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Critical Swim Speed (CSS) Calculator

Swim a hard 400 and a fast 200, enter both times, and get your threshold pace, five training zones and projected race times.

Test

Wakayoshi two-test method: 400 + 200

Enter your 400 and 200 m test times to find your Critical Swim Speed.

How to Run the Test

  1. 1

    Warm up properly — 15–20 minutes easy swimming with drills, plus a few short efforts — so the starts are not a shock to your system.

  2. 2

    Swim the long test — a genuine all-out 400 m (or 400 yd). Hold back just enough that the last 50 is still fast.

  3. 3

    Recruit for 3–5 minutes, then swim the short test — an all-out 200, attacking the last length.

  4. 4

    Enter both times and read your CSS pace, the training zone paces and the projected times below.

Understanding Your Zones

Zone 1–2 (Recovery / Aerobic): the bread and butter of base training. Swim these paces comfortably; you should be able to hold a conversation in short bursts.

Zone 3 (Tempo): firm but controlled. Use it for pace work and longer threshold-style sets.

Zone 4 (Threshold / CSS): your money pace. Around 80–85% of an all-out effort. Sets like 8 × 100 on a tight send-off belong here.

Zone 5 (VO₂ / Speed): short repeats at or faster than CSS on generous rest — quality over quantity.

CSS is a tool, not a daily prescription. Retest every few weeks and recalculate when your other test times improve. For training-set planning, pair this with the Interval & Send-off Calculator, and compare your projected race times against the Speed & Race Projection Calculator.

Why Use the Critical Swim Speed Calculator?

A training pace guessed by feel drifts; a critical swim speed calculator does not. CSS — the fastest pace you can hold with controlled fatigue — is the single most useful number an age-group, Masters, open-water or triathlon swimmer can own. From two all-out swims you get a threshold pace, a full training-zone system and honest race projections.

The classic 400 + 200 CSS test is already the most searched test on the "critical swim speed swimming" topic, and this calculator walks it backwards: enter your two times, get your numbers instantly, no spreadsheets.

Five training zones from two test times

The swim training zones built from CSS cover the whole session: easy recovery and aerobic pace in Zones 1–2, tempo work at Zone 3, the threshold sets at your CSS pace in Zone 4, and short higher-speed repeats in Zone 5. That structure is exactly what stroke-based "effort" scales cannot give you — here every zone is a concrete pace per 100.

Projected race times that respect your threshold

CSS is the anchor for realistic expectations. The projected times from 50 to 1500 — who swims open water for the first time and wonders "what can I actually hold?" — convert a 400 + 200 effort into an honest 1500 target. Compare those projections against the Race Projection calculator and the Pace Calculator to build and verify your plan.

Free, private and repeatable

Retest monthly and watch the numbers move — no sign-up, no server upload, no guesswork. Because it lives in your browser, you can recompute your CSS between sets on the pool deck, then drop the result straight into the Interval Calculator to build that week's threshold set.

CSS Calculator FAQ

What is Critical Swim Speed (CSS)?
CSS is the fastest pace you can hold with minimal fatigue — your "aerobic maximum". It comes from a short two-test set: one long swim (400 m or yds) and one short swim (200), swum all out. The mathematical gap between them reveals your sustainable threshold pace, which age-group and open-water swimmers use to set training paces.
Which test should I swim: 400 + 200, or 800 + 400?
The Wakayoshi 400 + 200 method is the classic test and fits in most practices. You need a long swim that is genuinely hard and a short swim that is very fast. Some coaches swap in a 500 + 200 in yards, or rely on an all-out 1000 when a sprint test is inconvenient — all of them feed the same formula.
How do I use the training zones?
Swim your recovery and aerobic sets at Zone 1–2. Do tempo work at Zone 3 and threshold sets (like 8 x 100 on a set send-off) around Zone 4 — your CSS pace. Save Zone 5 for short, high-quality repeats on full rest. Retest every 4–6 weeks or after a training block to keep the numbers honest.
Is my CSS the same as my race pace?
Not exactly. CSS is a training threshold, not a racing tactic. In a 400 you can hold slightly better than CSS; a 1500 or 1-mile open-water race usually settles very close to CSS. Use the projected times below as targets, then refine them against real race results.
How often should I retest my Critical Swim Speed?
Retest every 4–6 weeks or at the start of a new training block. After a solid aerobic block your CSS is usually faster, and training zones should shift with it. If your race results keep coming in outside the projected range, redo the 400 + 200 test — the formula is only as good as the effort you feed it.
Does CSS work for open-water and triathlon swimmers?
Yes — it is one of the most used pacing tools in open water and triathlon. A 1500 or 1-mile swim settles very close to CSS pace, so the zone you train in is the pace you race at. Sighting, currents and drafting still change the actual times, but the intensity guide holds.

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New here? Start with the Pace Calculator for any race, or read the course conversion guide before comparing times across pools.