How to find out the size and length of brake line

What size are my brake lines?

This is the easy part. The two most common brake lines found are 3/16"/4.75mm and 1/4". The smaller size is 3/16"/4.75mm brake line,
CNF-3; and the larger size is 1/4", CNF-4.

3/16" is found on virtually all cars and trucks made in the world.

1/4" brake line is most frequently found in short pieces between the master cylinder and ABS pump, the long run to the rear of full sized cars, trucks and hydraulic clutch lines.

Cars and trucks in the 50's frequently had all 1/4" line. On some late model GM and most European built cars the larger line will be 6mm. Also, some Japanese trucks actually have 1/4" tubing.

How do I tell what the diameter of my brake lines are?

If you have a caliper you can measure them, 3/16" is .1875", and 4.75mm is the same. 4.75mm is the metric equivalent of 3/16"

1/4" tubing measures .250"/6.35mm.

6mm tubing is .236".

Production tolerance for the o.d. of brake tubing is +/- .003".
Please bear in mind that a cheap caliper can easily be .010" off the mark.

How do I tell what size (diameter) my brake lines are
without fancy tools?

Look at the master cylinder where the lines are near each other to see if there are two sizes of tubing. You can also use a 1/4" open-end wrench to gauge your lines. It will fit easily onto 1/4" line and will bang back and forth on 3/16" line. A stack of three pennies is about 3/16" thick and three pennies and a nickel are around 1/4" thick.

On some late model GM and most European built cars the larger line will be 6mm (.236") which is a little bit smaller than 1/4". What I suggest is that you use 1/4" line with our nuts which are sized for 1/4" tubing.
The 1/4" flare will fit the flare seat, but the 1/4" line will not fit in the 6mm nut. You can drill 6mm nuts out to 1/4" but be sure to deburr and chamfer the nut where it seats on the flare.

How do I tell how long the brake line is?

The easiest way is to use a roll of common plumbing solder. Simply snake the solder into position on the car (or along the line if it's removed). Tuck it into the corners and lay it where you want it. Do not skimp or stretch the solder into position unless you want to come up short.

Mark the solder with tape or such and pull it out, straighten it out and measure it. Your can snake Cunifer™ virtually anywhere you can snake solder, but be careful, modern cars are very tight and sometimes you cannot fit the nut or it is impossibly tight.