The basics of a chest transformation

How do I know if a transformation is necessary?

Key point All the indications in this tutorial apply only in the case of a transformation of a pattern to the chest and are not valid to define its size in lingerie.

The majority of sewing pattern bodices on the market are based on a B cup. Based on this principle, we conclude that all other cups deserve to be adapted to the bust.

Let's start by defining some important terms that will be discussed in this tutorial.

- Cap difference in measurement between the top of the chest and the circumference of the chest.

- Chest top A line passing above the chest and flush with the armpits.

- Chest size Line passing through the tip of the breast, also called the breast protrusion, at the strongest point of the breast.

Depending on the cup, the difference is equal to :

2.5 cm - cup A

5 cm - B cup

7.5 cm - C cup

10 cm - cup D

Example 1:
Circumference of the upper chest: 85 cm

Chest size: 89 cm

Difference between the 2 measurements: 89 - 85 = 4 cm

Closest cup: B + 1 cm

In this case, it is not necessary to make any changes to the pattern.

Example 2:
Circumference of the upper chest: 88 cm

Chest size: 96 cm

Difference between the 2 measurements: 96 - 88 = 8 cm

Closest cup: C + 0.5 cm

In this case, it is necessary to make a modification on the pattern.

At this stage, it is possible to define your cup and to know if a transformation is necessary.


How to choose your pattern size?

For A, C and D cups, it is not advisable to choose your pattern size based on your bust size alone, as the garment will not fully fit your own body type.

It is therefore advisable to choose your size according to your chest measurement and not your chest size.

To do this, refer to the pattern measurement table, always in the "bust size" column. Note that the size chosen is always based on a B cup.

How to calculate the transformation measure?

The next step is to calculate the measurement to enlarge or reduce the pattern at the chest to fit an A, C or D cup.

Here is the calculation to apply:
- cup corresponding to her morphology - cup B.
- Divide the measurement obtained by 2, in order to add only half to the pattern of the half front.

Let's go back to our example 2 above:
Top of the chest: 88 cm.
Size to be chosen according to the table of measurements: 38 designed for a B cup (i.e. 5 cm difference between the chest and the top of the chest)
Measurement to be added to the pattern: cup corresponding to your morphology - B cup = (C cup + 0,5 cm) - 5 cm = 8 cm - 5 cm = 3 cm/2 = 1.5 cm


Hello, we don't have a video to help you with this type of fitting, but our free downloadable guide to made-to-measure sewing does have diagrams. pattern-according-to-my-morphology
have a nice day

Maison Fauve February 14, 2023

Hello Émilie

These explanations are very clear and if I understood well concerning the example, it is necessary to lengthen the front between the armpit and the breast clip.
Do you have a tuto?
I just finished sewing a blouse Atlas ; I will send you a picture soon.
Have a nice weekend

Lechat February 14, 2023

Thank you for all the tutorials and online help you provide. But nature has not given us all big advantages. I have searched in vain for a video or explanations to reduce the cup of a pattern, but I can't find it.You might gain by making an example 3.

Filing September 13, 2022

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