How to identify the correct inner diameter of a hose using a string?

You measure a circumference and divide by pi. That gives the diameter. The trick is knowing what to wrap the string around, because automotive hoses are sold by inner diameter, not outside diameter.

Why hose size is inner diameter

Hose size is inside diameter. A 5/8" heater hose is not 5/8" across the outside. It is 0.625 inches through the center. If you measure the outside and order that number, you will get a hose that is loose on the barb. That causes weeping coolant, vacuum leaks, or a popped line under boost.

Two hoses can share an outside diameter and still have different internal bores. Wall thickness changes from one hose type to the next. Fuel hose, heater hose, silicone hose, and emissions hose all have different construction. The measurement that matters is the ID.

The string method, step by step

Used correctly, a length of string gives you a circumference measurement you can convert to diameter with one division. The best string is soft, non-stretchy, and no thicker than carpet thread. Don't use elastic cord or a shoelace. Both stretch and lie.

1. Cut a piece of string about 12 inches long.
2. Wrap it once around the metal fitting the hose will slip onto, not the old hose.
3. Stay perpendicular to the fitting. If the string spirals, you get a falsely long reading.
4. Pinch the crossing point or mark it with a pen.
5. Lay the string flat against a ruler or tape measure.
6. Record the length in inches or millimeters.
7. Divide by 3.14.

That last step matters. If the string length is 1.96 inches, the diameter is 1.96 divided by 3.14, which gives 0.625 inches, a 5/8" hose. In metric, a 50mm circumference gives about 16mm ID. Keep the circle constant nearby: 3.1416. This works because circumference equals pi times diameter, so diameter equals circumference divided by pi.

Measure the fitting, not the old hose

The most reliable string measurement comes from the round metal barb or nipple the hose will push onto. The outside diameter of that barb is designed to match the hose's inside diameter for a proper interference fit. Wrap the string around the barb at its widest straight section, not at the raised shoulder where the hose stops.

If you're measuring a vacuum line on a plastic nipple, the same rule applies. Wrap the string around the nipple. A 3/16" vacuum barb gives a circumference near 0.59 inches. Divide by 3.14 and you get roughly 0.1875 inches, which is 3/16". That's the ID of the line you need. A plastic caliper or drill bit used as a go/no-go gauge is more precise, but the string method gets you close enough to order when a caliper isn't within reach.

Measuring the outside of an existing hose

Sometimes you can't reach the fitting. You may be replacing a hose that connects to lines buried under an intake manifold. In that case, measure the outside circumference of the old hose and subtract twice the wall thickness.

If the old heater hose has an outside circumference of 2.75 inches, that gives an outside diameter of 0.875 inches, and after subtracting two 1/8-inch walls you land back at 0.625 inches, a 5/8-inch ID. The weak point is the wall thickness estimate. You need a clean cut end, not a squashed, cracked, or swollen end. Heat and oil swell rubber over time, so an old hose's outside diameter often reads larger than the original size. Use the fitting measurement whenever you can.

Common errors

Dirt, mold lines, and raised lettering on the outside of a hose will fool the string. Wrap on the smoothest section and align the string by feel before marking it. Don't pull hard enough to cut into a soft hose. Don't let the string droop. In a tight engine bay, take the fitting off the vehicle and measure on the bench when possible. A short strip of masking tape can hold the string in place while you check that the wrap is square.

When to skip string

String is a field method. It works for identifying a hose ID when you lack a caliper, but it is not the final word on a high-pressure fuel line or an oil cooler hose. For anything carrying fuel, oil, or pressurized coolant, buy a cheap digital caliper or use a set of stepped hose gauges. The difference between a 5/16" and 3/8" fuel barb is only 0.0625 inches, and a string reading can be off that much if the wrap isn't square.

What to do with that ID number

Once you know the ID, use it to order the right hose. If you're replacing a vacuum line, match the ID to the quick coupler or fitting on the engine. If you're replacing a coolant or heater hose, choose a silicone hose rated for coolant use in the same ID, not a fuel or emissions line. And if the old hose was just ugly rather than failed, the ID tells you which size of hose skin or wrap to buy. A caliper is faster, but the string method works anywhere and needs no battery. It just needs a clean wrap and one division.

When you have the ID written down, match it to the relevant silicone hose or vehicle kit if you want the guesswork removed.