Why your best 100m changes your whole training

Most swimming apps hand you a programme. ForSwim gives you one that looks like you. The difference comes down to a single number: your best 100m. Here is why that time changes everything about the quality of your training.

The problem with generic programmes

Imagine two swimmers: Marie, 100m in 1'20", and Thomas, 100m in 2'10". Give them the same programme, "6×100m, rest 20 seconds", and what happens?

Marie finishes each 100m in 1'20" and has only 20 seconds to recover. She leaves again before she is ready. The session turns into piled-up fatigue, her technique falls apart, and she risks finishing flat.

Thomas finishes in 2'10". With 20 seconds of rest, a send-off every 2'30" leaves him plenty of time to catch his breath. He recovers too much. The session is not hard enough to make him better.

The same programme, two opposite experiences. That is why the 100m is the keystone of a smart plan.

How your 100m is used in every session

Your best 100m freestyle is your reference speed, what swimming calls your base pace. From there, each intensity zone is calculated with a coefficient:

Easy zone (Z1/Z2), coefficient ×1.35 If you swim the 100m in 1'30" (90 seconds), your Z1/Z2 pace is 90 × 1.35 = 2'01"/100m. That is a pace you can hold for a long time, where you could hold a conversation, and build endurance without burning out.

Threshold / CSS zone (Z3/Z4), coefficient ×1.08 Same example: 90 × 1.08 = 1'37"/100m. That is your anaerobic threshold, the fastest pace you can hold over a long distance. Working just under this zone, or right on it, is where real progress happens.

Sprint zone (Z5/Z6), coefficient ×0.95 90 × 0.95 = 1'25"/100m. Maximal effort on short repeats (25m, 50m). This zone works your power and your top-end speed.

Without your 100m, these numbers are estimated from a stated level (beginner, intermediate, advanced). It works, but it is an estimate. With your 100m, your real physiology drives the training.

Send-offs on the clock: a concrete example

Take a classic threshold session: 8×100m at CSS pace.

Without your 100m filled in (level "intermediate"): Send-off every 1'55". That is the average value for this level.

With your 100m at 1'30": Calculated CSS pace = 1'37"/100m. Estimated swim time = 1'37". Add 15" of rest. Send-off every 1'55". It matches here.

With your 100m at 1'15": CSS pace = 1'21". Rest 15". Send-off every 1'40", which is 15 seconds less. Over 8 repeats, the difference in work density is huge.

With your 100m at 2'00": CSS pace = 2'10". Rest 15". Send-off every 2'25". If you had been left on 1'55", you would have swum too fast, or had no time to recover.

That grain of precision is what separates training that moves you forward from training that only tires you out.

How to measure your best 100m

You do not need perfect conditions. Here is the simple protocol:

1. Pick the right moment Mid-session, after a solid warm-up (400m minimum). Never at the start, with cold muscles. Never at the end, if you are exhausted.

2. The specific warm-up 200m easy + 4×25m build (the last one almost all-out). You should be ready to leave hard from the first metre.

3. The 100m Push off the wall (do not dive: it skews the result). Swim at maximal effort, but steady. A lot of swimmers leave too fast on the first 25 metres and fall apart. The goal: make your 4 lengths as even as possible.

4. Write the time down Stop the clock when you touch the wall. That is your reference 100m.

Tip: repeat this test every 6 weeks. If you have improved, update your time in ForSwim: your send-offs are recalculated automatically, and your plan matches your new form.

Why it also changes your endurance (not only the sprints)

People often assume that personalising paces only matters for hard sessions. That is wrong. It may matter even more for endurance.

Too much training sits in the "grey zone": not slow enough to truly restore you, and not fast enough to truly stimulate you. That is the zone where many swimmers spend 80% of their time, without knowing it, and it explains progress plateaus.

When your Z1/Z2 pace is calculated properly from your 100m, you avoid that trap. Your endurance sessions are truly easy, which lets your body recover, lock in the adaptations from the hard sessions, and come back stronger.

That is the principle of polarized training: 80% of sessions very easy, 20% very hard. For it to work, you need to know where "very easy" sits for you, not for a hypothetical swimmer at your level.

What if I do not know my 100m?

No panic. ForSwim also works without it. Paces are estimated from your level (beginner, intermediate, advanced). You still get a solid programme.

If you want to go further, here is how to run a first test quickly:

Option 1, pool test: Next time you swim, put 3×100m all-out into the session, with 5 minutes of rest between each. Take the average of the 3. That is a good estimate of your best 100m in training conditions.

Option 2, estimate from a recent test: You swam a race or a time trial recently? Over 50m, multiply your time by 2.1 to estimate your 100m. Over 200m, divide by 2.3.

Enter that time in your ForSwim profile and your sessions adjust straight away. You do not need to regenerate the plan. The calculation is automatic.