Free Reference Tool

Brass Olive Size Guide

Find the right compression olive for metric & imperial copper pipe — instantly.

Olive Size Finder — Select Your Pipe Size

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Full Reference Chart

Pipe OD → correct brass olive size for compression fittings

Metric Brass Olives — 4mm to 12mm

Olive Size	Pipe OD	Pipe OD (inch approx.)	Common Use	Type
4mm	4.0mm	~5/32″	Microbore, underfloor heating	Metric
5mm	5.0mm	~3/16″	Specialist refrigeration, gas	Metric
6mm	6.0mm	~15/64″	Central heating, domestic plumbing	Metric
8mm	8.0mm	~5/16″	Central heating primaries	Metric
10mm	10.0mm	~3/8″	Hot & cold water supply	Metric
12mm	12.0mm	~15/32″	Less common — check vs 1/2″	Metric

Imperial Brass Olives — 1/8″ to 5/8″

Olive Size	Pipe OD (inch)	Pipe OD (mm exact)	Common Use	Type
1/8″	0.125″	3.18mm	Refrigeration, pneumatics	Imperial
3/16″	0.1875″	4.76mm	Brake pipes, automotive hydraulics	Imperial
1/4″	0.250″	6.35mm	Central heating, domestic plumbing	Imperial
5/16″	0.3125″	7.94mm	Central heating, older pipework	Imperial
3/8″	0.375″	9.53mm	Older UK plumbing, hot water cylinders	Imperial
1/2″	0.500″	12.70mm	Most common UK domestic plumbing	Imperial
5/8″	0.625″	15.88mm	Larger domestic & commercial supplies	Imperial

How to Measure Your Pipe for the Correct Olive

Brass compression olives are sized to match the outside diameter (OD) of your copper pipe — not the bore (inside diameter). Always measure OD.

Calipers — the most accurate method. Wrap around the pipe and read the measurement directly.
Tape measure — measure the circumference and divide by π (3.1416) to get OD.
Check existing fittings — a compression fitting already on the pipe will tell you the size (it's marked on the fitting body).

If you're unsure whether your pipe is metric or imperial, remember that UK domestic plumbing installed before the 1970s is almost always imperial. Post-metric pipework is typically 15mm, 22mm, or 28mm OD — if you have those sizes, you need a copper pipe olive rather than these sizes listed here.

What Is a Brass Olive?

A brass olive is a small ring-shaped sealing component used in compression pipe fittings. When the compression nut is tightened, the olive is compressed against the pipe and the fitting body, creating a watertight seal without soldering or welding.

Brass olives are preferred over copper alternatives in many applications because brass is softer and deforms more predictably, giving a more reliable seal — especially on refitted joints.

Frequently Asked Questions

Can I reuse a brass olive?

Generally, no. Once an olive has been compressed it permanently deforms to the shape of the pipe. Re-tightening on the same pipe occasionally works for a very minor drip, but for a reliable seal you should always fit a new olive when dismantling a joint.

What's the difference between metric and imperial olives?

Metric olives are sized in millimetres (4mm, 6mm, 8mm, 10mm, 12mm) and match modern metric copper pipe. Imperial olives are sized in fractions of an inch (1/4″, 3/8″, 1/2″) and match older imperial copper pipe. They are not interchangeable — always match the olive to the pipe OD exactly.

Do brass olives work with plastic (Speedfit / JG) pipe?

Brass compression fittings with brass olives are designed for copper pipe. Plastic push-fit systems (Speedfit, JG Speedfit, Hep2O) use different internal seals and do not use traditional olives. Some compression fittings can be used on plastic pipe with an internal support insert, but the olive must match the OD of the plastic pipe.

Which olive size is most common in UK homes?

In homes plumbed post-1970s with metric pipe, 6mm, 8mm, and 10mm olives are the most common. In older properties with imperial pipe, 1/4″, 3/8″, and 1/2″ are the most frequently encountered sizes.

Do I need jointing compound with a brass olive?

For water systems, a properly tightened brass olive should seal without compound. However, many plumbers apply a small amount of PTFE tape or jointing compound to the olive as additional insurance — particularly on gas and heating systems where regulations may require it.