400/200 testing protocol

Critical Swim Speed test, done properly.

A repeatable pool protocol for collecting trustworthy 400 and 200 times, calculating CSS, and turning the result into useful threshold training.

At a glance

Efforts
400 + 200
Order
Long first
Output
Pace /100

The protocol

Make the test repeatable before making it hard.

CSS becomes useful when changes reflect training rather than different equipment, starts, recovery, or timing. Control the setup first; then ask for maximal efforts.

  1. 01

    Prepare one repeatable setup

    Choose SCY, SCM, or LCM and use the same pool, lane, timing method, start type, and equipment for both efforts. Record conditions you want to reproduce next time.

  2. 02

    Warm up without creating fatigue

    Swim easy, add technique work, then complete short pace builds with generous recovery. Finish feeling ready to race—not as if the test has already started.

  3. 03

    Swim a maximal, controlled 400

    Start firmly but avoid an opening sprint you cannot sustain. Record the exact total time and, when possible, intermediate splits to evaluate pacing quality.

  4. 04

    Recover, then swim a maximal 200

    Use easy swimming and rest until breathing and stroke control return. The 200 should be a genuine best effort in the same course and with the same timing method.

A clean timing checklist.

Write these conditions beside the result. They are part of the test, not administrative detail.

  • Same pool course and measurement unit for both swims
  • Same start type: push or dive, never one of each
  • Exact times recorded to at least one tenth when available
  • No paddles, fins, buoy, drafting, or pace assistance
  • Enough recovery for a maximal 200
  • Notes saved for the next retest

CSS formula

Speed comes from the difference between efforts.

Convert both times to seconds. Subtract the short distance from the long distance, then divide by the difference between their times.

CSS speed = (Dlong − Dshort) ÷ (Tlong − Tshort)

Worked example

400m in 6:00 · 200m in 2:50

(400 − 200) ÷ (360 − 170) = 1.0526m/s

CSS = 1:35 /100m

Calculate your CSS

Read CSS as a training benchmark.

Your calculated CSS is an estimate of sustainable threshold pace, not a promise that every threshold repeat should land on the same number. Repeat length, recovery, stroke, course, fatigue, and session purpose still matter.

Begin with the calculator’s training ranges. During the set, watch whether pace remains repeatable while stroke control and recovery stay appropriate. If quality collapses early, the target may be too aggressive for that session.

Use the same course unit throughout. A CSS result from yards is not interchangeable with a metres result, and a pool CSS should not be treated as a direct open-water prediction.

Racing the first 100 of the 400

An explosive opening can turn the long test into a survival swim and make the CSS estimate less representative. Aim for the fastest pace you can distribute across all four hundreds.

Starting the 200 before recovery

If the short test is suppressed by fatigue, the difference between efforts narrows and the calculated CSS can become misleadingly fast.

Mixing timing or start methods

A dive for one effort and a push for the other changes the comparison. Manual and automatic timing should not be mixed inside the same test.

Treating CSS as an exact physiological border

CSS is a practical training estimate. Use repeat quality, stroke control, recovery, and coaching judgment to refine the pace for real sessions.

Retest when the comparison can teach you something.

Four to eight weeks is a useful cadence during consistent training. Repeat sooner when a test was clearly compromised; wait longer when training has been interrupted or your current zones still describe the work accurately.

Repeat

Same course

Record

Same method

Compare

Pace and quality

Critical Swim Speed test questions

What is the 400/200 Critical Swim Speed test?

It is a two-effort swimming test that uses maximal 400 and 200 times to estimate sustainable threshold speed. The difference in distance is divided by the difference in time, then converted to pace per 100 metres or yards.

How much recovery should I take between the 400 and 200?

Recover long enough to make the 200 a genuine maximal effort. A practical starting point is roughly 8 to 12 minutes of easy swimming and rest, but readiness matters more than hitting an exact minute.

Should the 400 or the 200 come first?

Swim the 400 first, recover fully, then swim the 200. Keeping the order, course, timing method, and warm-up consistent makes future tests easier to compare.

Can I use a 500 and 200 instead?

The calculator accepts custom long and short distances, but the 400/200 protocol is the clearest standard for repeat testing. Do not compare results from different distance pairs as if the protocols were identical.

How often should swimmers retest CSS?

A four-to-eight-week interval is practical during consistent training. Retest sooner only when the previous test was compromised, or after a major change in fitness, training phase, or time away from swimming.

Use the result

Calculate CSS, then turn it into exact pool targets.

Enter both times to get CSS pace, starting training zones, and projections. Continue to the pace calculator when you need 25, 50, or 100 splits.

Open CSS calculator