How to Measure a Hydraulic Hose

Why hose size matters

Hydraulic systems rely on the accurate transfer of fluid under pressure. An incorrectly sized hose can compromise flow, increase pressure drop, and result in heat build-up.

These issues are not always immediately obvious, but over time they reduce the efficiency of your system and increase the risk of failure. Unfortunately, many hose installers lack formal training in measurement or specification, leading to replacements that differ slightly, but significantly, from the original.

Unfortunately, we see this all too often. That’s why we’re committed to making hose selection simpler and more accurate.

Need help choosing the right hose? Contact our team today.

Measuring hose length

When ordering a hydraulic hose, it’s important to understand how hose length is measured. The two common methods are overall length and cut length.

Overall length refers to the complete hose assembly, including the end fittings. It is the most widely used measurement method when ordering replacements.

Cut length refers to the hose body only, excluding the fittings. Misunderstanding the difference can easily lead to misorders and delays, so always double-check how your supplier defines hose length.

If you’re unsure how to measure or specify length, use our free Technical Downloads to assist with planning and ordering.

Understanding hose size

Hydraulic hoses are specified by their internal diameter (ID), not the external diameter. Two systems are commonly used: nominal diameter (in millimetres) and inches.

The nominal diameter, abbreviated as DN, refers to the internal diameter in millimetres. For example, a DN12 hose has a 12mm bore. The imperial system uses inches, where one inch equals 25.4mm.

Both measurements are used widely, depending on the country and industry. Measuring the outside of the hose won’t give you an accurate size because wall thickness varies between hose types.

Always measure or specify the internal diameter when identifying or ordering a hose.

Decoding hose markings

Most hydraulic hoses come with printed markings showing the hose type, size, pressure rating, and manufacture or cure date.

These are helpful when identifying replacements, but don’t rely on markings alone. If your system has been modified or is experiencing higher-than-expected pressures, those printed ratings may no longer be sufficient.

Pressure can vary throughout the hydraulic system. Some areas may be low pressure and low flow, while others, particularly near cylinders, can see pressure spikes that exceed pump ratings.

Always check the actual system requirements before selecting a hose. If you’re unsure, contact our team before ordering.

The Importance of internal diameter

It’s a common mistake to measure the external diameter of a hose when trying to determine size. But the correct measurement is always the internal diameter.

For example, a 3/4″ hose refers to the ¾ inch internal bore, not the outer wall. Because hose wall thickness varies depending on pressure rating and construction, using the outer diameter can result in choosing the wrong hose entirely.

This can cause poor system performance or unsafe operating conditions.

Hose construction & pressure ratings

Different hoses are designed for different applications. The number of wire braids inside the hose affects its pressure rating. For example, an R2T hose contains two layers of wire braid and is suitable for low to medium pressure applications.

A 4SP hose has four internal wire braids and is built for high-pressure use. The hose’s rubber compound, fittings, and environmental exposure also influence its suitability.

You won’t always be able to see these details from the outside, so if in doubt, speak to a specialist or refer to product datasheets.

Protecting hose fittings from corrosion

While most hose fittings are made from plated steel, stainless steel is often used for marine or corrosive environments. Stainless fittings are more resistant to rust but require care during installation, specifically, a lubricating paste should be used to prevent thread galling.

For hoses located on boat decks or in exposed environments, additional protection such as Denso tape can help extend service life and prevent failure due to corrosion.

What happens if you get it wrong?

Installing the wrong size hose can lead to serious system issues. A hose with too small a bore restricts fluid flow, increases pressure drop, and generates heat, affecting performance and safety.

On the other hand, a hose with too large a bore can reduce fluid velocity, create turbulence, and cause noise or vibration in the system.

Precision matters. Choosing the correct size helps maintain system stability and prolongs the life of both hoses and machinery.

Accurate hose identification is not just a matter of convenience, it’s essential to safe and efficient hydraulic system operation. By understanding how to measure length correctly, specify internal diameter, and interpret hose markings, you can ensure your system performs as intended and avoid costly mistakes.

Whether you’re replacing a hose or planning a new system, our team at HOS is here to help. From product selection to technical advice and custom hose assemblies, we offer everything you need to get the job done right.