Swim Interval Calculator

A swim set lives or dies on the send-off. Get it wrong and you either rest too long and lose the training effect, or you rest too little and your stroke falls apart by rep four. This calculator does the arithmetic for you. Give it a target pace per 100, the rep distance, and the rest you want. It returns the send-off time you read off the pace clock.

What the calculator computes

It works in two steps. First it scales your target pace to the rep distance. A 1:30 per 100 pace over a 100 m rep is 1:30 of swimming. Over a 50 m rep it is 45 seconds. Then it adds your rest and rounds up to the next 5 seconds, because pace clocks run in 5 second blocks and you want a clean number to leave on.

So a 1:30 pace, a 100 m rep, and 15 seconds rest gives a 1:45 send-off. You push off, swim your 100, and the next rep starts when the clock hits 1:45 again. The faster you finish, the more rest you bank.

Where to get your target pace

The honest input here is a pace you can hold for the whole set, not your flat-out 100. Most swimmers anchor it to their CSS, the critical swim speed that marks the line between aerobic and anaerobic work. Run the CSS calculator first, then set your interval pace at or a touch under that number for threshold sets.

If you only have a single swim recorded, the swim pace calculator breaks a total distance and time into a clean per 100 figure you can drop in here. Either way, the pace you enter should be repeatable, since the send-off is built on the assumption that you hit it every rep.

Using the send-off to train

The rest you choose sets the character of the work. Tight rest of 10 seconds or less keeps your heart rate high and trains pace under fatigue. Longer rest of 20 seconds or more lets you swim each rep faster and cleaner, which suits sharper quality days or longer reps. The calculator shows the true rest inside the cycle so you can see what your rounding bought you.

As you get fitter, hold the send-off steady and let your swim times drop. When the rest starts feeling long, tighten the send-off by 5 seconds. That is the same progression a coach would write, and it pairs straight into a build week from Your First Triathlon Training Plan.