Short answer: most prescription swimming goggles are sold in fixed diopter increments (commonly -1.5, -2.0, -3.0, -4.0, -5.0, -6.0, -7.0 and so on) rather than a continuous custom range like eyeglasses, so the goal is to round to the nearest available strength using your spherical (SPH) power — cylinder (astigmatism) correction is usually not available in stock Rx swim goggles.
An eyeglass prescription has several numbers, but for swim goggles only one matters: SPH (sphere), the main correction for near- or far-sightedness. A prescription reading "-3.25" means you'd typically choose the nearest stocked strength, usually -3.0, since Rx swim goggles are manufactured in fixed steps rather than custom-ground to a decimal.
CYL and AXIS values on a prescription correct for astigmatism, and correcting for this in a swim goggle lens requires the lens to sit in a fixed rotational position on the face — something goggles, which shift and reseat with every stroke and every time they're removed and put back on, can't reliably guarantee the way a spectacle frame can. For this reason, most Rx swim goggles on the market are sphere-only. Wearers with significant astigmatism will get a noticeably softer result than their glasses provide, though most find it a large improvement over swimming with no correction at all.
If your prescription falls exactly between two available strengths, the general guidance is to round toward the weaker (lower absolute) power rather than the stronger one — slightly under-correcting produces a softer, more comfortable image, while over-correcting a swimmer's vision underwater can cause eye strain and headaches over a longer swim session. This matters more the stronger the prescription is; someone at -1.5 barely notices a half-step difference, someone at -7.0 will.
Most quality prescription swim goggles allow the left and right lenses to be different strengths, matching each eye's individual prescription rather than forcing a single averaged power across both — always check this before ordering, as some budget Rx goggles only offer a single matched-pair power and can't accommodate a real prescription where the two eyes differ.
For a first pair of Rx swim goggles, or for a child whose prescription may still be changing, it's generally safer to choose a slightly lower strength than the eyeglass prescription rather than rounding up — clear enough vision to swim safely and confidently matters more than perfect optical correction in the water.
Browse Salvador Eyewear's full range of prescription swimming goggles by diopter strength.