You’ll want to be completely topless for this to ensure you’re measuring your breasts at their most natural, Harrington says — no padding, binding, lifting, or constricting. Take the measuring tape and wrap it around your ribcage, just under your bust. The measuring tape should be parallel to the ground.
You should be able to fit a finger under the measuring tape. “If you’re pulling your tape measure too tightly, you might wind up getting a bra band that’s too tight,” Harrington says. “If you have a larger or heavier bust, you probably do want a bra band that’s a little bit tight, but let’s start off with accurate measurements first. If you have a smaller bust, having a bra band that’s too tight can be very uncomfortable, especially if you are thinner and have less cushion on your rib cage because the bra band is resting directly on those bones.”
Round the measurement to the nearest whole number and you’ve got your band size. Depending on the manufacturer, you may need to add four if the measurement is even and add five if the number is odd. So if you measured 34 inches, your band size would be 38; if you measured 27 inches, your band size would be 32. Historically, this has been standard practice in bra measuring and manufacturing — and no one is really sure why. Some speculate the “plus-four rule” was to accommodate for breathing room when bras were made out of un-stretchy material like silk and satin. Popular bra-fit community r/ABraThatFits traced the plus-four method back to the 1960s when bra measurement practices changed. Again, it’s important to consult each manufacturer's fit guide to determine if you’ll need to do this addition.