What Size Brake Line Do I Need?

What Size Brake Line Do I Need?

Brake lines are designed to last for the life of the car, but we know that life doesn't always work out that way. Leakage is the most common symptom that a brake line needs to be replaced. Be sure to check out our Knowledge Garage for helpful brake line replacement tips, such as 'When Is It Time For A Brake Line Repair?' Sometimes brake lines collapse or bend because of a car accident. Other damage like holes and cracks can be caused by rocky roads or by hitting debris. Moisture and road salt can also create rust that eats through the line. Last but not least, brake fluid itself can corrode the line from the inside.

When a replacement line is needed, the first question is what size to get. For brakes it almost always comes down to two diameters: 3/16" and 1/4".

The 3/16-inch line — also called a CNF-3 — is the standard on the vast majority of cars, SUVs, and light trucks on the road today. If you're working on a typical passenger vehicle, there's a very good chance your brake lines are 3/16".

That doesn't make it the only size you'll meet. 1/4-inch line is still in active use — you'll find it on longer runs and full-size trucks, on some rear-axle lines, and on certain master-cylinder lines, and it's also common for transmission cooler lines. (1/4" hydraulic systems were widespread back in the 1950s, but it's far from a vintage-only size.) Step up again and 5/16" and 3/8" lines turn up mainly on fuel and transmission systems rather than brakes. Because the correct size is set by your vehicle's hydraulic system, the safe rule is simple: replace your line with the same diameter it came with.

Keep in mind that tube diameter is only half the equation — the fitting threads on each end are a separate, larger measurement. For every tube diameter and its matching SAE and metric thread sizes in one place, see our brake line size chart.

Between 3/16" and 1/4", both diameters deliver pressurized brake fluid equally well, so there's nothing to gain by switching sizes — match what the manufacturer used.

If you find yourself with custom brakes installed and you can't determine the size of the brake line, you can measure it in a few steps. You'll need a caliper and your owner's or shop's manual.

First, consult the owner's manual. It should tell you the exact location of the master cylinder and the brake lines.

Second, clamp the caliper over a part of the brake line you can access easily. Rotate the dial until the jaws close completely and snugly around the line.

Third, read the measurement on the caliper — it should be in inches, millimeters, or sometimes both.

Finally, you might need to convert the measurement to find the right brake line size. Manufacturers typically use standard or metric measurements. To convert inches to millimeters, multiply by 25.4; to convert millimeters to inches, divide by 25.4. Having both measurements with you when you come to 4LifetimeLines will prepare you to find the right brake line kit. Be sure to check out one of 4LifetimeLines' newest products, RightQuick, which offers a correct, complete replacement brake line for your vehicle's year, make, and model. Forget measuring and cutting your brake lines — each coil in the kit is cut to length for your vehicle. All kits use a 90-10 copper-nickel brake line and are fitted with black-oxide-coated fittings. Now you're ready when asking, "What size brake lines does my vehicle need?"

The experts at 4LifetimeLines are always here to help you with all your automotive line replacement needs.