What to do when two sizes almost fit

Short answer: Choose the smaller size if the fabric has stretch and you air-dry the item. Choose the larger one if there is no stretch, or if you tumble dry. That rule settles most cases where two sizes fit almost equally well.

Stretch creates room

With a few percent of elastane, the fabric gives and follows the body, so the smaller of the two sizes often works fine in practice. Without stretch, the fabric will not do that, and then it is safer to go with the size that fits without straining. See bamboo underwear for men.

The drying method decides

If you tumble dry, a piece typically shrinks a little with each wash, so you should size up from the start to leave room. If you air-dry instead, the fabric shrinks less, and you can often stay with the smaller of the two sizes. See washing and care.

Underwear up, bra band down

For underwear generally, the rule is to size up, because a too small size digs in and becomes uncomfortable quickly. For a bra band, the opposite applies, size down, because a loose band does not support properly and slips out of place. See bamboo underwear for women.

Socks up

If you are at the top end of a size range, close to the next step up, go with that step rather than squeezing into the smaller size. A sock that pulls tight over the toes wears out faster and feels uncomfortable inside a closed shoe. See all bamboo socks for men.

Try both if you can

If you are still unsure after weighing up stretch and drying method, buy both sizes, try them at home and return the one that does not fit. It costs a little time but removes any doubt, and you avoid guessing wrong on something you wear daily. See bamboo briefs.

Measure instead of guessing between two

If you are choosing between two sizes, quickly measuring a piece you already own can settle it. If your measurements sit close to the line between two sizes on the chart, the other factors, stretch and drying method, often matter more than the measurement alone.

Body changes shift the choice

If your body has changed recently, for example after a period of more or less training, the size you used to take may no longer be the right one. In that case it is better to measure again than to rely on what used to fit.

Differences between brands

Two brands with the same size label can fit differently, so a rule that worked with one brand does not automatically apply to another. Always check the size chart for that specific brand, rather than assuming a letter size means the same everywhere.

When you still choose wrong

If you choose wrong despite the rules, that is not a sign you did anything wrong, only that the fabric and your body behaved slightly differently than expected. Exchange the size and use the experience to choose more precisely next time you face the same situation.