How to Measure Pipe OD Correctly

Accurate Measurement Techniques for Selecting Insulation Bore Size

Choosing the correct insulation bore size starts with accurately measuring your pipe's outside diameter (OD). This simple guide shows you three methods to measure pipe OD correctly, ensuring you order the right insulation size first time.

📐 Why Accurate Measurement Matters

• ✅ Correct fit — Insulation must fit snugly without gaps or compression
• ✅ Thermal performance — Gaps reduce efficiency, compression reduces R-value
• ✅ Condensation control — Poor fit allows vapour ingress
• ✅ Cost savings — Avoid ordering wrong size and wasting money

Key principle: Insulation bore size = pipe outside diameter (OD), NOT nominal pipe size.

🔧 Method 1: Digital Caliper (Most Accurate)

What You Need:

• Digital caliper (vernier caliper)

Step-by-Step:

1. Open caliper jaws wider than pipe
2. Place jaws across widest part of pipe (diameter)
3. Close jaws gently until they touch pipe surface
4. Read measurement on digital display
5. Round up to nearest standard insulation bore size

Accuracy: ±0.1 mm

Best for: Professional installations, precise measurements

🔧 Method 2: Tape Measure (Quick Estimate)

What You Need:

• Flexible tape measure
• Calculator

Step-by-Step:

1. Wrap tape measure around pipe circumference
2. Note circumference measurement (e.g., 69 mm)
3. Divide by 3.14 (π) to get diameter
4. Example: 69 mm ÷ 3.14 = 22 mm OD
5. Round up to nearest standard bore size (22 mm)

Accuracy: ±1-2 mm

Best for: Quick estimates, accessible pipes

🔧 Method 3: Pipe Gauge (Professional)

What You Need:

• Pipe diameter gauge (wrap-around gauge)

Step-by-Step:

1. Wrap gauge around pipe
2. Read diameter directly from gauge scale
3. Match to insulation bore size

Accuracy: ±0.5 mm

Best for: Professional installers, multiple measurements

📊 Matching OD to Bore Size

Once you have the OD measurement, match it to the correct insulation bore size:

Rule: If measurement falls between sizes, round UP to next size for snug fit.

⚠️ Common Mistakes

1. Using Nominal Pipe Size

❌ Wrong: "It's a 15mm pipe, so I need 15mm insulation"

✅ Right: Measure the actual OD — nominal size ≠ actual OD

2. Measuring Internal Diameter

❌ Wrong: Measuring inside the pipe

✅ Right: Measure OUTSIDE diameter (OD)

3. Not Accounting for Pipe Type

Different pipe types have different ODs:

• Copper 15mm = 15mm OD
• Steel 1/2" = 21.3mm OD (needs 22mm bore)
• MDPE 15mm = 16-18mm OD (needs 18mm or 22mm bore)

See full pipe bore size chart →

4. Measuring Over Existing Insulation

❌ Wrong: Measuring over old insulation

✅ Right: Remove old insulation first, measure bare pipe

📊 Pipe Type Quick Reference

Copper Pipe (UK Standard)

Nominal size = OD (mostly)

• 15mm copper = 15mm OD → 15mm bore
• 22mm copper = 22mm OD → 22mm bore
• 28mm copper = 28mm OD → 28mm bore

Steel Pipe (Imperial)

Nominal size ≠ OD

• 1/2" steel = 21.3mm OD → 22mm bore
• 3/4" steel = 26.9mm OD → 28mm bore
• 1" steel = 33.7mm OD → 35mm bore

MDPE / Multi-Layer

OD varies by manufacturer — ALWAYS measure

• 15mm MDPE = 16-18mm OD → 18mm or 22mm bore
• 20mm MDPE = 20-22mm OD → 22mm bore
• 25mm MDPE = 25-28mm OD → 28mm bore

✅ Pro Tips

• 📍 Measure in multiple places — Pipes can be slightly oval
• 📍 Measure bare pipe — Remove paint, rust, or old insulation first
• 📍 Round up if unsure — Slightly loose is better than too tight
• 📍 Check fittings too — Valves and elbows may have larger OD

🛒 Shop by Bore Size

• 15 mm Bore
• 22 mm Bore
• 28 mm Bore
• 35 mm Bore
• View All Sizes

📚 Related Guides

• Pipe Bore Size Chart
• Insulation Thickness Selection Guide
• How to Choose Pipe Insulation
• Back to Knowledge Centre

Still not sure? Contact our technical team for help.