Home FAQ

What size brake line fitting do I need?

In most passenger vehicles, you’ll be choosing between a 3/16-inch or 1/4-inch outer-diameter brake line with SAE-style flare fittings. The exact size depends on your car’s design, so the safest approach is to measure the line you’re replacing or consult the vehicle’s service manual or OEM diagrams.

Understanding brake line sizes

Brake lines are defined by two main dimensions: the outer diameter of the tubing and the flare type used at the ends. The two most common tubing diameters on passenger cars are 3/16" and 1/4", while trucks and some specialty vehicles may use larger diameters such as 3/8" or 5/16". The flare type—most often SAE double flare in the U.S. or bubble flare on certain European models—determines which fittings and seats you need. Because the nut size and seating surface are matched to both the tube diameter and the flare design, getting the right combination is essential for a proper seal and safe operation.

Common brake line tube diameters

These diameters cover the sizes you’re most likely to encounter. Use the one that matches the brake line you’re replacing.

3/16 inch (0.1875") outer diameter — the most common size for many sedans and light trucks.

1/4 inch (0.25") outer diameter — used on some models and certain routing configurations.

3/8 inch (0.375") outer diameter — more common on full-size trucks and some heavier applications.

5/16 inch (0.3125") outer diameter — seen on some older or specialty systems.

Note: The correct size depends on your specific vehicle and the section of the brake system you’re repairing. If you’re unsure, bring the old line or a measurement to a parts counter or consult your vehicle’s service manual.

Identifying the correct fitting

To ensure you purchase the right fittings, identify both the tube diameter and the flare style, then match them to the parts you plan to buy. This prevents leaks and ensures proper seating in the joint.

Measure the outer diameter of the hard brake line you’re replacing with calipers or a precise ruler.

Inspect the end of the line to determine the flare type (double SAE flare versus bubble flare). The shape and seating surfaces guide the nut and seating you need.

Check your vehicle’s service manual or OEM parts diagram for the exact line size and flare specification for your year, make, and model.

With the measurements in hand, you can buy a replacement brake line kit that specifies the tube diameter and flare type, or order by OEM part numbers to ensure exact compatibility.

What to buy and how to install

Shop options range from vehicle-specific pre-formed lines to generic segments that match your measured size and flare. If you can, use a kit designed for your exact model to ensure ends, bends, and mounting points align perfectly.

Choose a vehicle-specific kit when possible to guarantee fitment of ends and bends.

If replacing a segment yourself, ensure the new line is straight, clean at the ends, properly flared, and securely clamped during installation.

After installation, bleed the brake system to remove air and check for leaks. If you’re not confident in identifying the correct size or completing the installation, seek help from a qualified mechanic. Safety first: braking systems are critical to vehicle safety.

Summary

Brake line fittings are defined by two factors: the tubing outer diameter (commonly 3/16" or 1/4" for passenger cars, with larger sizes on trucks) and the flare type (SAE double flare versus bubble flare). To determine the right size, measure the existing tube, identify the flare style, and refer to the vehicle’s service manual or OEM diagrams. When in doubt, bring measurements or the old line to a parts counter to ensure you pick the correct kit or fittings, then install with proper bleeding and safety precautions.

How do I know my brake line fitting size?

To measure brake line fittings, first measure the outer diameter (OD) of the brake line with calipers for compression or flare fittings, and then use a thread pitch gauge to determine the thread size and pitch. For compression, inverted flare, and flare fittings, focus on the tubing's OD. For the threads on external fittings or bolts, use a thread gauge or try fitting it with a reference bolt to find the match.
1. Measure the brake line diameter

For compression, inverted flare, and flare fittings: Use a caliper to measure the outer diameter of the brake line itself.
For banjo bolts: The size is based on the bolt's diameter (e.g., 10mm or 12mm).

This video demonstrates how to measure brake line diameter: 54sMoto WrenchersYouTube · Mar 22, 2024
2. Identify thread size and pitch

Use a thread gauge: This is the most accurate method. A thread gauge has a set of blades with different numbers of threads per inch (TPI) or millimeters.
- For inch threads, find the blade that perfectly matches the spacing between the threads on your fitting.
- For metric threads, find the corresponding millimeter size.
Use a caliper for external threads: If you can't use a thread gauge, a caliper can give you the outer diameter of the fitting, but you'll still need to determine the pitch separately.
Try a reference bolt: You can also use a reference bolt with a known thread size (e.g., a 3/8" x 24 UNF bolt) to see if it threads smoothly into the fitting.
For metric vs. imperial threads: Be aware that metric and inch threads are different. A 3/8" x 24 thread is not the same as a 10mm x 1.0 metric thread.

This video explains how to measure thread sizes: 58sCompressor SourceYouTube · Jul 19, 2019
3. Verify the fit

Test-fit by hand: Before tightening, try hand-threading the fitting. It should go in at least three full turns smoothly without binding.
Check for a tight fit: A correctly matched fitting will thread in tightly and smoothly. If there's any looseness or shaking, the fit is wrong.
Match metal types: If possible, match the fitting and line materials. If you need to adapt between different materials, use a brass adapter and ensure the connection is sealed properly.

Are all brake line fittings the same?

There are several types of brake line fittings, each designed for specific applications. Understanding the differences between these fittings is crucial for selecting the right type for your vehicle: Double Flare: Used in most automotive applications, double flare fittings are known for their durability and secure fit.

Can I use 1/4 brake line instead of 3/16?

The other most common brake line is the ¼ inch. This size was routinely used in the 1950s, so if your truck or car was manufactured in that era, you would need the ¼ inch. All other vehicles that have been manufactured since then should use the 3/16-inch brake line.

What size fitting on 5-16 brake line?

1/2-20 thread
1/2-20 thread. To be used with 5/16" brake line tubing.

Kevin Bennett

Company Owner

Kevin Bennet is the founder and owner of Kevin's Autos, a leading automotive service provider in Australia. With a deep commitment to customer satisfaction and years of industry expertise, Kevin uses his blog to answer the most common questions posed by his customers. From maintenance tips to troubleshooting advice, Kevin's articles are designed to empower drivers with the knowledge they need to keep their vehicles running smoothly and safely.