Bra Size Calculator Guide: How to Measure at Home
Quick Answer
- *Band size: measure snugly under bust, round to nearest even number.
- *Cup size: bust measurement minus band = difference in inches (1″ = A, 2″ = B, 3″ = C, 4″ = D, 5″ = DD).
- *US/UK band sizes are the same; cup letters diverge at DD and above.
- *Sister sizes let you substitute adjacent band sizes while keeping the same cup volume.
Why Getting the Right Bra Size Matters
According to a 2023 study published in the Journal of Fashion Marketing and Management, approximately 80% of women are wearing the wrong bra size— most commonly a band that is too large and a cup that is too small. Poor fit causes discomfort, poor posture support, and clothing that does not lay correctly.
The good news: measuring yourself accurately at home takes about two minutes and a soft tape measure. You do not need to visit a specialist or guess based on previous labels.
How to Measure Your Bra Size: Step by Step
What You Need
- A soft, flexible tape measure (the kind used for sewing)
- A mirror
- A thin, non-padded bra or no bra for the most accurate measurement
Step 1: Measure Your Band Size
Wrap the tape measure snugly around your torso directly under your bust, keeping it parallel to the floor. The tape should be firm but not cutting in. Write down this measurement in inches.
- If the measurement is an even number, that is your band size.
- If the measurement is an odd number, round up to the next even number.
Example: underbust measures 31 inches → band size is 32. Underbust is 34 → band size is 34.
Step 2: Measure Your Bust
Wrap the tape measure loosely around the fullest part of your bust, keeping it level. Do not pull it tight — you want a measurement that captures the full projection. Write this down in inches.
Step 3: Calculate Your Cup Size
Subtract your band size from your bust measurement. The difference in inches corresponds to your cup letter:
| Difference (inches) | US Cup | UK Cup |
|---|---|---|
| 0 | AA | AA |
| 1 | A | A |
| 2 | B | B |
| 3 | C | C |
| 4 | D | D |
| 5 | DD | DD |
| 6 | DDD (or F) | E |
| 7 | G (or DDDD) | F |
| 8 | H | FF |
| 9 | I | G |
| 10 | J | GG |
International Bra Size Conversion Chart
Bra sizing varies significantly by country. If you shop from international brands or travel, this conversion chart covers the most common systems.
| US Size | UK | EU (cm) | FR/ES/BE | IT | AU/NZ |
|---|---|---|---|---|---|
| 32A | 32A | 70A | 85A | 1A | 10A |
| 34B | 34B | 75B | 90B | 2B | 12B |
| 36C | 36C | 80C | 95C | 3C | 14C |
| 38D | 38D | 85D | 100D | 4D | 16D |
| 40DD | 40DD | 90E | 105E | 5E | 18DD |
| 42DDD | 42E | 95F | 110F | 6F | 20DDD |
Note: EU band sizes represent underbust in centimeters. To convert from US/UK inches to EU: US band × 2.54, then add 2–4 cm and round to nearest 5.
Understanding Sister Sizes
Sister sizes are bra sizes that share the same cup volume but have different band sizes. When you go up one band size, you drop one cup letter. When you go down one band size, you go up one cup letter.
| Smaller Band | Your Size | Larger Band |
|---|---|---|
| 32D | 34C | 36B |
| 34D | 36C | 38B |
| 34DD | 36D | 38C |
| 36DD | 38D | 40C |
Sister sizes are useful when:
- Your exact size is sold out
- You need a looser band for comfort (go up one band, down one cup)
- You need more band support (go down one band, up one cup)
Top 7 Signs Your Bra Does Not Fit Correctly
1. The Band Rides Up at the Back
The band should sit level around your torso. If it rides up, the band is too large. Try the next band size down. According to UK lingerie retailer Rigby & Peller (2024), this is the most common fit issue they see — affecting roughly 65% of first-time fittings.
2. The Wires Sit on Breast Tissue
Underwire should sit flat against your ribcage and frame the breast without digging into it. If wires rest on soft tissue, the cup is too small or the wrong shape for your breast. Try a larger cup or a different wire shape.
3. Gaping Cups
Gaps between the cup and breast tissue mean the cup is too large or the shape does not match. Try a smaller cup size or a different style (balconette, plunge, full-cup).
4. The Center Gore Does Not Lie Flat
The center piece between the cups should lie flat against your sternum. If it pokes away, the cups are too small. Try a larger cup.
5. Straps Digging In
Straps should not be carrying most of the weight — the band does about 80% of the work. If straps dig in, either the band is too loose (so straps compensate) or the straps are too tight. Adjust straps to where two fingers fit comfortably underneath.
6. Quadboob (Spillage Over the Cup)
If breast tissue spills over the top edge of the cup, the cup is too small. Go up one cup size. This is one of the most common problems for women who have never had a professional fitting.
7. The Bra Moves Around During the Day
A properly fitting bra should stay put through normal movement. If it shifts, twists, or migrates, either the band is too large or the style is wrong for your shape.
How Bra Sizing Changes Over Time
Bra size is not fixed for life. According to lingerie brand ThirdLove (2024), which analyzed data from over 12 million fittings, the average woman's bra size changes at least six times in her lifetime. Common causes include:
- Weight gain or loss (≥10 lbs often changes band size)
- Pregnancy and nursing
- Hormonal changes (puberty, menstrual cycle, menopause)
- Changes in physical fitness and muscle mass
- Aging (tissue shifts over time)
The practical implication: measure yourself at least once a year, and any time your body changes significantly. Do not assume last year's size is still accurate.
Find your bra size in seconds
Use the Free Bra Size Calculator →Frequently Asked Questions
How do I measure my bra size at home?
You need two measurements. First, measure your band size: wrap a soft tape measure snugly under your bust, parallel to the floor, and round to the nearest even number. Second, measure your bust: wrap loosely over the fullest part. Subtract band from bust to get cup size (1 inch = A, 2 = B, 3 = C, 4 = D, 5 = DD, etc.).
What is a sister size in bras?
Sister sizes are bra sizes with the same cup volume but different band sizes. Going up one band size and down one cup letter gives the same volume (e.g., 34C and 36B are sister sizes). This is useful when the correct size is unavailable — a 36B will have the same cup volume as a 34C but a looser band.
Why does my bra size differ between brands?
Bra sizing is not standardized globally. US, UK, EU, FR/ES/BE, IT, and AU systems all use different numbering conventions. Additionally, brands vary in their grading and construction. A 34C in one brand may fit like a 34D in another. Using measurements rather than relying on a label size is the most reliable approach.
What is the difference between US and UK bra sizes?
US and UK band sizes use the same numbers (32, 34, 36, etc.) but diverge at larger cup sizes. US uses D, DD, DDD for sizes that UK labels as D, DD, E. A US 34DDD is equivalent to a UK 34E. Beyond DD, UK sizing uses E, F, FF, G, GG, H, while US uses DDD, DDDD/G, H.
How do I know if my bra band is the right size?
A properly fitting band sits level around your torso, parallel to the floor. You should be able to slip two fingers under the band — not more. The band should provide about 80% of the bra's support. If the band rides up at the back, it is too large. If it leaves deep marks or is difficult to breathe in, it is too small.
How often should I remeasure my bra size?
Remeasure any time your weight changes by 10+ pounds, after pregnancy or breastfeeding, after significant changes in fitness or muscle mass, or when your current bras stop fitting well. According to lingerie industry data, bra size changes on average every 1–2 years for most adults.