A clothing size is a label within a particular range, not a universal body measurement. Before ordering activity clothes, open the chart for the actual garment, check its units and find out whether it describes the body or the finished item. Those details are more useful than assuming that your usual letter size will fit every brand.
Choose the right chart
Check the product category and regional sizing. A chart for a close-fitting top may not be the chart used for trousers, a unisex sweatshirt or another collection from the same company. Follow the chart linked from the product rather than a size-conversion image saved months ago.
Nike's dual-sizing chart, for example, explicitly says its figures are body measurements. It also provides an inches-and-centimeters control. That does not mean every retailer follows the same system; it shows the kind of information worth finding before comparing numbers.
Measure what the instructions ask for
Use a flexible tape and follow the stated landmarks. Nike's guide describes measuring around the fullest bust and hips and keeping the tape horizontal. Its waist instruction refers to the narrower part where the body bends. Another garment may ask for an inseam or a different measurement, so read rather than guessing.
Keep the tape comfortably in place instead of pulling it tight to reach a preferred number. If measuring alone makes the tape slope, use a mirror or ask for help. Record the unit beside the number so that a later comparison does not mix centimeters with inches.
Take the measurement again if the tape slipped or the result seems inconsistent. That is different from repeatedly tightening it until a smaller size appears to work. The purpose is information for choosing clothes, not judging your body.
Do not confuse body and garment dimensions
A finished shirt measured flat is not the same as a measurement around a chest. Nor does doubling every flat measurement automatically explain the intended fit of a stretchy garment. Find the retailer's definition and ask if it is unclear.
Read the fit description alongside the chart. Relaxed, fitted and compression-style garments are intended to feel different even when the recommended body range overlaps. A model photograph can show the design, but it cannot replace your own measurements and the product's instructions.
Resolve a split between sizes
If different measurements point to different sizes, use the retailer's guidance for that item. Do not invent a universal rule to always size down for exercise or always size up for movement. The garment's cut and intended use matter.
Check the return and exchange conditions before ordering, especially for a new brand or a close-fitting item. Keep the product details and your measurements together so that a replacement choice is based on something more useful than remembering that the first parcel felt wrong.
When it arrives, try the clothing through the ordinary movements you plan to do, following the return policy. The chart helps narrow the choice; the final question is whether this particular garment lets you move comfortably in its intended role.