is the slope of the line through two points (distance against time), which is the speed you could hold with a steady effort over a long swim in this simple model. The table scales that speed by a share and shows the pace for each.
A 400 in 6:10 and a 200 in 2:55
The 400 takes 370 seconds and the 200 takes 175. The difference in distance is 200 and the difference in time is 195 seconds, so the critical speed is 200 / 195 = 1.0256 meters per second. The pace per 100 meters is 97.5 seconds, or 1:37.5, and a 25 meter length takes 24.4 seconds.
At 90% of that speed the pace is 1:48.3 per 100 (27.1 seconds a length), at 95% it is 1:42.6, at 105% it is 1:32.9 and at 110% it is 1:28.6. Entering the same times for yards works the same way and gives the pace per 100 yards.
What the number is and is not
Critical swim speed is a way to describe your swimming with one number from two simple tests. Coaches and swimmers use it to compare paces and to set repeat times for sets. It is an estimate: it depends on how well you paced each trial, on turns and the pool, and it changes as fitness changes.
The shares in the table are examples to compare efforts, not recommended zones. This page is informational and is not training or medical advice; talk to a coach before changing how you train.
Related pace tools
For running paces and splits use the Pace Calculator, for a climb on a bike the Cycling Climb Speed Calculator, and to study laps from a timing sheet the Lap Time Analyzer. An Interval Timer helps time the sets themselves.
More sport and game tools
For pairings, payouts and music, try the Musical Interval Calculator, Swiss Pairing Generator and Tournament Prize Payout Calculator.
Assumptions and limits
- The two time trials are maximal, well paced, from a push start in the same pool.
- Distances are exactly 400 and 200, in meters or in yards.
- The model treats speed as constant between the two efforts, which is a simplification.
- The 200 must be much faster per length than the 400; if not, the page warns you.
- The example times are an illustration. Replace them with your own.
Frequently asked questions
Why 400 and 200?
These two distances are long enough to measure endurance and short enough to swim at full effort. Other distance pairs exist, but this is the common choice and the one this page uses.
What if my 400 is faster per 100 than my 200?
The page warns you, because that usually means one of the swims was not at full effort. The result is still computed from the numbers you gave.
Can I use a short course pool?
Yes. Enter the pool length to get times per length. Turns help in short pools, so a short course trial can look faster than a long course one.
Is this safe to use for planning training?
It is only a pace estimate. The page gives no advice on how hard or how often to train, and anyone with a health concern should check with a professional first.