Bra Size Calculator USA
The bust size is the loose circumference measured around the chest over the fullest part of the breasts.
The band or frame size is the firm circumference, fitted not tightly, measured directly underneath the breasts.
Bra Size Converter USA
Convert bra sizes between different international sizing standards.
Bra Size Calculator: The Guide to Finding Your Perfect Fit
A bra size calculator is a measurement tool that determines your correct band size and cup size using bust and underbust measurements for an accurate fit.
Accurate bra sizing reduces shoulder strain, improves posture, and eliminates the pressure caused by tight bands or oversized cups. Modern calculators use precise inch-based measurements, replacing outdated methods such as the “+4 rule” and providing results consistent with USA lingerie standards from brands like Wacoal,
ThirdLove, and Victoria’s Secret. A reliable bra size calculator for home use guides you through measuring your ribcage and fullest bust point with a soft tape measure to generate your true size and sister-size alternatives for better comfort and support.
Why Accuracy Matters: The Science of Support
Before we count and calculate breast size, it is crucial to understand why precision matters. A bra is an engineering marvel designed to combat gravity.
The support should come primarily from the band, not the straps.
The Band’s Role: The band should provide 80-90% of the support. If it is too loose, the weight of the breast tissue pulls down on the straps.
The Strap’s Role: Straps are merely for stability. When they bear too much weight, they dig into the trapezius muscles, potentially causing headaches and shoulder grooves.
Health Implications: According to research often cited by institutions like Dartmouth Geisel School of Medicine and ergonomic studies, insufficient breast support is frequently linked to upper back pain (thoracic pain) and poor posture.
By using a precise bra measurement technique, you are not just improving your silhouette; you are investing in your musculoskeletal health.
The Tools You Need
To get the most accurate result, forget the “guess and check” method at the department store. You will need:
A soft tape measure: If you don’t have one, use a non-stretchy string and a ruler.
A mirror: To ensure the tape is level around your back.
Your best-fitting, non-padded underwire bra: Or go braless if gravity allows you to get an accurate nipple position. Avoid push-up bras as they artificially alter your breast sizes and shape.
How to Calculate Bra Size Step-by-Step
The modern, “expert” method of sizing differs from the traditional method used by many old-school retailers. We will teach you the modern method, which prioritizes a snug band for better support.
Step 1: The Band Measurement (Underbust)
This is the anchor of your bra.
Stand straight and wrap the measuring tape around your ribcage, directly underneath your breast tissue (where the bra band usually sits).
Ensure the tape is parallel to the floor. It should be snug, tight enough that it doesn’t slide, but not so tight that it cuts off circulation.
Exhale deeply. This ensures you get the smallest measurement possible for a firm fit.
Record this number in inches.
Expert Note: If you get an odd number (e.g., 33 inches), round to the nearest even number. However, unlike the old days, do not add 4 inches automatically unless the band feels uncomfortably tight. The “plus 4” method is a relic of the 1950s when fabric had no stretch. Modern elastics are advanced enough to fit true-to-size.
Step 2: The Bust Measurement (Overbust)
This determines your cup volume.
Wrap the tape measure around the fullest part of your bust (usually across the nipple).
Keep the tape straight across your back.
The tape should barely touch the breast tissue, do not pull it tight. You want to measure the volume, not compress it.
Record this number in inches.
Step 3: Count and Calculate Breast Size
The “cup size” is simply the difference between your band and your bust. It is not a static size (a “D cup” is not the same volume on every band).
Use this formula:
Bust Measurement – Band Measurement = Difference in Inches
Refer to the cup size chart below to convert that difference into a letter.
| Difference (Inches) | US Cup Size | UK Cup Size |
| 1 inch | A | A |
| 2 inches | B | B |
| 3 inches | C | C |
| 4 inches | D | D |
| 5 inches | DD / E | DD |
| 6 inches | DDD / F | E |
| 7 inches | G | F |
| 8 inches | H | FF |
| 9 inches | I | G |
Example:
Underbust: 34 inches
Bust: 38 inches
Calculation: 38 – 34 = 4 inches.
Result: 34D.
The "Sister Size" Secret
One of the most powerful tools in a professional fitter’s arsenal is the concept of sister sizes.
If you use a bra size calculator and get a 34C, but the band feels too tight, you cannot just jump to a 36C. Why? Because a 36C has a larger cup volume than a 34C. If you increase the band size, you must decrease the cup letter to maintain the same volume.
The Rule of Thumb:
Up a Band, Down a Cup: If the band is tight but the cup fits, go from 34C rightarrow 36B.
Down a Band, Up a Cup: If the band is loose but the cup fits, go from 34C rightarrow 32D.
Common Sister Size Groups (Same Cup Volume)
Volume Group 1: 30D = 32C = 34B = 36A
Volume Group 2: 30DD = 32D = 34C = 36B
Volume Group 3: 32F = 34E = 36DD = 38D
Understanding this helps you navigate different brands. If a brand runs small in the band, use your sister size to find the perfect fit without sacrificing cup volume.
Decoding Breast Shapes
A bra size chart gives you a starting point, but it cannot tell you your shape. Two women can both be a 34DD but require completely different bra styles.
1. Shallow vs. Projected
Shallow: Breast tissue is spread over a larger area of the chest wall. You might have “stealth boobs” that look smaller than they measure. Best Fit: Balconette or Demi-cup styles.
Projected: Breast tissue sticks out significantly from the chest wall. Best Fit: Seamed, unlined cups that allow for depth.
2. Full on Top vs. Full on Bottom
Full on Top: You have more tissue above the nipple. Best Fit: Plunge bras or styles with open tops to avoid “quad-boob” (cutting in).
Full on Bottom: You have more tissue below the nipple. Best Fit: Full coverage or balconette styles that taper in at the top to prevent gaping.
Troubleshooting Common Fit Issues
Even the most accurate bra size calculator USA creates a starting point, not a verdict. You must judge the fit on your body. Here is how to diagnose common problems.
The “Quad-Boob” Effect
Symptom: Your breast tissue spills over the top or sides of the cup.
Diagnosis: The cup is too small.
Solution: Go up a cup size. If you are a 34C, try a 34D.
The Floating Gore
Symptom: The center piece of the bra (the gore) does not sit flat against your sternum.
Diagnosis: The cups are too shallow or too small to accommodate your breast depth.
Solution: Try a larger cup size or a “projected” style with vertical seams.
The Riding Band
Symptom: The back band pulls up between your shoulder blades.
Diagnosis: The band is too big. This is the most common mistake.
Solution: Size down in the band and up in the cup (Sister Sizing). If you wear a 36B, try a 34C.
The Slipping Strap
Symptom: Straps constantly fall off your shoulders.
Diagnosis: The band is likely riding up, causing the straps to lose tension, or you have sloping shoulders.
Solution: Tighten the band (size down). If the band is firm, look for “racerback” or “leotard back” styles where straps are set closer together.
US vs. UK Sizing: A Crucial Distinction
When shopping online, you must check if the bra size calculator is using US or UK sizing. They are identical up to a DD cup, but diverge significantly after that.
US Sizing: Uses single letters (D, DD, DDD, G, H). The progression is often inconsistent between brands (e.g., Wacoal vs. Victoria’s Secret).
UK Sizing: Uses double letters (D, DD, E, F, FF, G, GG). This system is generally more standardized and consistent for larger breast sizes.
Pro Tip: If you see “FF” or “GG” as an option, you are looking at a UK size chart. If you see “DDD,” it is likely a US chart.
The "Add 4" Myth Explained
You may encounter older advice suggesting you add 4 inches to your underbust measurement (e.g., measuring 30 inches and buying a 34 band).
Why does this exist? Decades ago, bra materials were stiff and unforgiving. A 30-inch band was literally 30 inches long with no stretch, so a woman with a 30-inch ribcage needed a 34-inch band to breathe.
Why it fails today: Modern bras are made with elastane and Spandex. A 30-inch band stretches to accommodate a 30-inch ribcage comfortably. Adding 4 inches results in a band that is too loose, providing zero support and forcing the straps to do all the heavy lifting. To count and calculate breast size accurately in 2025, ignore the “+4 rule” unless a specific brand’s size chart explicitly demands it.
Periodical Resizing
Your bra measurement is not a permanent ID number. It is a snapshot of your body at a specific moment. You should use a bra size calculator every 6 months, or after any of the following:
Weight fluctuation: Gaining or losing as little as 5-10 lbs can change your cup or band size.
Hormonal changes: Pregnancy, breastfeeding, and menopause drastically alter breast density and shape.
Exercise changes: heavy pectoral exercises can increase your underbust measurement, while cardio might decrease your bust volume.
FAQs: Bra Size Calculator USA
How do I know if my bra size is correct?
A correctly fitted bra should feel like a firm hug. The band should be level around your body, the center gore should lie flat against your breastbone, and your breasts should be fully enclosed in the cups without spilling over or gaping. You should be able to run two fingers under the band, but no more.
What is the most accurate bra size calculator in the USA?
While many brands offer their own calculators, the “most accurate” method is the one described in this guide: measuring the underbust snug (without adding inches) and calculating the cup based on the difference. Independent fitting communities often refer to this as the “ABTF” (A Bra That Fits) method, which is generally superior to the “+4” method used by some department stores.
Can I calculate my bra size without a tape measure?
It is difficult to get a precise bra measurement without a tape, but you can use a piece of string or ribbon. Wrap the string around your torso for the band and bust measurements, mark the length with your finger, and then measure the string against a standard ruler or construction tape measure.
Why is my cup size different in different brands?
Shape mismatch is usually the culprit. A 34C in a plunge style fits differently than a 34C in a full-coverage style. Additionally, US and UK brands define cup progressions differently after the D cup. Always check the specific bra sizes chart for the brand you are purchasing.
Does a loose band cause back pain?
Yes. If the band is loose, the weight of the breasts is transferred to the shoulder straps. This downward pressure on the shoulders can compress nerves and strain the trapezius muscle, leading to neck and upper back pain. A tighter band distributes the weight across the sturdy ribcage.
How do I measure for a sports bra?
The bra measurement process is the same, but the fit preference might differ. For high-impact activities (running, HIIT), you may prefer a tighter band and a compression-style cup. For yoga, an encapsulation style in your standard size is usually best.
The Bottom Line
Finding your true size is a journey of trial, error, and measurement. By using the tools and methods in this guide, you are already ahead of the 80% of women wearing the wrong size. Remember, a bra size calculator gives you a number, but your comfort gives you the answer.
Your Next Step: Grab your tape measure right now and follow the three steps above. Once you have your calculated size, look at the labels of the bras you are currently wearing. If they don’t match, it is time to schedule a fitting or order a “test bra” in your new size to feel the difference a supportive band can make.