A shirt can feel comfortable over a light T-shirt and pull when you wear it open over another layer. Sizing up without checking the measurements can also leave you with drooping shoulders or sleeves that are longer than you want. Start by deciding how you will use it.
This guide focuses on a lightweight transitional shirt, not a padded or thermal garment. The label is a starting point; the cut, measurements and way you plan to wear it complete the decision.
Compare a shirt with a similar fabric and cut, measure the same points on each garment and test the room with the layer you will actually wear. “Overshirt” is not an automatic instruction to size up.
Decide how you want to wear it
If you will button it like a shirt, think about the fit you find comfortable across the shoulders, chest and arms. If you will wear it open over a T-shirt, check that the chest does not pull when you move and that the sleeves do not restrict your arms. The aim is not to make it cling or look oversized; it is to make it work with the layer you have in mind.
Words such as regular, relaxed and oversized help describe the silhouette, but they are not a shared sizing scale across retailers. “Overshirt” can describe how a garment is styled rather than a fixed measurement. Read the exact product description and look at its front and side photos.
Compare garment measurements with garment measurements
Choose a shirt from your wardrobe that fits the way you want. If possible, pick one with a similar weight and cut: comparing a fitted shirt with a roomy overshirt can lead you to the wrong conclusion. Lay it flat, fasten the buttons, smooth creases without stretching the fabric and note:
- Shoulders: from one shoulder seam to the other. With a dropped shoulder, also check where the seam sits relative to your shoulder.
- Chest: measure pit to pit. If the product page gives the garment's full chest circumference, double the flat width; do not compare it directly with a body measurement.
- Length: from the high point of the shoulder to the hem, following the retailer's stated method.
- Sleeve: check where the measurement starts; it may run from the shoulder seam or from the centre back.
Brands do not always publish finished-garment measurements. Some show a body size chart; others add measurements for the product. Read the diagram or note beside the chart: a body measurement and a measurement taken from the shirt answer different questions. If you are between options, this guide to deciding between two sizes can help.
Test the room with the layer you will wear
Try your reference shirt over the same T-shirt or top you have in mind. Button it and raise your arms; then wear it open and cross your arms in front of you. Notice whether the yoke shifts, the buttons pull or the back feels tight. These signs can help you tell a lack of ease from a style preference.
If the new shirt has a roomy cut, it may give you enough space without sizing up. If it has a straight cut and you want to use it as a layer, comparing it with a similar shirt worn over your T-shirt is more useful than relying on the word “overshirt”. A larger size can add length to the body and sleeves even if the original issue was only at the chest.
Look at the key measurements together. If the shoulders already sit too low on your reference garment, adding chest room may not be what you want. If the shoulders fit but the buttons pull when fastened, review the chest measurement and cut before changing size.
Read the product page without guessing what it leaves out
Check whether the retailer gives the model's height and the size they are wearing. That helps you picture the drape, but it cannot guarantee the same result for you: proportions, pattern and fit preference matter. Also check whether the item is presented as a shirt or a layer, and which measurements are actually available.
Composition and care instructions describe that particular garment; on their own, they do not tell you how much it will stretch or change in the wash. Follow the maker's label and do not choose a size on the assumption that the shirt will shrink or stretch. For more on choosing useful room without over-layering, see our guide to transitional clothing sizes.
Quick checklist before you buy
- Decide whether you will wear it buttoned or open over another layer.
- Choose a reference garment with a similar cut and fabric weight.
- Measure the shoulders, chest, length and sleeve without stretching the fabric.
- Check whether the chart describes your body or the finished garment.
- Test movement with the layer you usually wear.
- Review the photos, description and care notes for the exact style.
- Save the brand, style, size and fit notes in SIZES.
The right shirt size is not determined by the word “flannel” or “overshirt”. It depends on the garment's measurements, its pattern and how you want to wear it. Keep a reference shirt that works for you and the next purchase can start with real information instead of a guess.
Frequently asked questions
How do I choose a flannel shirt size online?
Decide how you will wear it, measure a shirt with a similar cut and fabric that fits well, and compare those measurements with the product guide. Check whether the figures describe your body or the finished garment.
Should I size up if I want to wear it open?
Not automatically. Check the chest and shoulders over a T-shirt and review the shirt's cut. Sizing up may also add length to the sleeves and body.
Which parts of a shirt should I measure?
When comparing two garments, check shoulders, pit-to-pit chest, back length and sleeve, using the same measuring points.
Does an overshirt fit the same as a regular shirt?
Not always. Overshirt can describe how a garment is worn and often its cut, but each brand defines its own fit. Check the measurements and description for the exact style.