For how to size a shirt to tuck in, start with the space you need when the shirt is inside a waistband, not just the way it looks untucked. The shoulder seam should sit close to the shoulder, the buttons should lie flat across the bust, and the body should have enough ease to stay smooth when you reach forward. A shirt that is too tight will pull free as soon as you sit, while one that is too large may create bulk under the skirt or trousers. COS options in this edit range from $99 to $189. Tie-front Knitted Wool Shirt has a softer, more flexible character, while a satin shirt can slide and need a firmer tuck. Check the hem length as well. It should extend far enough into the waistband to stay anchored, but not so far that several layers gather at the stomach. Try the shirt with the exact bottom you plan to wear, then lift your arms and sit down. If the hem rises out, size or fabric may be the issue rather than the waistband. A fuller shirt can work with a straighter skirt or trouser, while a closer shirt leaves more room for a wider leg or fuller shape. The right size supports movement and keeps the tuck looking clean.

The bottom half matters as much as the shirt when you want the tucked line to stay neat. Look for a waistband that sits securely without pressing into the body, and choose a fabric with enough structure to keep the shirt from slipping straight back out. COS skirts in this edit range from $139 to $289. Pleated Merino Wool Midi Skirt brings warmth and surface movement, so a lighter shirt can keep the top half clear. A lace or jacquard skirt adds texture near the waist, making a smooth tuck more useful than a bulky one. Before choosing a size, fasten the waistband and sit down. It should remain comfortable without gaping at the back or cutting across the stomach. A high rise usually gives the shirt more fabric to grip, but the best rise is the one that stays in place through normal movement. Tuck the shirt fully, then gently pull a little fabric back out around the waist. This small release prevents a stiff, over-tight appearance and gives the body room to move. If the skirt is fitted, keep the shirt thin or softly draped. If the skirt has volume, a more substantial top can balance it. The aim is a stable base that lets the shirt remain part of the outfit instead of becoming a repeated adjustment.

A tucked shirt looks deliberate when the fastening is clear, but definition does not mean discomfort. Check the waistband while standing and seated, and make sure the fastening does not create a ridge under a fine shirt. COS trousers here range from $129 to $189. Corduroy Straight-leg Pants give the outfit a more tactile, casual direction, while a wool-blend or contrast-piped pair can read more tailored. Seamless Sheer Knitted Silk Pants need particular care with underlayers and the line of the tuck, since extra bulk will show through more readily. Consider the rise, too. A mid or high rise offers a clearer place for the hem to sit, while a lower rise may need a shorter shirt or a lighter partial tuck. The leg shape affects the balance. Straight trousers work with a fuller sleeve or softly oversized body, while a wider leg usually benefits from a closer shoulder and clearer waist. Keep the shirt fabric in mind when choosing a belt. A slim belt suits a delicate top, while a wider belt can support a heavier weave. Check the back view before leaving the house, because a shirt can look neat from the front and bunch at the spine. A good waistband holds the outfit together without announcing itself.

Sizing a tuck-in shirt is mostly a question of movement. The shoulders and bust need to fit cleanly, the body needs enough ease to let you sit and reach, and the hem needs enough length to stay inside the waistband. The bottom half should hold the tuck without pinching, while the fabric determines how much bulk the outfit can handle. COS prices here run from $99 to $289, with room for lighter shirts and more structured skirts or trousers. Try the complete outfit before deciding between sizes. If you take one thing away, buy for the movement you need after the tuck, not only the outline you see in a mirror.