How to identify the correct clamp size for a metric hose?

You need a clamp whose listed working range includes the installed outside diameter of the hose. Measure the outside diameter at the clamp seat with the hose in its final position over the fitting. Then choose a clamp that puts that measurement in the middle of the range. The metric size printed on the hose is the inside diameter. That number describes flow capacity. Clamp fit uses the outside diameter.

Measure the installed outside diameter at the clamp seat

If the hose is already on the fitting, measure it there. Use a caliper across the hose at the point where the clamp will sit. The clamp seat is on the shank of the fitting, behind the raised barb or bulge. Not on top of it. If the hose is off the car, slide it onto the fitting first or hold it in place while you measure, because a hose changes outside diameter when it stretches over a fitting and the number you get from a loose hose will read too small.

No caliper? Wrap a strip of paper around the hose, mark where it overlaps, flatten it, and measure the length. Divide that length by 3.14. That number is the outside diameter in millimeters.

Use the metric label only as a starting point

A 13mm heater hose is 13mm across the inside bore. The wall adds to that. A 13mm hose with a 4mm wall measures 21mm outside diameter at rest, while a 13mm hose with a 2.5mm wall measures 18mm outside diameter, and once the hose is stretched over a fitting the outside diameter grows again, which is why the printed size cannot tell you the clamp size.

The quick formula is outside diameter equals inside diameter plus twice the wall thickness. For a 13mm ID hose with a 4mm wall, that is 13 + 8, or 21mm. Use that as a check only. The installed measurement is the one that matters.

Pick a clamp range with room to adjust

Clamp makers list a working range for each size. A worm-drive clamp marked 19-32mm fits hoses with outside diameters between 19mm and 32mm. A 26mm installed hose falls near the middle of that range. That leaves adjustment on both sides. A 31mm hose in the same clamp sits too close to the fully open limit. A 19mm hose sits too close to the fully closed limit. Pick a clamp where the measured outside diameter sits in the middle of the range. The middle position lets the clamp compress the hose without bottoming out and leaves room for thermal expansion or hose swelling.

Convert metric measurements to imperial clamp sizes

Many aftermarket clamps list ranges in inches. Divide your millimeter number by 25.4. A 21mm outside diameter is 0.83 inch. A 24mm outside diameter is 0.94 inch. A clamp listed as 3/4 inch to 1-1/4 inch covers 19.1mm to 31.8mm. Check your measurement against that converted range before buying. Metric spring clamps vary by manufacturer, and the number stamped into the band can refer to the outside diameter of the installed joint or to a nominal hose inside diameter, so the stamp alone doesn't tell you which sizing system you're holding. Measure the old clamp in its installed position and match the new one to that measurement.

Sizing Boa Clamps on a metric hose

For Boa Clamps, start with the same measurement. Measure the installed outside diameter in millimeters, divide by 25.4, and compare that to the Boa sizing chart. Boa Clamps run from roughly 1/2 inch to 3-1/2 inch, so the metric range covers about 13mm to 89mm. Pick a Boa that slips over the installed hose and fitting with light resistance before heat. The heat-shrink band then conforms to the joint. You don't need the preload adjustment of a worm-drive clamp. Once shrunk, Boa Clamps grip tighter as underhood temperatures drop and they don't require retightening. If your measurement sits at the top end of one size, check the next size up in the chart before ordering. A clamp that has to be forced over a fitting is too small.

Example measurements for metric hose IDs

Run these numbers as arithmetic checks, then measure the installed joint before ordering.

• 8mm ID hose with 3mm wall: 8 + 6 = 14mm outside diameter at rest.
• 13mm ID hose with 4mm wall: 13 + 8 = 21mm outside diameter at rest.
• 19mm ID hose with 4.5mm wall: 19 + 9 = 28mm outside diameter at rest.
• 32mm ID hose with 5mm wall: 32 + 10 = 42mm outside diameter at rest.

Remember the installed fitting raises these numbers. A 13mm hose with a 4mm wall can measure 23mm or 24mm over a barbed fitting, not 21mm. Measure on the assembled joint.

Common mistakes when sizing a metric hose clamp

• Buying a clamp based on the hose ID printed on the hose.
• Measuring the old clamp when it's loose and rusted.
• Picking a clamp that fits at the extreme end of its range.
• Reusing a stretched or corroded clamp on a new hose and expecting a consistent seal.

Once you have the outside diameter number, the rest is a parts counter task. Compare it to the range on the clamp you want. For a heat-shrink Boa Clamp, use the same number and match it on the size chart. If you're measuring several hoses for a full dress-up kit, list each installed outside diameter and order the sizes that cover those numbers. That is the difference between a clamp that leaks and one you can install and forget. Find the Boa Clamp sizing chart on the Boa Clamps page.