Choosing the right cup size can feel confusing. Every brand uses different names for their sizes. One brand's "small" might be another brand's "large." That's why Shiro created a universal sizing system that makes comparing cups simple and clear.

How Brands Name Their Cups

Different menstrual cup brands use all kinds of names for their sizes. You'll see "Model 0," "Size A," "Small," "Regular," or even just numerical codes. A brand might call their cup "Small" even though it's actually wider than another brand's "Large."

This inconsistent naming makes it nearly impossible to compare cups across brands. You can't tell if a "Model 0" from one company will fit like a "Size A" from another. This is where Shiro's universal system changes everything.

Shiro's Universal Sizing System

Shiro measures every menstrual cup and places it into one of four universal sizes: S, M, L, or XL. We base this on the cup's width, also called diameter. This is the measurement across the widest part of the cup.

Here's how Shiro's universal sizing works:

Size S: 0-40 mm width
Size M: 41-43 mm width
Size L: 44-46 mm width
Size XL: 47 mm and above

Every cup in our database gets measured and assigned a Shiro size. This means you can compare any cup to any other cup using one simple scale. A brand's "Model 0" with 38 mm width becomes a Shiro "S." Their "Model 1" with 42 mm width becomes our "M."

Why Cup Width Matters Most

Width is the primary factor in cup sizing because it determines how the cup fits inside your vaginal canal. The vaginal canal is naturally elastic and can stretch to accommodate different widths. However, there's a comfort range that works best for each woman.

Smaller widths (S and M) tend to feel more comfortable for women who are younger, have not given birth vaginally, or prefer a less noticeable fit. Larger widths (L and XL) work better for women who have given birth vaginally or who need a firmer seal for active lifestyles.

Cup Length: The Secondary Factor

After width, cup length matters—especially if you have a low cervix. Length measures from the rim of the cup to the very bottom, not including the stem.

Women with a low cervix need shorter cups so the cup sits comfortably below the cervix without putting pressure on it. Women with a high cervix can wear any length comfortably. If you're not sure about your cervix height, read Chapter 6: Cervix Height to learn how to measure.

Size and Capacity

Larger cups generally hold more fluid, but this isn't always true. A tall, narrow cup might have the same capacity as a short, wide cup. That's why Shiro shows you both size and capacity for every cup.

Your flow level helps determine what capacity you need. Heavy flows need higher capacity cups. Learn more in Chapter 5: Understanding Your Flow.

Common Size Selection Mistakes

Many women choose cup size based only on age or whether they've given birth. While these factors matter, they're not the whole picture. Your cervix height, pelvic floor strength, and activity level all play important roles too.

Another common mistake is choosing the smallest cup possible thinking it will be most comfortable. Sometimes a slightly larger cup creates a better seal and actually feels more secure. Your body is unique, and the right size for you might surprise you.

Finding Your Perfect Size

The best way to find your perfect cup size is to consider all your factors together: age, childbirth history, cervix height, flow level, and activity level. Shiro's Cup-Figurator quiz takes all these factors into account and recommends cups in the universal size that fits your body best.

Remember, Shiro's universal sizing system means you're never locked into one brand. You can explore different cups across different brands, all within your recommended size category. That's the power of a standardized sizing system—more choices, less confusion, and better results.