Swim pace:
time per 100 m, set targets and race predictions

Why swimmers think in 100 metres

Runners talk in minutes per kilometre, cyclists in kilometres per hour, and swimmers in time per 100 metres. The unit suits the sport: pool lengths are 25 or 50 metres, and a single length is too short to compare while a kilometre is too long to hold in mind during a session.

A per-100 pace turns a vague session into a plan. If you know your comfortable pace, you can set intervals, work out how long a continuous swim should take and notice when you are slowing down after the second kilometre.

Pace is also the common language between swimmers. A time of 1:45 per 100 means the same thing to everyone in the lane, regardless of whether they swam 400 metres or 4 kilometres.

The formula

A second example with an uneven number: 1,500 m in 28 minutes 30 seconds is 1,710 seconds. Divide by 1,500 to get 1.14 seconds per metre and multiply by 100 for 114 seconds, which is 1:54 per 100 m.

Digital watches and apps do this conversion for you, but the formula is worth knowing because the numbers they show depend on how they count. Some devices count pool lengths by arm motion, which can drift, and some include wall rests while others pause the clock. If your logged pace looks too good to be true, check the settings before you believe it.

Using a pace to set targets

Run the formula the other way to predict a time: multiply the pace by the distance and divide by 100. At 1:54 per 100 m, a 400 m swim takes 4 × 114 = 456 seconds, or 7:36.

Treat predictions as a ceiling for a rested swimmer in a similar pool. Pace usually drifts slower as distance increases, because fatigue, a poorer turn technique and loss of focus come in late in a long swim. A rough planning guide is to expect your 1,500 m pace to be slower than your 400 m pace.

A race plan can use the same table backwards. If your goal for 1,500 m is 25 minutes, the required pace is 1,500 seconds ÷ 15 = 100 seconds, or 1:40 per 100 m. Compare that with a recent 400 m effort: if you need to hold nearly the same pace you swam for 400 m, the goal is ambitious and you may want a staged target for this season.

Critical swim speed from two time trials

Coaches often estimate an aerobic threshold pace from a short and a long all-out swim. The method needs two time trials on separate days, such as 200 m and 400 m.

Retest every six to eight weeks under similar conditions. A fall of a couple of seconds per 100 m in the result is a sign that the training is working, while a rise can mean fatigue or an unrepresentative trial.

Turning a pace into a training set

Coaches write sets as a repeat count, a distance and an interval, such as 10 × 100 m on 2:00. The interval is the time from the start of one repeat to the start of the next, so rest is whatever remains after the swim.

To make the set harder without changing the distance, cut the interval to 1:55 so the rest falls to 10 seconds. To make it easier, lengthen the interval or reduce the repeat count. Because the clock restarts on the interval, swimming faster earns longer rest, which is why a steady pace is easier to hold than a fast first repeat followed by a collapse.

In a 25 m pool, a pace of 1:45 per 100 m means 26 seconds and a quarter per length. Writing the 25 m split on your hand or a wall card is a simple way to check pacing during a continuous swim, and the same split doubles to 52.5 seconds for a 50 m pool.

Converting pace to speed

Pace per 100 m is a time over distance, so it is the inverse of speed. To compare with other sports, divide 100 metres by the seconds and convert metres per second to kilometres per hour by multiplying by 3.6.

At 2:00 per 100 m, speed is 100 ÷ 120 = 0.833 m/s, which is 3.0 km/h. At 1:30 per 100 m it is 1.11 m/s, or exactly 4.0 km/h. That is why a triathlete who swims 1.5 km at 2:00 per 100 m takes 30 minutes in the water while the same person might cover 40 km on a bike in about an hour.

Small pace changes add up on long swims. Gaining 5 seconds per 100 m over 1,500 m saves 75 seconds, which is often more time than you could gain from a quicker turn alone.

What makes two paces hard to compare

• Pool length: a 25 m pool gives more push-offs than a 50 m pool, which can make the same effort look faster in the short pool.
• Open water: current, waves, wetsuits and sighting change the pace and the true distance.
• Lap counting: one missed length changes the distance by 25 or 50 metres and shifts the pace by several seconds.
• Rests: if you stopped at the wall, state whether the pace includes rest time.
• Equipment: fins, paddles and pull buoys flatter the time.