How current enters the estimate
A current pushing you the way you're going adds to your speed; one against you subtracts from it. The model holds when the current is aligned with your route — with you or against you, in a straight line. A current that hits you side-on pushes you sideways and forces you to angle your swim to compensate, lengthening the real path compared with the straight line — a different problem, not modelled here. Where the current shifts direction or comes from the side, common in many straits, treat this number as a rough order of magnitude and ask someone who knows the local waters.
Why a pace held over a mile gets slowed down
The pace you hold over a mile is the pace of a fresh effort: over a long crossing, fatigue brings it down. If you declare your pace "over a mile", we slow it down according to the distance: the cost of each metre swum rises gradually up to 21% at 20 km and beyond, and at the same effort your sustainable speed is divided by the same factor (1.21 at 20 km).
The 21% is a measured figure, but not ours: it comes from Zamparo and colleagues (2005), who found that in elite long-distance swimmers the energy cost was on average 21% higher, at the same speed as before, after a 2 km effort at race pace. That study gives a single before and after, not a curve over hours of swimming: that the rise is gradual up to 20 km is our own editorial choice. If instead you declare the pace you expect to hold for the whole distance, the correction isn't applied, so it isn't counted twice.
Why a wetsuit changes the time
A wetsuit makes you float higher: your legs do not drop as they would with bare skin, and a more horizontal body meets less drag in the water. The advantage is not the same for everyone: those who already swim with an efficient position gain less, while those starting from a slower pace gain more, because they have more to gain from the extra buoyancy. That a wetsuit helps less experienced swimmers more is also written by Terry Heggy for U.S. Masters Swimming (2015).
The correction we apply starts from the towed drag measurements made by Chatard and colleagues (1995) on eight triathletes: with a wetsuit, their drag drops by 22% at 1.1 m/s (1'31" per 100 m), by 14% at 1.3 m/s (1'17"), and by 9% at 1.6 m/s (1'03"). In competitive swimmers from the same study, it did not change. About one-third of that drop translates into speed: Toussaint and colleagues (1989), who measured 14% less drag, estimated a swimmer would be about 5% faster. Hence the 3% at 1'03" and just over 7% at 1'31".
At slower paces, drag measurements run out and we do not extend the curve. However, the gain continues to grow: in a 30-minute test in 18 °C water, at about 2'09" per 100 m, triathletes in wetsuits swam 7% further, and a 2022 review reports another study, over 1,500 m, that reaches 10%. For this reason, from 1'31" we make the correction rise more slowly, up to 9% at 2'30", and from there we hold it steady. This last stretch is our own choice and rests on only two studies, both conducted in cold water. Your actual gain also depends on the wetsuit model and how well it fits you.
Salt water: a shakier advantage
The sea is denser than a lake due to dissolved salt, and this makes you float a little more: the same principle as a wetsuit, on a much smaller scale. Here the data is weaker. Unlike the wetsuit, the numbers come from our own field tests, not a lab study, and the effect is smaller. A review on the link between buoyancy and performance considers it beyond dispute that those who float higher encounter less drag, but it does not say by how much and does not discuss salt water: we have not found a study measuring the advantage of the sea over a lake. The correction we apply is for this reason lower than the wetsuit one.
What's left out of the sum (and why)
Waves and wind stay out of the sum: their effect depends on direction relative to your route, wave period, how well you handle them — too many variables for one number.