38DDD Is the Same As What? Understanding Sister Size

Bra sizing can feel confusing, especially when you encounter labels like DDD, E, or F. One of the most common questions we hear is: “38DDD is the same as what?” The short answer is that 38DDD has multiple equivalents, depending on whether you’re comparing sister sizes or international sizing systems.

This guide explains exactly what 38DDD means, which sizes are equivalent in volume, and how to use that knowledge to find your best fit.

What Does 38DDD Mean?

To find your band size, measure around your rib cage directly under your bust. You should do this while wearing a non-padded bra, and ensure the measuring tape is straight and level across your back. Knowing your current band size is important as a starting point for finding your best fit. If your rib cage measurement plus additional inches results in an odd number, add 5 inches to determine your band size. For example, if your measurement is an odd number like 33, round up by adding 5 to get a band size of 38.

A 38DDD bra consists of:

• 38 → Band size (band's width, determined by ribcage measurement in inches)

• DDD → Cup size, representing a 6-inch difference between the fullest part of the breasts (bust) and the band in most US sizing systems. For a comfortable night's sleep, it's also important to choose the best pajamas for women that offer the right fit and style.

Bra sizing is based on the ratio between the band size (band's width) and the bust measurement, which reflects the volume of the breasts. This ratio helps ensure the cup volume matches the support provided by the band. Think of bra sizing like waist and inseam measurements in clothing: changing one measurement (like the band or waist) while keeping the other the same can alter the fit, but the overall proportional volume can remain similar.

This size indicates a full, heavy bust on a wider ribcage, requiring strong band support and well-shaped cups.

How Bra Size Is Calculated

Finding your perfect bra size starts with two key measurements: your band size and your cup size. Getting these right is the first step toward a well fitting bra that offers the support and comfort you need for your unique body shape.

Measure Your Band Size Begin by wrapping a soft measuring tape snugly around your torso, directly under your bust at the rib cage. For the most accurate band measurement, wear a non padded bra or no bra at all, and make sure the tape is level and not too tight or loose. Note the number of inches. If your measurement is an even number, add 4 inches; if it’s odd, add 5 inches. This total gives you your band size, typically an even number like 32, 34, 36, or 38.

Step 2: Measure Your Bust Next, measure around the fullest part of your bust, usually at nipple level, with your arms relaxed at your sides. This bust measurement should be taken with the tape straight and parallel to the floor.

Step 3: Calculate Your Cup Size Subtract your band measurement (after adding 4 or 5 inches) from your bust measurement. The difference in inches determines your cup size:

• 0 inches = AA cup
• 1 inch = A cup
• 2 inches = B cup
• 3 inches = C cup
• 4 inches = D cup
• 5 inches = DD cup
• 6 inches = DDD cup, and so on

Example:If your bust measures 37 inches and your band (after calculation) is 34 inches, the difference is 3 inches. This means your bra size is 34C.

Why This Matters Knowing your correct band and cup size is essential for finding bras that fit perfectly and provide the right support. However, sizing can vary between different brands and styles, so it’s always a good idea to try on a few options to see what fits best.

Sister Sizes: Expanding Your Options If your current bra size isn’t available or doesn’t feel quite right, sister sizes can help. Sister size charts show which bra sizes have the same cup volume but a different band size. For example, if you wear a 34C, your sister sizes would be 32D and 36B, these all have the same cup size but different band measurements. Understanding your sister size gives you more flexibility to find a bra that fits your body shape and feels comfortable, even when trying new styles or different brands.

Taking the time to measure accurately and explore sister sizes ensures you’ll find a bra that fits perfectly, supports your bust, and helps you feel confident every day.

38DDD Sister Sizes (Same Cup Volume)

Sister sizes, also known as bra sister size or sister bra size, share the same cup volume but have different band and cup letter combinations.

The sister sizes shown in the size chart below have the same cup volume, but the band and cup sizes are adjusted to provide alternative fits. These sizes are considered the same size in terms of cup volume, even though the band and cup letters differ. Sister sizes usually run in opposite directions with one size shifts, meaning if you go down a band size, you go up a cup size, and vice versa.

38DDD Sister Sizes

• 36G (band down, cup up)
• 40DD (band up, cup down)

Primary Size	Sister Sizes
38DDD	36G, 40DD

These sizes hold the same breast volume, but the fit will feel different depending on band tension.

When to Use a Sister Size

• Band is too tight or feels a bit snug? → Try 40DD (You can also try fastening your bra on the loosest hook or tightest hook to see if it improves comfort.)
• Band feels too loose? → Try 36G (Adjusting to the tightest hook or loosest hook may help, but a different size might be needed.)

If you notice the band is too tight, go up a band size and down a cup size to maintain the same cup volume. If the band is too loose, go down a band size and up a cup size. For example, if you wear a 34B and feel that the band is too tight, you can try a 36A, or if the band is too big, you can try a 32C. Going down a band size increases the cup size, while going up a band size decreases the cup size.

Sister sizing is ideal when cup fit is good but the band feels uncomfortable. Finding the correct size and ideal fit may require trying a larger cup size or adjusting the band for a perfect fit.

38DDD in Other Sizing Systems

UK Size Equivalent

• 38DDD (US) = 38E (UK)

UK brands do not typically use “DDD” and instead progress to E, F, FF, and so on.

EU Size Equivalent

• 38DDD (US) ≈ 85F (EU)

EU sizing uses centimeters for bands and single letter cup progression.

DDD vs F vs G: Why the Letters Change

Cup letters are not standardized globally:

• US: DD, DDD, G
• UK: DD, E, F, FF
• EU: E, F, G

This means a DDD cup in one system may be labeled as E or F in another, even when the volume is identical.

Common Fit Issues With 38DDD

Women wearing 38DDD often experience:

• Spillage if cups are too shallow
• Strap digging if band support is insufficient
• Underwire discomfort if wires are too narrow
• Band riding up if the band is overstretched

Proper bra fit is essential for comfort and support. Finding a well-fitting bra can sometimes seem like a tough task, whether you're trying a new style or just getting yourself some new bras. Sometimes, achieving the right bra fit requires trying a new style or brand to better suit your body shape and preferences.

These issues are usually solved by proper sizing and style selection, not by sizing down. If you continue to have fit issues, consider consulting a bra expert for personalized advice.

Best Bra Styles for 38DDD

• Full Coverage Bras: Maximum containment and lift
• Balconette Bras: Structured lift with shaping
• Side-Support Bras: Pull breast tissue forward
• Wide-Band Bras: Distribute weight evenly
• Underwire Bras with Firm Bands: Essential for support

Exploring different lingerie styles can help you find the most comfortable and flattering fit for your body.

Final Thoughts

So, 38DDD is the same as 36G or 40DD in cup volume, and it converts to 38E (UK) or 85F (EU) internationally. Understanding these equivalents gives you more flexibility, better fit options, and less frustration when shopping.

Unlike most clothing, bras do not come in a universal 'one size', finding the right fit requires careful measurement and consideration of sister sizes, since bra sizing is more complex than standard clothing sizes.

At Olivia Paisley Intimates, we design bras with real support and real cup volume in mind because the right fit is about proportion, not labels.