How to Choose the Right Size Silicone Breast Forms: A Complete Decision Guide

Choosing the wrong size is the most common and most costly mistake in breast form buying. A breast form that's too large creates a silhouette that looks augmented rather than natural. One that's too small fails to create the chest presence the wearer is looking for. And because breast forms are a significant investment, getting the size right the first time matters.

This guide walks through the complete size decision — from taking measurements to understanding projection, from matching body proportions to assessing the result in a trial wearing session. It's designed to be used before purchase, not after.

The Three Numbers That Drive the Size Decision

Breast form sizing is driven by three measurements taken from your own body. Take these with a soft measuring tape, wearing only a bra or no top at all.

Chest circumference (band size). Measure around the ribcage, directly under where the breast forms will sit — the same position as a bra band. Keep the tape parallel to the floor and snug but not tight. This number determines the bra band size, which in turn determines how the breast forms are supported. A breast form that's correctly sized for the chest circumference will sit naturally within the bra cup; one that's too large will overflow the cup regardless of bra size.

Chest width. Measure across the chest from one side to the other — the width of the area where the breast forms will sit. This measurement determines the base width of the breast form. A breast form whose base width is wider than the chest width will extend beyond the natural breast boundary, creating an unnatural silhouette. A breast form whose base width is narrower than the chest width will leave gaps at the sides.

Desired cup size. This is the only measurement that involves a decision rather than a physical fact. The desired cup size — A, B, C, D, or larger — determines the volume and projection of the breast form. The decision should be made in the context of the chest circumference: a D cup on a 32-inch chest creates a very different silhouette from a D cup on a 40-inch chest. Think in terms of the overall silhouette rather than the cup letter in isolation.

Step 1: Establish the Base Width Boundary

Before considering volume or projection, establish the base width boundary — the maximum base width of breast form that will sit naturally on your chest.

Stand in front of a mirror with your arms relaxed at your sides. Place your hands flat on your chest, fingers pointing toward each other, and identify the natural boundary of the chest — where the chest meets the armpit on each side, and where it meets the sternum in the centre. The distance between the armpit boundaries is the maximum base width for the breast forms.

A breast form that fits within this boundary will sit naturally on the chest. One that exceeds it will push into the armpit area, creating discomfort and an unnatural silhouette. This boundary is non-negotiable — it's determined by the body's anatomy, not by preference.

Step 2: The Projection Decision — How Much Curve?

Within the base width boundary, the projection decision determines how much the breast forms extend forward from the chest. Higher projection creates a more dramatic silhouette; lower projection creates a subtler one.

Projection is related to cup size but not identical to it. Two breast forms with the same cup size can have different projections depending on their shape — a teardrop shape has lower projection than a round shape of the same volume. When comparing breast forms, look at the projection measurement (the depth from the base to the highest point) as well as the cup size.

For first-time buyers, lower projection is generally the safer choice. A subtler silhouette is easier to manage in everyday clothing, less likely to create fit problems with existing wardrobe, and less visually conspicuous in contexts where discretion matters. The silhouette can always be increased with a larger size at the next purchase; it can't be reduced once the breast forms are in place.

Step 3: Matching Projection to Body Frame

The same projection looks different on different body frames. A breast form with 12cm of projection on a petite frame creates a dramatically different silhouette from the same breast form on a larger frame. The goal is a silhouette that's proportionate to the body — one that looks like it belongs on the frame rather than sitting on top of it.

A useful reference point: look at women whose body frame is similar to yours and whose chest silhouette you find natural and proportionate. The cup size that creates that silhouette on a similar frame is a reasonable starting point for your own size decision. This reference-point approach is more reliable than working from cup letters alone, because it grounds the decision in a visual outcome rather than an abstract measurement.

For detailed guidance on matching breast form size to specific body proportions — petite, tall, plus-size, and athletic frames — see our body proportion guide.

The Wardrobe Compatibility Check

Before finalising a size, run a wardrobe compatibility check. The breast forms need to work with the clothing you actually wear — not just with the clothing you imagine wearing.

Go through your existing wardrobe and identify the clothing you wear most frequently. For each category — fitted tops, loose tops, structured jackets, formal wear — consider whether the intended breast form size will create a silhouette that works with that clothing. Fitted tops are the most revealing test: a breast form that creates a natural silhouette under a fitted top will work under almost anything. One that creates an obviously augmented silhouette under a fitted top will require significant wardrobe adjustment.

If the intended size creates wardrobe compatibility problems — if it's too large for the fitted tops you wear regularly, or if it requires a completely different bra wardrobe — consider whether a smaller size would be more practical for everyday wearing. A breast form that's worn regularly in everyday clothing provides more value than a larger breast form that's only comfortable in specific outfits.

For clothing selection guidance, see our bra guide — bra selection is inseparable from size selection, because the bra determines how the breast form sits and how the silhouette reads.

Size Decisions for Specific Contexts

Certain wearing contexts have specific size considerations that go beyond the general framework above.

Post-mastectomy wearing. Post-mastectomy breast form sizing is driven by symmetry with the remaining natural breast rather than by aesthetic preference. The goal is a breast form that matches the remaining breast in volume, projection, and position — creating a symmetrical silhouette. For detailed guidance on post-mastectomy sizing, see our post-mastectomy guide.

Wearing during HRT. For transgender women on hormone replacement therapy, the chest is changing — which means the optimal breast form size is also changing. A size that's appropriate at the beginning of HRT may be too large or too small after 12 months of hormonal change. For guidance on sizing during transition, see our HRT guide.

Mature wearers. For wearers over 50, the chest shape and skin elasticity change in ways that affect how breast forms sit and how the silhouette reads. Softer, lower-projection shapes often work better for mature wearers than high-projection round shapes. For age-specific sizing guidance, see our mature wearers guide.

A Simple Decision Framework

If the measurements and considerations above feel overwhelming, use this simplified framework to narrow the decision:

Start with the chest width measurement and identify the maximum base width. Eliminate any breast form whose base width exceeds this measurement. From the remaining options, choose the lowest projection that achieves the silhouette you're looking for — not the highest. If you're uncertain between two sizes, choose the smaller one for the first purchase. A smaller breast form that's worn confidently every day is more valuable than a larger one that stays in the box.

This framework produces a conservative first choice — one that's more likely to be wearable across a range of everyday contexts and less likely to create the fit and wardrobe problems that come with oversizing. The size can always be increased at the next purchase, informed by the experience of wearing the first pair.

The Trial Wearing Session: What to Assess Before You Commit

Once the breast forms arrive, conduct a structured trial wearing session before committing to the size — and before removing any tags or packaging that might affect the return or exchange process.

Put on the bra and breast forms and stand in front of a full-length mirror. Assess the silhouette from the front, both sides, and the back. The breast forms should sit naturally within the bra cups, with no overflow at the top or sides. The silhouette from the side should show a natural curve — not a shelf-like projection that's clearly separate from the chest. From the back, the bra band should be level and the straps should not be pulling.

Then put on the clothing you wear most frequently — a fitted top, a loose top, a jacket — and assess the silhouette under each. The breast forms should create a natural-looking chest silhouette under each clothing type, not an obviously augmented one.

Finally, move. Raise your arms overhead, bend forward, sit down, walk around. The breast forms should move naturally with the body — not shift independently or create discomfort during movement. If they shift significantly during normal movement, the bra fit needs adjustment before the size assessment is meaningful.

If the trial wearing session reveals that the size is wrong — too large, too small, or the wrong shape — the size decision needs to be revisited. Common trial wearing problems and their solutions are covered in our troubleshooting guide.

Frequently Asked Questions

Q: I'm between two sizes — which should I choose?
A: Choose the smaller size for the first purchase. A smaller breast form that's worn comfortably every day provides more value than a larger one that creates fit or wardrobe problems. The experience of wearing the smaller size will inform whether a larger size is warranted at the next purchase.

Q: My chest is asymmetric — how does this affect size selection?
A: Minor asymmetry — which is extremely common — doesn't significantly affect breast form size selection. The breast forms will create a symmetrical silhouette regardless of minor underlying asymmetry. Significant asymmetry — such as post-mastectomy asymmetry — requires a different approach: see our post-mastectomy guide for specific guidance.

Q: I want a dramatic silhouette — should I just choose the largest size available?
A: Not necessarily. A dramatic silhouette is created by the combination of size, projection, and bra — not by size alone. A high-projection breast form in a correctly fitted bra creates a more dramatic silhouette than a larger low-projection breast form in a poorly fitted bra. Before choosing the largest available size, assess whether a high-projection option in a more moderate size achieves the silhouette you're looking for.

Q: How do I know if the breast forms are the right size or if the bra is the wrong size?
A: If the breast forms overflow the bra cups, the bra cup size is too small — not necessarily the breast forms too large. If the breast forms sit correctly in the cups but the silhouette looks wrong, the breast form size may need adjustment. Assess the bra fit first — a correctly fitted bra is the foundation of a correct silhouette assessment. See our buying guide for first-time buyers for more on this distinction: first-time buying guide.

Q: My skin tone doesn't match the breast forms — is this a size problem?
A: No — skin tone and size are separate decisions. If the size is correct but the skin tone doesn't match, see our skin tone guide for guidance on selecting the right tone.

Further Reading

• First-Time Buying Guide — the complete purchase decision beyond size
• Body Proportion Guide — size selection for specific body frames
• Bra Guide — finding the right bra for the chosen size
• Skin Tone Guide — the second decision after size
• Troubleshooting Guide — when the trial wearing session reveals a problem
• Post-Mastectomy Guide — sizing for symmetry after mastectomy
• HRT Guide — sizing during hormonal transition
• Mature Wearers Guide — age-specific sizing considerations