Brake Line Size Chart: Every Tube & Fitting Thread Size Explained
If you've ever stood at the parts counter — or stared at a search bar — trying to figure out what size brake line or fitting you need, you're not alone. Brake line sizing trips people up because there are really two separate measurements at play: the tube diameter (the outside diameter of the line itself) and the fitting thread size (the threads on the tube nut that screw into the component). They're related, but they're not the same thing — and mixing them up is the most common reason a fitting won't thread in or a connection won't seal.
This guide puts every common size in one place: standard tube diameters, the SAE and metric thread sizes that go with them, and how to measure what's actually on your vehicle before you order parts.
Standard Brake Line Tube Sizes
Brake line tubing is measured by its outside diameter (OD), not inside diameter. In North America, you'll run into four common sizes:
| Tube OD | Metric Equivalent | Typical Use |
|---|---|---|
| 3/16" | 4.75 mm | The standard for most cars, SUVs, and light trucks — the vast majority of hydraulic brake lines on the road |
| 1/4" | 6.35 mm | Longer runs on trucks and full-size vehicles, some rear axle and master cylinder lines; also common for transmission cooler lines |
| 5/16" | 7.94 mm | Primarily fuel and transmission lines rather than brakes |
| 3/8" | 9.53 mm | Fuel supply lines, larger transmission cooler lines |
If you're working on a typical passenger vehicle, chances are very good your brake lines are 3/16". For a deeper look at choosing between 3/16" and 1/4", see our guide on what size brake line you need.
Brake Line Fitting Thread Size Chart
Here's where most of the confusion lives. The tube nut that slides over your line has its own thread size, and it's always larger than the tube itself. A 3/16" line does not use a 3/16" fitting — it most commonly uses a 3/8"-24 inverted flare fitting.
| Tube OD | Common SAE Threads | Common Metric Threads | Typical Flare |
|---|---|---|---|
| 3/16" | 3/8"-24 (most common) | M10x1.0, M10x1.25, M12x1.0 | SAE 45° double flare or ISO/DIN bubble flare |
| 1/4" | 7/16"-24, 1/2"-20 | M12x1.0 | SAE 45° double flare or bubble flare |
| 5/16" | 1/2"-20 | — | SAE 45° double flare |
| 3/8" | 5/8"-18 | — | SAE 45° double flare |
A few things worth knowing when you read this chart:
- Thread size is written as diameter × pitch. "3/8"-24" means a 3/8" thread diameter with 24 threads per inch. "M10x1.0" means a 10 mm thread diameter with 1.0 mm between threads.
- One tube size, several possible threads. Domestic vehicles usually use SAE threads; European and many Asian vehicles use metric. Some vehicles mix both on the same car.
- The thread only holds the connection together. The actual seal is made by the flare on the end of the tube pressing against its seat — which is why the flare type matters as much as the thread.
Browse our full range of brake line fittings, tube nuts, and adapters in every size above, or grab a fitting assortment kit and keep the common sizes on hand.
How to Measure the Brake Line and Fittings You Have
Before ordering parts, take five minutes to confirm what's on your vehicle:
- Measure the tube OD. A caliper is ideal, but a wrench works in a pinch: if a 3/16" line fits snugly in the jaws of a 3/16" open-end wrench, that's your size.
- Use a thread gauge. Our SAE brake line fitting size gauge and metric fitting size gauge let you thread your existing fitting in and read the size directly — no guesswork, no measuring thread pitch by eye.
- Check both ends of the line. It's common for one line to have different fittings at each end, especially where a line meets a hose or an ABS unit.
- Identify the flare before you buy. A double flare looks folded in on itself like a small funnel; a bubble flare looks like a raised ring near the end of the tube.
Flare Types: The Other Half of the Equation
Matching threads gets the nut started; matching the flare is what stops the leak. The three flares you'll encounter:
- SAE 45° double flare — the standard on most domestic vehicles, with dimensions defined by SAE J533. The tube end is folded back on itself for strength.
- ISO/DIN bubble flare — common on European and many Asian vehicles, usually paired with metric threads and specified under ISO 4038.
- 37° AN flare — found in performance and racing applications, not on factory street brake systems.
Never mix flare types, even if the threads happen to match — the seat geometry is different and the joint will weep under pressure. If you're not sure which one you're looking at, our guide to bubble vs. double flares breaks it down with photos, and a quality flaring tool will let you make either flare cleanly on new line.
Putting It Together: A Quick Ordering Checklist
- Measure your tube OD (most likely 3/16").
- Identify the thread size on each fitting with a gauge or by checking the chart above.
- Identify the flare type (double or bubble).
- Order tubing, tube nuts, and unions to match — or a high-pressure compression union for a fast spot repair.
If you want a full walkthrough of how tube nuts, unions, and adapters work together, read Understanding Brake Line Fittings. And because brake lines are a safety-critical system, it's worth reviewing NHTSA's vehicle safety resources before taking on a full repair.
Frequently Asked Questions
What size brake line do I need for my car?
Most cars, SUVs, and light trucks use 3/16" (4.75 mm) brake line. Larger trucks and some long rear runs use 1/4". The most reliable answer is to measure the outside diameter of your existing line with a caliper.
What thread size is a 3/16 brake line fitting?
The most common thread for 3/16" brake line is 3/8"-24 inverted flare. On metric vehicles, 3/16" line typically uses M10x1.0 or M10x1.25 fittings, and occasionally M12x1.0.
Are brake line fittings universal?
No. Fittings vary by thread size (SAE vs. metric), thread pitch, and flare type. Two fittings can look nearly identical and still not seal. Always match thread size, pitch, and flare type to the component you're connecting to.
How do I measure a brake line fitting?
The easiest way is a thread size gauge — thread your fitting into the gauge until you find the size that engages smoothly. Alternatively, measure the thread diameter with a caliper and count threads per inch (SAE) or measure the distance between threads (metric).
What's the difference between a bubble flare and a double flare?
A double flare (SAE 45°) folds the tube end back into itself; a bubble flare (ISO/DIN) forms a raised ridge near the tube end. They require different seats and are not interchangeable, even when thread sizes match.
Get the Right Size the First Time
Brake line sizing isn't complicated once you know the system: measure the tube, match the thread, match the flare. Every size in the charts above is in stock at www.4lifetimelines.com — from single fittings to complete line repair kits — so you can get the job done right the first time.