Short answer: for SAE brake line, use double flare, the tube end is folded over on itself to double the wall thickness at the sealing surface, which is what a brake fitting is actually designed to clamp against. Single flare is a different, thinner shape used on other fittings entirely, and the two are not interchangeable.
What “double” actually means
An SAE 45° double flare isn’t just flared twice for luck, the process folds the very end of the tube back inside itself before forming the final 45° cone, so the surface that actually contacts the fitting is two layers of tube wall thick rather than one. That extra thickness is what lets a double flare hold up to the pressure spikes and vibration a brake line sees over years of use without cracking at the flare.
Where single flare shows up instead
A single flare, one 45° cone formed directly from the tube wall with no fold, is common on fuel and transmission cooler lines, where the pressures and duty cycle are gentler. It is not a substitute for a double flare on a hydraulic brake line, even though the two can look similar enough at a glance to fool someone flaring their first line. AN 37° and ISO/DIN bubble flares are different again, different angle, different shape, matched to their own fitting families rather than SAE double flare hardware.
How to tell which one you’re looking at
A double flare has a visible step or slight thickening right at the flared end, from the folded-over material. A single flare tapers smoothly from the tube’s normal wall thickness straight into the cone with no step. If you’re reworking an existing line and can’t tell which it is, don’t guess, a flaring tool built for double flare (like the ones below) forms the fold as part of the process, so using the right tool for SAE brake work makes the question moot.
What we stock
BrakeQuip Products carries flaring tools that form a proper SAE double flare, BQ351 and BQ350, plus AN 37° and DIN bubble-flare tooling in BQ352 for the fitting standards that call for it. See our step-by-step flaring guide for the full process.