How to Calculate Your Step-Diopter Swim Goggle Power
Step Diopter or Pre-made off the shelf prescription swim goggles correct close to the prescription not exact. Use this guide to help determine what power to order.
Definition of terms:
Your Rx may not say Right or Left. If it uses the latin, OD is right, OS is left.
The first number (column) of your prescription is the "Sphere". Premade goggles only correct the sphere.
The second column is the "cylinder" (cyl). This is your astigmatism. If your Cyl column is above 1.50, skip to step 3 first.
1. Add half the cylinder to the sphere
| SPH | CYL | Calculation | Estimated power |
|---|---|---|---|
| −2.00 | None | −2.00 + 0.00 | −2.00 |
| −3.00 | −1.00 | −3.00 + (−0.50) | −3.50 |
| −4.00 | −0.50 | −4.00 + (−0.25) | −4.25 |
| +3.00 | −1.00 | +3.00 + (−0.50) | +2.50 |
| −4.00 | +1.00 | −4.00 + (+0.50) | −3.50 |
The result is called the spherical equivalent.
2. Round Down
- Between two minus powers? Most of the premade goggles come in 0.50 or 1.00 steps. If you are in between powers (i.e. -4.25) round down to the nearest power. Don't round up. If you round up, you risk getting used to that power making your regular glasses seem weak when you put them back on.
3. Check the cylinder before ordering
Premade step-diopter lenses don’t correct astigmatism. If either eye has about 1.50 diopters of cylinder or more (plus or minus), ask us about custom goggles before ordering. That’s a point to review your options; even smaller amounts can leave noticeable blur.
Want to see why? Try the Eyesight Simulator. For an example prescription of −2.00 SPH / −2.00 CYL, enter 0.00 SPH / −2.00 CYL to illustrate what remains after correcting sphere alone. A −3.00 spherical-equivalent lens instead leaves an approximate +1.00 SPH / −2.00 CYL. Keep the same axis in both examples. These settings illustrate remaining blur; they don’t preview exactly what you’ll see in the water.
Shop Prescription Swim Goggles
Ready to find your next pair? Browse our prescription swim goggles and compare the available lens powers to the estimate you calculated above. If you’re unsure which option to choose, ask us for help before ordering.
