How to Find Your Full Cup Bra Size at Home: A Step-by-Step Fitting Guide

Quick answer

Measure your underbust snugly for the band, then measure your fullest bust point to get your cup. In Australia, add roughly 10 cm to your underbust measurement to get your band size, then check the difference between band and bust to estimate your cup. Always confirm with a fit check: the band should sit firm and level, the wires should sit flat on your ribcage, and the cup should fully contain your breast tissue without spilling or gaping.

A full cup bra only works when the size is right. If the band rides up, the wires dig in, or you're constantly adjusting, the measurements are probably off — not your body. The good news is you can get a reliable starting size at home with a soft tape measure and five minutes.

This guide walks you through measuring your underbust and bust, converting that to an Australian size, and then checking the fit points that matter most for fuller cups. Use it as a starting point, then adjust based on how the bra actually feels and sits.

Before you measure: what you'll need

You need a soft tape measure, a mirror, and a well-fitting unlined bra (or no bra at all). Measuring over a padded or sports bra adds bulk and throws off your numbers. Stand in front of a mirror so you can keep the tape level and parallel to the floor.

Write your measurements down in centimetres. If you're between sizes, note both numbers — you'll use them to decide whether to size up or down after the fit check. Don't round aggressively; a centimetre or two can change your cup.

• Measure in centimetres for Australian sizing, or inches if you prefer to convert later.
• Keep the tape snug but not tight — it should sit flat against your skin without compressing tissue.

Step 1: Measure your underbust

Wrap the tape around your ribcage directly under your bust, where the bra band sits. Keep it level all the way around. Breathe out normally and read the number where the tape meets. This is your underbust measurement.

If the number is odd, round to the nearest even number. In Australia, add about 10 cm to your underbust measurement to get your band size. For example, a 75 cm underbust points to a band size around 85 cm — but always confirm with a fit check, because band preference varies.

• Common mistake: pulling the tape too tight. The band needs to grip, not squeeze.
• Checkpoint: the tape should be level at the back — if it dips, your band size will read too small.

Step 2: Measure your fullest bust point

Wrap the tape around the fullest part of your bust, usually across the nipple line. Keep it level and don't pull it tight — you want the tape to rest on the tissue, not compress it. Stand straight with your arms down.

Read the number at the front where the tape meets. This is your bust measurement. If you're fuller on one side, measure the fuller side for the most accurate cup estimate.

• Common mistake: measuring over a padded bra, which inflates the cup size.
• Checkpoint: the tape should touch your back at the same height as the front — if it rides up, remeasure.

Step 3: Work out your cup size

Subtract your underbust measurement from your bust measurement. The difference in centimetres gives you a rough cup estimate: around 10 cm is often an A cup, 12.5 cm a B, 15 cm a C, 17.5 cm a D, 20 cm a DD/E, and so on. Cup sizing varies between brands, so treat this as a starting point rather than a final answer.

Full cup styles are designed to contain more tissue, so if you're between cup sizes, start with the larger cup and check the fit. A cup that's slightly too large will gap; a cup that's too small will spill or dig into the breast tissue.

• Common mistake: assuming the same cup letter means the same volume across band sizes. A 12D and a 16D are not the same cup volume.
• Checkpoint: if the centre gore doesn't sit flat against your sternum, the cup is often too small.

Step 4: Check the band fit

Fasten the bra on the loosest hook and check the band. It should sit firm and level all the way around, with no more than two fingers of give under the band. If it rides up at the back, the band is too big or stretched out. If it digs in and leaves red marks, it's too tight.

A firm band does most of the support work in a full cup bra, so this is the most important fit check. If the band feels right but the cups don't, adjust the cup size rather than the band.

• Common mistake: wearing a band that's too loose and relying on the straps for support.
• Checkpoint: if you can pull the band more than 5 cm away from your back, size down.

Step 5: Check the cup and wire fit

Scoop your breast tissue into the cup, then check the wires. They should sit flat against your ribcage, follow your natural breast root, and not dig into breast tissue. The cup should fully contain your breast without spilling over the top or gaping at the edges.

Look for wrinkles in the cup fabric — some wrinkling at the top edge is normal, but deep wrinkles usually mean the cup is too big or the shape isn't right for you. If the wires sit on breast tissue rather than behind it, go up a cup size.

• Common mistake: judging cup fit without scooping and swooping first.
• Checkpoint: the centre gore should touch your sternum. If it floats, try a larger cup.

Step 6: Test movement and adjust

Wear the bra for a few minutes and move around — reach overhead, bend forward, sit down. The band shouldn't shift, the straps shouldn't fall off your shoulders, and the cups shouldn't gap or spill. Adjust the straps so they sit flat without digging in.

If the band is right but the cup isn't, change only the cup size. If the cup is right but the band isn't, change only the band size. Making one change at a time makes it much easier to find your true size.

• Common mistake: overtightening straps to fix a band that's too loose.
• Checkpoint: if you're still adjusting after five minutes, the size or style isn't right.

Your at-home full cup fitting steps

1. Measure underbust: Wrap the tape snugly under your bust, keep it level, and note the number in centimetres. Round to the nearest even number, then add about 10 cm for your Australian band size.
2. Measure fullest bust: Measure around the fullest point without compressing tissue, keeping the tape level. If you're fuller on one side, use the fuller side for your cup estimate.
3. Calculate cup difference: Subtract underbust from bust to estimate your cup size, then treat it as a starting point. Between sizes? Start with the larger cup and check the fit.
4. Check band fit: Fasten on the loosest hook and confirm the band sits firm and level with no more than two fingers of give. If it rides up, size down; if it digs in, size up.
5. Check cup and wire fit: Scoop tissue into the cup, then check that wires sit flat and the centre gore touches your sternum. Floating gore or spilling usually means go up a cup size.
6. Move and adjust: Reach, bend, and sit to test stability, then adjust straps so they sit flat without digging. Change one size element at a time to isolate what needs adjusting.

Full cup styles worth checking your size in

• Peony Lace Full Cup Bra - Black: Full cup design with lace detailing, available in back and cup sizes. A good everyday option to test your measured size in a classic full cup shape.
• Print Full Cup Bra - Mystic Navy: Ditsy floral print with navy lace, full cup support, available in back and cup sizes. Lets you check the same full cup fit in a printed style if you want variety.
• Peony Lace Premium Support Bra - Savvy Red: Premium support design in savvy red, available in back and cup sizes. Useful if you want extra support while confirming your cup and band measurements.
• Radiate Lace Wirefree Bra - Lavender Spritz: Wirefree construction with lace design, available in back and cup sizes. A wirefree alternative if wires tend to dig in, while still offering support.

Finding your full cup bra size at home comes down to two measurements and a careful fit check. Measure your underbust and bust, convert to an Australian size, then test the band, wires, and cup in a style like the Peony Lace Full Cup Bra - Black. If the fit isn't right, adjust one element at a time until it is.