Heats are a ceiling, and ceilings step
Entries divided by lanes, rounded up. That is the whole calculation, and its behaviour is the reason meets run long. Thirty-six entries in six lanes is six full heats. Thirty-seven is seven heats, the last of which holds one swimmer, and that heat consumes a full start, a full swim and a full changeover before the next event can begin.
So the events that cost you time are not the biggest ones. They are the ones sitting one or two entries above a multiple of the lane count. On a session with eight events, two of them landing badly is two extra heats, and on a 200 free that is around six minutes of clock for two swimmers.
The fill rate, and why it is worth printing
Multiply the heats by the lanes and you get lane slots — the number of swimmer-places the session physically ran. Compare that with the entries and you have a fill rate. A session with 231 entries running 42 heats in 6 lanes used 252 slots, so 21 lanes were empty, and the water travelled past those empty lanes at exactly the same rate as the full ones.
That figure is the honest measure of how efficiently a session is packed, and it is the one that moves when the entry list changes shape. Deck seeding, combining heats and combining age groups all attack the same number, and none of that is a decision a page can make for you — the rules for it come from whoever is sanctioning the meet.
Seconds per heat is a measurement
The field asks for the seconds one heat takes, and the figure to put in it is the slowest lane plus the touch, from your own timing sheet or from last year, not a winning time. The difference is not small: a 50 free heat where the winner goes 28 and the last lane goes 42 takes 42 seconds of clock, not 28. Meets that run late almost always ran late because the heat time was taken from the fast end of the field.
The gap between heats deserves the same treatment. Water cleared, next heat behind the blocks, starter ready, timers reset. Twenty-five seconds is an ordinary figure and it is what the field starts at, but the only version worth trusting is one somebody measured on your deck, at your meet, with your officials.
What one more lane does
The calculator reruns the whole session at one lane more and one lane fewer, because that comparison is not intuitive. Adding a lane does not reduce the session time proportionally — it reduces it by whatever heats it removes, which depends entirely on where each event count sits relative to the multiples. Some events lose a heat, some lose none, and the total is a lumpy number rather than a percentage.
For lane geometry — whether a seventh lane fits across your width at all — see the pool lane layout calculator, and for the lines to divide it, the lane line take-off calculator. For the same throughput problem in a different sport, the trial ring schedule calculator works an entry list into a ring finish time, and the swimming pace calculator converts a swim into a pace.
Every figure on this page came from you. Lane counts, per-lane loadings, lane widths, course lengths, send-off intervals, spacings and session lengths are fields you fill in from your health department, your facility operator, the governing body you compete under, the event organiser or your coach. The page holds none of them, does not know which apply to you, and nothing it prints is a statement of what any pool, programme, event or competition requires.
Starting blocks over shallow water, and diving into shallow water generally, cause catastrophic spinal injuries every year. Where blocks may be used, at what depth, and who may start from them are set by your governing body and your facility operator. Nothing on this page addresses it and no depth is stated anywhere on it.
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
How do I work out how many heats an event has?
Divide the entries by the racing lanes and round up. Forty-two entries in six lanes is exactly seven heats with no empty water; forty-three is eight heats, the last of which is one swimmer. Because the division rounds up, the heat count steps rather than slides, and one extra entry can cost a whole heat of clock time. The calculator also prints how many swimmers are in the last heat of each event, which is the number that tells you where the session is losing time. How heats are actually seeded, and whether they can be combined, is decided by the body sanctioning the meet.
What should I put for seconds per heat?
The time from the start to the last touch, plus nothing else, taken from your own timing sheet or last year's meet. Use the slowest lane, not the winner. A 50 free where the fast lane goes 28 and the last lane goes 42 occupies 42 seconds of the session, and building a timeline on the 28 is how a five-hour session becomes a seven-hour one. If you have no history at all, time a few heats at your next meet and write the numbers on the sheet — one meet of measurement replaces every estimate you would otherwise make for years.
Why does the session run longer than the events add up to?
Because of everything between them. Each event carries a gap between its heats and there is another gap between events, and on a session with 42 heats and 8 events those gaps are around twenty minutes before the break is counted. The calculator separates racing time, heat gaps, event gaps and the break so you can see which one is doing the damage. On most sessions it is the heat gap, because it is small and it happens dozens of times, which is exactly the shape of number people leave out of a mental estimate.
How much time does an extra lane save?
It depends on where your entry counts sit, which is why the calculator reruns the whole session rather than applying a ratio. Going from six lanes to seven removes a heat from an event with 37 entries and removes nothing from one with 35, so the saving is lumpy. On the default entry list the seventh lane removes six heats and takes about nine minutes off the session, and the finish time moves accordingly. Whether a seventh lane exists in your pool is a separate question, and the lane layout calculator answers it from your wall-to-wall width.
Does this handle deck seeding, scratches or combined heats?
No. It takes an entry count per event and packs it into lanes, which is the arithmetic underneath all three of those but is not any of them. Scratches change the entry count, so rerun it with the number that is actually swimming. Combining heats or age groups changes what an event even is, and whether either is allowed is a rule of whichever body is sanctioning your meet, not something this page knows. The value of the page is upstream of all that: it shows you where the empty lanes are, which is where combining would pay if the rules permit it.