The board does not change, the set does
A send-off is a departure time, not a rest period. You leave the wall every interval regardless of how the last one went, so the rest is whatever is left over — and what is left over shrinks as the swim times grow. That is the whole mechanism, and it is why a set can get harder repeat by repeat while the number written on the board stays exactly where it was.
Put figures on it and the effect is stark. A hundred at 1:35 on a 1:50 send-off leaves fifteen seconds. Drift six per cent by the tenth repeat, which is a completely ordinary amount of drift, and the same send-off leaves a little over nine. The last repeat has nearly forty per cent less rest than the first, and nothing on the board records that.
The failure point is worth naming too. If the swim time ever exceeds the interval, the swimmer arrives after the next send-off has already gone and there is no rest left to shrink. From that repeat onward the set is continuous swimming with a stopwatch attached, and the clock stops describing what is happening. The calculator flags the repeat where that first occurs rather than pretending the arithmetic still holds.
Reading a sixty-second face
A pace clock has one hand and a sixty-second face, which means it tells you where in a minute you are and nothing else. Intervals that are whole minutes always leave from the same point — on a 2:00 send-off the departure is on the top every single time, which is why coaches like them and why they are the easiest to run a large group on.
Anything else walks. A 1:50 send-off is 110 seconds, and 110 and 60 share a factor of 10, so the departures land on the 0, then 50, then 40, then 30, 20, 10, and back to 0 — six repeats to come back round. The calculator prints that cycle because it is the thing you cannot work out while swimming, and because knowing your set returns to the top every sixth repeat is what stops you counting wrong at the wall.
Fade is a measurement, not a target
The fade field asks how much slower the last repeat is than the first, and the honest source for it is your own splits from a set like the one you are planning. Zero means holding the opening pace throughout. The calculator distributes the slowdown evenly across the repeats, which is a simplification — real sets fade unevenly, often flat for a while and then all at once — but an even distribution gives the right total and the right last-repeat number, which are the two figures the rest depends on.
Nothing here says what your fade should be, what pace you should hold, or what interval you should be on. Those come from a coach who has watched you swim. This page multiplies and subtracts.
Where the pace itself comes from
The pace this page consumes is produced by the swimming pace calculator, which converts lengths and a stopwatch reading into time per hundred yards and per hundred metres — including the eight and a half per cent gap between the two that ruins swim logs. Feed the per-hundred figure from there into the pace field here.
For pace arithmetic in other sports, the running pace calculator and the speed and pace converter do the same job on land. For lane space rather than time, the swim lane capacity calculator.
There is no coaching on this page. Nothing here is training guidance, a workout, a conditioning plan, a stroke correction or anything medical, and no number it prints should be read as what any swimmer needs or can do. It converts times, distances and counts into other times, distances and counts. What a swimmer should actually be doing in the water is a question for a coach.
Drowning is the hazard behind all of this, and it is quiet and fast. Who watches the water, how many of them there are, what they are qualified to do and when the pool may open at all are decided by your health department and your facility operator. This page contains no ratio, no procedure and no requirement of any kind on that subject, and a number it prints is never a reason to believe an activity is adequately watched.
Questions people ask
What interval should I be on?
This page has no answer to that and will not offer one. The send-off is a coaching decision made by somebody who has watched you swim, and it depends on what the set is for, where you are in a season, what you did yesterday and half a dozen things a calculator cannot see. What the arithmetic does is show you the consequence of an interval you have already been given: how much rest it leaves at the pace you actually hold, how that rest shrinks as you fade, and whether it collapses altogether before the last repeat. That is a useful thing to look at before a set. It is not a recommendation.
Why does my rest get shorter when the interval never changes?
Because rest is the leftover. You leave on the send-off no matter what, so the time you get at the wall is the interval minus however long the swim took, and the swim takes longer as the set goes on. On a 1:50 send-off with a 1:35 opening hundred you start with fifteen seconds. A six per cent drift by the last repeat makes it a 1:40 or so, and the rest is down to nine or ten. That halving of the rest is the set doing what a send-off set is designed to do, and it is completely invisible on the board, which still says 1:50.
What happens if I miss the interval?
You arrive after the next send-off has already gone, so there is nothing left to rest and you either leave immediately behind schedule or drop out of the rotation. The calculator finds the first repeat where the swim time exceeds the send-off and says so, then carries on modelling the set as continuous departures from the moment you get back, which is the honest version rather than pretending the schedule survived. In practice what happens next is a coaching decision — move down a lane, drop a repeat, change the interval — and none of that is on this page.
Where does the clock face column come from?
It is the seconds hand position at each departure, on a sixty-second face, starting from wherever you told it the first send-off was. Whole-minute intervals never move: every repeat of a 2:00 set leaves on the same second. Anything else walks round the face and eventually comes back, and how many repeats that takes is sixty divided by the greatest common factor of the interval and sixty. A 1:50 send-off comes back every sixth repeat, a 1:45 every fourth, a 1:20 every third. It is the sort of thing you cannot compute while breathing every three strokes, which is why it is printed.
Does the total set time include the rest?
Yes. The clock total runs from the first send-off to the moment the last repeat touches, so it contains all the swimming and all the rest except whatever happens after the final touch. The page also breaks it into time swimming and time on the wall, and gives the share of the set actually spent moving — which for a set with fifteen seconds of rest on a hundred is around ninety per cent, and for a short-rest sprint set can be a good deal less. That share is the figure to compare between two different sets of the same total distance.