Bra straps that slide off the shoulder

Short answer: Straps slip when the band under the bust is too loose. Tighten the band so it sits level, then adjust the shoulder straps so they only help with the last bit of support.

The band carries, the straps guide

Most of the support comes from the band under the bust, not from the straps. If the band is loose, the straps take on the full weight, which is why they slip down through the day. See bamboo bra without underwire for styles with a good band fit.

The two finger test under the band

You should be able to fit two fingers under the band, no more. If you can pull it far away from the body, it is too big, and you lose support exactly where it matters most, right under the bust.

Shoulder shape matters for the fit

Narrow or sloped shoulders make straps slip regardless of adjustment, because there is not enough flat surface to hold them in place. A style with straps set closer together at the back helps in these cases. See bamboo underwear for women.

Size changes more often than you might think

Weight, age and cycle affect how the band fits over time, even when the cup size stays largely the same. Measure again from time to time, rather than assuming the same size always fits.

Adjust the straps, do not just tighten them

Many tighten the straps when they slip, but that only solves the problem temporarily and presses on the shoulders. Check first whether the band fits correctly, then adjust the straps so they only add a little extra hold.

The stretch of the material affects the band fit

A soft bamboo knit with good elasticity follows the ribcage more closely than a stiff material, giving the band better contact with the skin all the way around. This reduces the risk of straps having to compensate for a loose band. See bamboo tops for women in the same soft fabric.

Activity increases the need for a firm band

If you move a lot through the day, pressure on the straps increases if the band is not sitting fully firm. On active days it can pay off to choose a band that sits slightly firmer than for rest.

Washing can weaken the elastic in the band

The elastic in the band loses tension over time, especially when washed at high temperature or in the tumble dryer. A band that feels loose after many washes is often a sign it is time for a new bra, not just an adjustment.

Try several styles to find the right band

Band and cup fit differently from brand to brand, even at the same stated size. So try several styles before deciding, and put more weight on how the band feels than on the number on the label.