CSS calculator (Critical Swim Speed)

What it does

CSS (Critical Swim Speed) is your sustainable threshold pace in the water, used as the anchor for every swim training zone and swim pacing prescription. The CSS calculator takes the results of a structured swim test and outputs your CSS pace in seconds per 100 m plus your SDI score (a swim economy proxy derived from stroke rate and distance per stroke).

URL: /calculators/ then open CSS. Or open it directly at /static/calculators/css_calculator.html.

It is also wired into step 1 of the race planning wizard.

The calculator's three input modes

The CSS calculator supports three input shapes. Pick the one that matches the test you ran — distances and exact times are yours to choose.

1. Two-distance time trial — any pair of distances

• Distance 1 in metres (whatever you ran).
• Distance 1 time (mm:ss).
• Distance 2 in metres.
• Distance 2 time (mm:ss).

The math: CSS pace per 100 m = (d2 - d1) / (t2 - t1), scaled to a per-100 figure. As long as the two distances differ enough to give a useful slope, any pair works.

2. 3-rep set with SR + DPS (optional)

For each of three repeats:

• Time (mm:ss).
• Stroke rate (SR).
• Distance per stroke (DPS).

Outputs an averaged CSS plus the SDI economy score. Use this when you want a stroke-economy reading alongside the threshold pace — useful for tracking technique drift across a season.

3. Single 1000 m time trial

If only one TT is available, enter the total time and the calculator estimates CSS from it. Less accurate than a real two-distance test but useful as a baseline when a structured protocol isn't possible.

Canonical two-distance protocols

The two most commonly used distance pairs:

Standard 400 + 200 (long rest)

• Pool warm-up, 800-1200 m easy with a couple of build 50s.
• 400 m all-out — fast steady, hold pace you can sustain.
• 5 to 10 minutes easy recovery.
• 200 m all-out — faster but still controlled.
• Plug 400 + 200 times into Distance 1 / Distance 2.

This is the protocol seeded in the CSS Test assessment and step 1 of the race planning wizard.

100 + 500 (60 seconds rest)

A session-friendly compressed variant — total test ~11 min after warm-up:

• Pool warm-up 600-1000 m + 4 x 50 build.
• 100 m all-out, fresh legs from the wall.
• 60 seconds at the wall — loose, no early pushoff.
• 500 m all-out — best sustainable pace for the longer leg.
• Plug 100 + 500 times into Distance 1 / Distance 2.

Notes specific to this variant:

• The 100 goes FIRST. With only 60 s rest the 500 is run partially recovered, which is the point — it intentionally drops the test into a regular squad session.
• The 500 must still be a true TT (sustained threshold effort). If the athlete blows up in the last 100 m, the CSS estimate underestimates true threshold; rerun on a fresh day if that happens.
• Tends to produce a slightly faster CSS estimate than 400+200 for athletes with strong VO2 max relative to threshold, because the 100 leans more anaerobic.

Other valid protocols (less common but supported)

The calculator doesn't care which distances you pick — pair anything you can pace reliably. A few that come up:

• 1000 m all-out single TT — feeds input mode 3 (single TT). Use as a baseline reading or when the pool session is rushed.
• 3 x 400 with full recovery, average pace — feeds input mode 2 (3-rep set) if you also log stroke rate and DPS, or input mode 1 if you only care about average pace (enter one rep as distance 1 and a different shorter rep as distance 2; or use the 3-rep mode and ignore SDI).
• 800 + 200 or 600 + 300 — any two distances that are far enough apart to give a stable slope work in input mode 1. Avoid pairs that are too close (e.g. 200 + 250) because tiny time deltas are dominated by reaction-time noise.

What to keep consistent

Whatever protocol you pick, stick with it across the season for the same athlete. CSS values from different protocols are NOT directly comparable for trend tracking — small differences in anaerobic load between protocols shift the CSS estimate by a few seconds per 100 m. The athlete's relative improvement month-over-month is the signal; protocol-swapping is noise.

Outputs

• CSS pace in seconds per 100 m, plus a formatted mm:ss string per 100 m.
• SDI score (Swim Distance per stroke Index) if you logged SR and DPS for the rep set.

When you run the calculator from inside the race planning wizard, the output is saved as a CalculatorResult linked to the race; the wizard dashboard shows CSS and SDI as the step 1 key values.

How often to retest

• Every 6 to 12 weeks during active swim build phases.
• After any major form change (new pool, new technique cue).
• Less often during off-season or recovery blocks.

See also: Yousuli swim zones, Glossary: FTP, CSS, threshold pace, CSS Test assessment.