CM to Cup Size Converter

The CM to Cup Size Converter calculates bra size from bust and band in centimetres — It converts CM to Cup Size.

What Is a CM to Cup Size Converter?

A CM to Cup Size converter translates body measurements in centimeters into a cup letter within a bra sizing system. Cup size is not a standalone value. It depends on the difference between your bust and your snug underbust. That difference maps to a letter, and the mapping rules vary by region.

This tool runs the math for multiple systems, such as US/UK, EU, and JP. It reads your inputs, converts centimeters to inches when needed, and rounds following common industry rules. The result shows a cup letter and a compatible band, with notes about brand variance and fit preferences.

Because sizing methods differ, expect slightly different letters across systems. The converter explains those differences. It also presents options so you can adjust rounding, choose fit tightness, and save notes for future use.

CM to Cup Size Formulas & Derivations

Under the hood, cup size comes from the difference between bust and underbust measurements. Systems compute that difference using centimeters or inches, then map it to a letter series. Here are the core formulas the converter applies.

• Difference (cm): Dcm = Bust_cm − Underbust_cm.
• Difference (in): Din = (Bust_cm − Underbust_cm) ÷ 2.54.
• US/UK cup index: one cup per 1.0 inch of difference. Letters advance roughly every 1 inch (A ≈ 1″, B ≈ 2″, C ≈ 3″, D ≈ 4″, DD/E ≈ 5″, DDD/F ≈ 6″, etc.).
• EU cup index: one cup per 2.0 cm of difference. Example bands use 5 cm steps and cups step at 2 cm differences (A ≈ 12–14 cm, B ≈ 14–16 cm, C ≈ 16–18 cm, D ≈ 18–20 cm, E ≈ 20–22 cm, F ≈ 22–24 cm, G ≈ 24–26 cm).
• JP cup index: one cup per 2.5 cm of difference (A ≈ 10 cm, B ≈ 12.5 cm, C ≈ 15 cm, D ≈ 17.5 cm, E ≈ 20 cm, F ≈ 22.5 cm, G ≈ 25 cm).
• Band handling: US/UK bands are usually inch-based and even-numbered; EU bands use centimeter ranges and often round to the nearest 5 cm step.

These formulas represent widely used conventions. Brands may set cut patterns differently, use half steps, or label double letters differently. The converter lets you select the target system and rounding behavior to align with your shopping context.

How the CM to Cup Size Method Works

The method starts with two measurements: underbust (snug) and bust (fullest). The tool converts units when needed, computes their difference, and maps that to a cup letter. Finally, it pairs the cup with a likely band size for the chosen system.

• Measure underbust snugly, parallel to the floor. Measure bust at the fullest point without compressing tissue.
• Compute the difference: Dcm = Bust_cm − Underbust_cm.
• For US/UK: convert Dcm to inches. Each full inch steps one cup letter. Example: 4.0–5.0 inches ≈ D to DD/E.
• For EU: use centimeters directly. Each 2 cm steps one cup letter starting from the A range around 12–14 cm.
• For JP: use 2.5 cm steps from A ≈ 10 cm upward.
• Choose band rounding that matches the system: even-numbered inches for US/UK, 5 cm steps for EU bands.

This approach standardizes the math across regions. It also exposes differences clearly, so you can compare sizes and explore sister size options with confidence.

What You Need to Use the CM to Cup Size Converter

You only need accurate measurements and a few quick choices. Gather your data and decide how you want the output displayed. The tool accepts entries in centimeters and handles the conversions for systems that use inches.

• Underbust circumference (snug) in centimeters.
• Bust circumference (fullest) in centimeters.
• Target sizing system: US/UK, EU, or JP (options available).
• Band rounding preference: nearest even inch (US/UK), nearest 5 cm (EU), or custom notes.
• Fit preference: firmer band or relaxed band.

The converter supports typical adult ranges, about 55–125 cm underbust and 70–160 cm bust. Very small or very large differences can produce rare letters beyond G/H. If your difference is below 1–2 cm, the tool may return AA/AAA or indicate that the cup index is near zero. Always review the notes for edge cases and brand-specific cautions.

Using the CM to Cup Size Converter: A Walkthrough

Here’s a concise overview before we dive into the key points:

1. Select your target system from the options (US/UK, EU, or JP).
2. Enter your underbust measurement in centimeters as a whole number or one decimal.
3. Enter your bust measurement in centimeters using the same precision.
4. Choose your band rounding preference and fit setting (firmer or relaxed).
5. Click Convert to process the inputs.
6. Review the suggested band and cup. Compare cross-system results if shown.

These points provide quick orientation—use them alongside the full explanations in this page.

Real-World Examples

Case 1: A user measures 78 cm underbust and 93 cm bust. The difference is 15 cm. EU method: 15 cm sits within the 14–16 cm range which maps to B in many EU charts. US method: 78 cm is 30.7 inches, which rounds to a 32 band; 93 cm is 36.6 inches; the difference is 4.6 inches, which suggests around a DD/E cup. What this means: EU ≈ 75B vs US ≈ 32DD/E, reflecting different mapping rules for the same body.

Case 2: A user measures 65 cm underbust and 89 cm bust. The difference is 24 cm. EU method: 24 cm usually maps to G in 2 cm steps (24–26 cm). US method: 65 cm is 25.6 inches; the nearest available even band is often 28; 89 cm is 35.0 inches; the difference is about 7 inches, which maps to a G cup in many US charts. What this means: EU ≈ 65G vs US ≈ 28G; the cup letter aligns while the band system differs.

Accuracy & Limitations

The converter follows common industry rules, but real garments vary. Pattern shapes, strap placement, wire width, and fabric stretch affect fit. Measuring technique also matters. Small changes in tape tension can shift the cup letter, especially near boundaries.

• Different brands use different grading scales and may skip or rename double letters.
• Band availability can force sister sizes (for example, moving from 30F to 32E).
• Bust shape, fullness distribution, and posture can change how a cup fits.
• Sports and wire-free styles may run tighter or looser than wired bras.

Use the output as a starting point, not a guarantee. When possible, try multiple sizes around the suggestion, read brand fit notes, and check return policies. Keep your measurement history so you can spot trends and refine your fit.

Units & Conversions

Units drive the mapping from measurements to cup letters. US/UK systems start with inches, while EU and JP use centimeters. Knowing the step size in each system helps you interpret small changes and compare sizes across regions.

To use the table, match your target system to its step size. For example, a 5.1 cm change in difference equals about two US/UK cup steps or roughly two and a half EU steps. Small rounding choices can shift the final letter in borderline cases.

Troubleshooting

If the output looks off, start by remeasuring. Use a flexible tape, level it around your body, and relax your shoulders. Keep the tape snug for underbust and comfortable at the bust. Round measurements consistently before entering them.

• Large gaps between systems? Check the band rounding options.
• Between sizes? Toggle the fit preference and compare.
• Unexpected letters? Verify units are set to centimeters for inputs.
• Edge outputs (AA or far past G)? Reconfirm numbers and decimals.

If a brand’s chart disagrees with the result, prioritize the brand chart for that purchase. Use the converter’s notes field to record the brand’s policy and your try-on feedback for next time.

FAQ about CM to Cup Size Converter

Which sizing systems does the Converter support?

It supports US/UK, EU, and JP. You can view one system or compare two, and adjust rounding to match brand charts.

How should I measure for best accuracy?

Measure underbust snugly on bare skin or a thin bra, and measure bust at the fullest point without compressing. Keep the tape level.

What if I fall between two cup letters?

Pick the smaller cup for shallow tissue or firmer bands, and the larger cup for fuller tissue or padded styles. Try both if possible.

Why is my cup letter different across bands?

Because cup volume changes with band size. Sister sizes trade band for cup to keep volume similar, so letters shift when bands change.

CM to Cup Size Terms & Definitions

Underbust

The circumference directly under the bust, measured snug. It guides band size selection and anchors the garment.

Bust

The circumference at the fullest point of the chest. Used with underbust to compute the difference that maps to cup size.

Band Size

The base number for a bra, in inches (US/UK) or centimeters (EU). It reflects underbust plus rounding rules.

Cup Size

The letter that represents the difference between bust and underbust. Larger differences yield higher letters.

Difference (Bust − Underbust)

The key calculation that drives cup mapping. Measured in centimeters or inches depending on the system.

Sister Sizes

Size pairs that preserve cup volume by trading band and cup steps, such as 32D, 34C, and 30DD.

Rounding Preference

An option to control how bands and differences round to system steps. It affects the final letter at boundaries.

Fit Preference

A setting indicating firmer or relaxed band tension. It nudges band rounding and may shift the recommended cup.

References

Here’s a concise overview before we dive into the key points:

• Wikipedia: Brassiere measurement methods
• Wikipedia: International bra size systems
• A Bra That Fits: Measurement guide and sizing tips
• HerRoom: Bra fitting center
• Wacoal: Bra fitting guide

These points provide quick orientation—use them alongside the full explanations in this page.