Swim CSS Calculator: free critical swim speed calculator.
Swim a 400 and a 200 all out. Get your CSS pace per 100 and the paces for your endurance, Red Mist and threshold sets.
Set paces per 100 m
These offsets are coaching practice from Swim Smooth's sessions, not physiology.
Swim Smooth zonesSwim Smooth Red Mist- EasyTechnique, drills and recovery · CSS + 8 to 10 s1:43 to 1:45
- EnduranceSteady aerobic sets · CSS + 6 s1:41
- Red MistLong repeats on short rest, like 6 × 400 · CSS + 3 to 4 s1:38 to 1:39
- ThresholdCSS sets, like 10 × 100 on short rest · at CSS1:35
- SprintShort reps, no agreed offsetFaster
Sprint sets have no agreed offset from CSS, so there is no number here. Your AI can set them from your own swims.
1,500 m at CSS pace takes 23:45: Swim Smooth's ballpark for a 1,500 m race. Longer races run slower than CSS.
Swim Smooth pacingHand it to your AI
Your AI can check these numbers against your training and adjust them. The prompt has your results filled in.
See the prompt
I swam a 400 m time trial in 6:00 and a 200 m in 2:50. A CSS calculator (Wakayoshi 1992, with Swim Smooth's set paces) put my critical swim speed at 1:35 /100 m, and my set paces at: easy 1:43 to 1:45 /100 m, endurance 1:41 /100 m, red mist 1:38 to 1:39 /100 m, threshold 1:35 /100 m. Look at my recent swims and build my next two weeks of swim sessions around these paces: the sets, the rest, and how often to retest.
Gemini, Grok Bot, Dots, Muse and Cue: copy the prompt and paste it in.
Your AI could build your swim sets around this. Join Memory.fit.
On Android now: get it on Google Play. One email when the iPhone app is on the App Store.
How it works
How the Swim CSS Calculator works.
Critical swim speed, or CSS, comes from a 1992 study by Kohji Wakayoshi and colleagues. Nine trained college swimmers swam to exhaustion at six set speeds in a swimming flume. Distance plotted against time made a straight line. Its slope, the critical velocity, tracked their speed at the onset of blood lactate accumulation and their 400 m freestyle closely.
You don't need a lab to find your line. Two all out swims give you two points on it. The slope between them is 200 divided by the time between your 400 and your 200, so your pace per 100 is that time gap halved. Swim the 400 first, rest until you are fully recovered, then the 200.
The set paces are coaching practice, not physiology. Swim Smooth builds its sessions around CSS: easy swims at CSS plus 8 to 10 seconds per 100, steady endurance sets at plus 6, Red Mist sets of long repeats at plus 3 to 4, and threshold sets like 10 × 100 at CSS itself. Sprint work is faster than CSS, and there is no agreed number for it.
Test again when your sets start to feel easy, in the same pool and the same way, so the numbers compare.
The formula
- CSS pace per 100 = (400 time − 200 time) ÷ 2
- set pace = CSS pace + the set's offset in seconds per 100
The science
Where the numbers come from.
Every number on this page links to the paper it comes from. How Memory.fit uses the science
Wakayoshi K, Ikuta K, Yoshida T, Udo M, Moritani T, Mutoh Y, Miyashita M. Determination and validity of critical velocity as an index of swimming performance in the competitive swimmer. European Journal of Applied Physiology and Occupational Physiology 64(2):153-157.
Research article · 1992 · doi:10.1007/BF00717953
Newsome P. Understanding Swim Training Zones for Endurance Swimming. Swim Smooth, July 24, 2025.
Coaching practice · 2025 · Read it
Newsome P. Red Mist vs. CSS Training: Why Both Are Vital. Swim Smooth, December 27, 2024.
Coaching practice · 2024 · Read it
Newsome P. Learn How to Swim Pace Like a PRO! Swim Smooth, April 24, 2025.
Coaching practice · 2025 · Read it
Example
A worked example.
A triathlete who swims a 400 m in 6:00 and a 200 m in 2:50.
- 16:00 minus 2:50 is 3:10. Halved, that is a CSS pace of 1:35 per 100 m, or 1:27 per 100 yd.
- 2Threshold sets, like 10 × 100 on short rest, go at 1:35. Red Mist repeats go at 1:38 to 1:39.
- 3Steady endurance sets go at 1:41, and easy swims at 1:43 to 1:45 per 100 m.
- 4Held for 1,500 m, CSS pace comes to 23:45, Swim Smooth's ballpark for a 1,500 m race.
Questions
Swim CSS Calculator: questions.
Warm up well. Swim 400 m all out from a push off and note the time. Rest until you feel fully recovered, then swim 200 m all out. Put both times in and the calculator does the rest.
There isn't one number. CSS depends on your background, your stroke and your fitness, and the spread between swimmers is wide. What matters is your own number, tested the same way each time, so you can see it move.
Yes. Pick Yards, swim a 400 and a 200 in yards, and the paces come out per 100 yd. Switch to Imperial to see both units side by side.
Close to it. In the original study, critical velocity tracked the speed at the onset of blood lactate accumulation very closely. Coaches use CSS as the pace for threshold sets on short rest.
Swim Smooth's ballpark is about CSS for 1,500 m, and slower for longer swims: around CSS plus 2 to 4 seconds per 100 for 5 km. Your AI can set your race pace from your own open water swims.
Not medical advice. These numbers come from published research on groups of athletes. Your own needs can differ; talk to a doctor or a sports dietitian about any health condition.