Why personal bests are the right starting point
Training zones built on personal bests are individualized by definition. A swimmer's 400 free PB reflects their own aerobic engine, stroke efficiency, and race history — not a generic standard pulled from a chart. If you set zones off PBs instead of age-group norms, the swimmer who tapers well and the swimmer who grinds through a slow-twitch season both get sets calibrated to what they can actually hold.
The catch: a single PB doesn't tell you everything. A 200 free time is loaded with anaerobic contribution. A 1000 free time is almost pure aerobic. Which PB you use — and how you adjust it — determines whether your zones are useful or just decoration on a workout card.
The math behind zone calculation
Most zone systems work off a threshold pace (sometimes called Critical Swim Speed, or CSS) and then set other zones as percentage offsets from it.
The cleanest way to get threshold pace from PBs is the two-distance method:
CSS = (distance 2 − distance 1) / (time 2 − time 1)
Use a middle-distance PB and a longer PB — 200 and 800 free, or 100 and 400 free work well. The math isolates the aerobic component because it subtracts out the anaerobic "head start" baked into the shorter swim.
Worked example
A swimmer's PBs: - 200 free: 1:52.40 (112.40 seconds) - 800 free: 8:52.00 (532.00 seconds)
CSS = (800 − 200) / (532.00 − 112.40) = 600 / 419.60 = 1.430 m/s
Convert to pace per 100: 100 / 1.430 = 69.9 seconds → threshold pace ≈ 1:09.9 / 100m
That number becomes Zone 3 (threshold, roughly lactate threshold / CSS pace). Everything else gets built around it.
Setting the other zones
Once you have threshold pace, common percentage offsets look like this:
- Zone 1 (recovery/easy aerobic): threshold pace + 15–20% → 69.9 × 1.18 ≈ 82.5s → 1:22–1:24 / 100m
- Zone 2 (aerobic base, EN1): threshold pace + 8–12% → 69.9 × 1.10 ≈ 76.9s → 1:16–1:18 / 100m
- Zone 3 (threshold, EN2/CSS): the calculated pace itself → 1:09–1:10 / 100m
- Zone 4 (VO2max, EN3): threshold pace − 3–5% → 69.9 × 0.96 ≈ 67.1s → 1:06–1:07 / 100m
- Zone 5 (sprint/anaerobic): not a percentage of threshold at all — pull this from a short PB (50 or 100 free) instead, since sprint speed and aerobic threshold don't scale together. A swimmer with a big 50 and an average 800 will have a Zone 5 much faster than the offset formula predicts.
This is the detail that trips coaches up when they build zones off a spreadsheet formula alone: threshold-based percentages work fine for Zones 1–4, but Zone 5 needs its own anchor.
Matching the PB to the zone you're setting
- Use a 400–1000 free PB (or the 400 IM for IMers) to anchor threshold and aerobic zones. These distances stress the same system you're trying to train in Zone 2–3 work.
- Use a 50–100 PB to anchor sprint and starting-speed work. Don't extrapolate top speed from a distance PB — it undersells true speed for sprinters and oversells it for distance swimmers.
- For backstroke, breaststroke, fly, calculate zones off the stroke's own PB where possible. A swimmer's 200 fly threshold pace is not their 200 free threshold pace minus a fixed stroke penalty — the ratio varies by swimmer.
Cautions when using PBs to build zones
- A PB from a full taper isn't representative of in-season fitness. If you set zones off a season-best swum at Provincials and then run threshold sets in October, the paces will feel unreasonably hard. Some coaches apply a flat 2–3% "in-season adjustment" to taper-time PBs, or simply update zones after every in-season time trial.
- Old PBs lie. A 12-year-old's 200 free from eight months ago says nothing about their current fitness. Re-test or re-anchor zones every 6–10 weeks for age-group swimmers, since their aerobic capacity moves fast.
- Short-course and long-course PBs aren't interchangeable. Convert or keep separate zone sets — turns add real time in a 25m pool that disappears in a 50m one.
- One bad PB skews everything downstream. If the 800 free PB was swum solo, off a poor start, or in traffic, your CSS calculation inherits that error. Sanity-check the two PBs against each other before trusting the pace they produce.
Keeping zones current
Zones aren't a one-time calculation. Treat them the way you treat best times — update after every meet where a swimmer improves a relevant PB, and re-anchor the whole set after a training block that meaningfully shifts fitness (a full aerobic base phase, a return from injury, a growth spurt). A zone chart from January has a shelf life.
Building this by hand for twenty swimmers, across strokes and distances, updated every time someone drops a best time, is where most coaches give up and default to generic pace charts. SwimIQ takes a swimmer's entered PBs and calculates their training zones automatically, so when you write a set calling for Zone 3 or EN2 pace, the app can check it against each swimmer's actual numbers instead of a guess.