Determining your suit jacket size requires measuring your chest size and understanding the corresponding size chart. Most suit jackets are sized in numbers, typically between 34 and 52, plus a letter or two, which relate to the chest size of the jacket. Here are some steps to help you determine your suit jacket size:

1. Measure your chest: Use a measuring tape and wrap it around the fullest part of your chest, under your armpits. Make sure the tape is snug but not too tight.

2. Find your size: Once you have your chest measurement, use a suit size chart conversion to estimate your suit jacket chest size. The number you get is your jacket size.

3. Determine the jacket length: Suit jackets usually come in three lengths: Short (S), Regular (R), and Long (L). You may sometimes also see XS and XL. This refers mostly to the sleeve length, but it may also affect the overall jacket length (depending on the brand). Generally, a "short" is suitable for people 5’8” and shorter; a "regular" is good for individuals 5’9” - 6’ feet tall; between 6’1” and 6’4” is considered "long;" people 6’5” and taller need "extra long".

4. Check the fit: A great fit will compliment the wearers silhouette. There should be no puckering, pulling, or excess fabric at your jacket chest or back. You should be able to move comfortably without the jacket straining, even when buttoned.

Its important to note that suit size charts are best used for finding a ballpark number for your suit sizes. They’re foolproof, but less likely to deliver a perfect fit than a suit size calculator.