The lap never divides into the race
Course legs get chosen for the water — where the depth is, where the marks will hold, where the sun is at the start, which line the safety boats can work. Race distances get chosen by the event. The two have no reason to agree, and they usually do not.
Three 240 metre legs make a 720 metre lap. A 1500 metre race is 2.08 laps. Two laps is 1440, sixty short. Three is 2160, far over. The usual resolution is the third one: put the start line sixty metres out from the first mark so the swim is 60 plus two laps, and now the course measures 1500 while the lap still measures 720. The calculator gives all three answers because which one an event picks depends on the beach, not the arithmetic.
Errors multiply by the lap count
This is the reason to measure carefully and the reason the calculator asks for an uncertainty. Every leg is swum once per lap, so ten metres of error on one leg of a two-lap course is twenty metres on the race — and on a four-lap sprint course it is forty, which is more than two per cent of a 1500. On a rectangle the effect is worse still, because each pair of sides is swum twice in a single lap.
Set marks from a measured baseline or from position fixes rather than from a boat log, and measure the long leg twice. The error you cannot see on the water is the one that turns up in the results.
Rode, and marks that move
Anchor rode is depth multiplied by a scope ratio, and the ratio is yours: it comes from whoever is setting the marks, from what the bottom is like and from what the wind is doing that morning. Twelve feet of water at three to one is thirty-six feet a mark, and five marks is 180 feet of line before any spare.
Carry more than the arithmetic. A mark that drags has to be reset, and it will be reset in worse conditions than it was laid in, with swimmers already in the water or about to be.
What the craft count is, and is not
Open water is not a pool with different scenery. Cold water takes a swimmer apart faster than most people believe, currents and boat traffic do not appear on a course drawing, and visibility on the water can go from ordinary to nothing in the time it takes weather to arrive. None of that is arithmetic and none of it is addressed here. An open-water event is planned with the water authority, the landowner or harbour master, the governing body sanctioning it, and people who have run events on that specific water.
The craft line on this page is one division: the course outline divided by a spacing you typed in. It is there because organisers are given a spacing and need to know how many boats that implies before they start asking clubs for volunteers. It is not a safety figure, it does not become one by being computed, and it says nothing about whether any event is adequately covered. That question belongs to the event organiser, the sanctioning body and the water authority, in that order and in writing.
Nothing here is a verdict. The page does not say a pool is adequately staffed, a lane is safely loaded, a session is properly supervised or a course is well laid out, because none of those is arithmetic and none of them can be settled by a page that has never seen your water. Supervision and lifeguarding are governed by your health department and your facility operator, and this page says nothing whatever about either.
For distance and pace once the swimming starts, see the swimming pace calculator. For getting a boat from one mark to another with a current running, the boat course to steer calculator. For what holds a mark up rather than down, the dock float buoyancy calculator.
Questions people ask
How do I make a course measure an exact race distance?
Usually by moving the start line rather than the marks. Pick leg lengths that suit the water and accept whatever lap distance falls out, then work out how many whole laps fit inside the race distance and put the start line the remainder away from the first mark. A 720 metre lap and a 1500 metre race means two laps plus a sixty metre run to the first buoy. The alternative — resizing every leg so the lap divides exactly — sounds tidier and is much harder to execute, because the marks then have to go where the arithmetic says rather than where they will hold.
How much anchor line does each buoy need?
Depth multiplied by whatever scope ratio the people setting the marks are working to, which is an input on this page rather than a figure it holds. Twelve feet of water at three to one is thirty-six feet per mark. Two things are worth adding to that total: spare line, because a dragging mark gets reset under worse conditions than it was laid under, and the fact that scope is a judgement about the bottom and the forecast rather than a constant. Whoever is laying the marks decides it, and if nobody on the team has done it on that particular water before, that is the person to find.
Does the triangle have to be equilateral?
No, and plenty of real courses are not. This calculator models the equal-sided case because it is the common one and because it needs only a single leg length, which keeps the measuring simple: three equal sides means one measurement checked three times rather than three separate measurements. If your course has unequal sides, add the three legs yourself and use the rectangle mode with the two sides that sum correctly, or work the lap distance out directly. The lap-into-race arithmetic and the error multiplication work the same way whatever the shape.
How many safety craft does a course need?
This page does not answer that and no page should. The number, the type of craft, who crews them and what they carry is decided by the event organiser, the body sanctioning the event and the water authority for that water, and it depends on conditions, the field, the water temperature and things nobody can put in a form. What the calculator does is divide the course outline by a spacing you have already been given, which tells you how many craft that spacing implies. Use it to plan volunteers. Do not use it as evidence about anything.
What does the leg uncertainty field actually do?
It multiplies. Enter how far out you think one leg might be and the page shows what that becomes across the whole race, because every leg is swum once per lap and the lap count multiplies the error before anyone sees it. Ten metres on one leg of a two-lap triangle is twenty metres on a 1500 — over one per cent, on a course that looked fine. It is there to make the case for measuring the long leg twice, and for setting marks from a measured baseline rather than from a boat travelling at an estimated speed for an estimated time.