Steel Tube vs Steel Pipe NZ: Galvanised Sizes Explained
Ask for “50mm galvanised pipe” at three different counters and you can walk away with three different products. One measures 50mm across the outside, one measures about 60mm, and one has a 50mm hole down the middle. Nobody got it wrong. Steel tube and steel pipe are simply named in two different ways, and the words get used for each other every day. This guide explains the difference, shows how to measure what you already have, and lists every size in our galvanised steel round tube range so you can order the right one the first time.
Steel Tube vs Steel Pipe: What Is the Difference?
Both are round, hollow steel sections. The difference is what they were made to do, and that decides how they are named.
| Steel tube | Steel pipe | |
|---|---|---|
| Made for | Structure and fabrication: frames, rails, posts | Carrying water, gas or air |
| Named by | Actual outside diameter x wall thickness | Nominal bore (NB), a trade size |
| What “50mm” means | 50mm across the outside | About 60mm across the outside |
| Ends | Plain cut | Often threaded or prepared for fittings |
In everyday use, plenty of people say “pipe” for anything round and hollow. That is fine, as long as you confirm the outside diameter in millimetres before you buy.
Why a “50mm Pipe” Is Not 50mm Across
Pipe sizes come from the nominal bore, a rounded figure for the hole through the middle. The outside diameter is fixed for each nominal size so that fittings always match, and it is always larger than the name suggests. A 50NB pipe is close to 60mm across the outside. A 25NB pipe is close to 34mm.
This matters in two common situations.
- You are matching existing pipework or fittings. If the old rail or frame was built from 50NB pipe, a Ø50mm tube will be loose in every fitting. You need the Ø60mm size.
- You are building something new from a drawing. If the drawing says 50mm OD, you need a true Ø50mm tube, and a 50NB pipe will be too big.
Galvanised Steel Tube Size Chart
These are the 16 sizes we stock, all in 6m lengths. Outside diameter and wall thickness are as listed on each product. The bore is the outside diameter less two wall thicknesses. The last column shows which sizes line up with a common nominal pipe size, which is why they suit jobs where the tube has to match existing pipe.
| Outside diameter | Wall | Bore (approx.) | Closest nominal pipe size | Price per 6m length |
|---|---|---|---|---|
| Ø20mm | 1.5mm | 17mm | Tube size | $23.00 |
| Ø21mm | 2.2mm | 16.6mm | 15NB | $34.50 |
| Ø25mm | 2mm | 21mm | Tube size | $37.95 |
| Ø26mm | 2.2mm | 21.6mm | 20NB | $42.55 |
| Ø32mm | 2.2mm | 27.6mm | Tube size | $54.05 |
| Ø38mm | 3mm | 32mm | Tube size | $77.05 |
| Ø42mm | 3mm | 36mm | 32NB | $87.40 |
| Ø48mm | 3mm | 42mm | 40NB | $96.60 |
| Ø50mm | 3mm | 44mm | Tube size | $103.50 |
| Ø60mm | 3mm | 54mm | 50NB | $125.35 |
| Ø60mm | 3.5mm | 53mm | 50NB | $142.60 |
| Ø76mm | 2mm | 72mm | 65NB | $120.75 |
| Ø76mm | 3mm | 70mm | 65NB | $169.05 |
| Ø89mm | 3.5mm | 82mm | 80NB | $217.35 |
| Ø114mm | 3.25mm | 107.5mm | 100NB | $258.75 |
| Ø165mm | 3.5mm | 158mm | 150NB | $404.80 |
Prices are from our online store at the time of writing and include GST. Check the product page for the current price before you order.
How to Measure the Tube You Already Have
If you are replacing or extending something, five minutes with a tape or calipers saves a wasted trip.
- Measure the outside diameter. Calipers are best. Without them, wrap a strip of paper around the tube, mark where it overlaps, measure that length and divide by 3.14.
- Measure the wall thickness at a clean cut end. If you cannot reach an end, measure the bore of a matching fitting or offcut instead.
- Compare with the chart above. A reading near 60mm means you have 50NB pipe and need the Ø60mm tube. A reading of 50mm means a true Ø50mm tube.
- Check anything that has to fit over or inside it. Clamps, caps, sleeves and brackets are made for a specific outside diameter, so match them before you order the lengths.
- Allow for coatings. Old paint and heavy galvanising can add a little to a reading, so measure in two or three places.
Choosing a Diameter and Wall Thickness
As a starting point, most jobs fall into one of four bands.
- Ø20mm to Ø26mm. Light frames, garden structures, racks and guards.
- Ø32mm to Ø50mm. Handrails, gate frames, trailer and ute racks, workshop frames.
- Ø60mm to Ø89mm. Fence and gate posts, sign posts, heavier frames.
- Ø114mm and Ø165mm. Heavy posts and site protection.
Two diameters come in a choice of wall. Ø60mm is available in 3mm and 3.5mm, and Ø76mm in 2mm and 3mm. A thicker wall is stiffer, heavier and holds a weld or a clamp screw better; a thinner wall is lighter to handle and costs less. On the Ø76mm size that choice is the difference between $120.75 and $169.05 a length.
Size charts do not replace design. For handrails, barriers, anything that carries load, or work that is part of a building consent, have the size confirmed by your designer or engineer and check requirements with your local council.
Joining Steel Tube Without Welding
You do not need a welder to build with round tube. Our cast iron key clamps slide over the tube and lock with a set screw, so a rail or frame can be assembled on site and taken apart later. They are made for five tube sizes: Ø26mm, Ø33mm, Ø42mm, Ø48mm and Ø60mm.
If you plan to use key clamps, choose your tube from the galvanised sizes that match, which are Ø26mm, Ø42mm, Ø48mm and Ø60mm. The range includes tees, elbows, crosses, corner clamps, straight joiners, base flange plates and handrail wall brackets, with prices from $24.15. See the full list under scaffolding and fencing accessories.
Round Tube, Square Hollow or Aluminium?
Round tube is not always the best section for the job.
- Round tube is comfortable to grip and has no flat face to catch water, which is why it is the usual choice for handrails and posts.
- Square and rectangular hollow give flat faces that are easier to bolt, bracket and clad. See our galvanised square hollow and galvanised rectangular hollow sections, and our comparison of steel and aluminium box section.
- Aluminium round tube is much lighter and does not rust, so it suits marine, caravan and decorative work where weight matters more than outright strength. Browse aluminium round tube.
A Note on Water and Gas
Our galvanised round tubes are supplied as plain cut lengths with no threads, and they sit in our range of structural steel sections. If your job carries water or gas, the pipe, the fittings and the installation all have to suit that service. Talk to your plumber or gasfitter before you buy.
Looking After Galvanised Tube
The zinc coating is what keeps galvanised steel from rusting, so the weak points are wherever the coating has been cut through.
- Seal cut ends and drilled holes with a zinc rich cold galvanising paint.
- Cap open tops on posts so water cannot sit inside.
- Take care when welding. Grind the coating back from the weld area, work with good ventilation because heated zinc gives off harmful fumes, and recoat the area afterwards.
- Near the coast, rinse and inspect more often. Salt air shortens the life of any coating. Our guide to galvanised steel and stainless steel explains when it is worth stepping up to stainless.
What Does Galvanised Steel Tube Cost in NZ?
At the time of writing, and including GST, a 6m length runs from $23.00 for Ø20mm x 1.5mm to $404.80 for Ø165mm x 3.5mm. The sizes most people ask for sit in the middle: $37.95 for Ø25mm x 2mm, $96.60 for Ø48mm x 3mm and $103.50 for Ø50mm x 3mm. Buying in quantity can bring the price per length down, which is worth knowing if you are pricing a fence line or a run of handrail. Check the product page for the current bulk offer before you order.
Every length is 6m, so plan how you will get it home. You can use the shipping calculator on each product page, or choose click and collect from one of our branches.
Steel Tube and Pipe FAQs
Is galvanised pipe the same as galvanised tube?
The words are often used for the same product. Strictly, pipe is sized by nominal bore and made to carry fluids, while tube is sized by its actual outside diameter and wall thickness and used for structure. Always confirm the outside diameter in millimetres.
What size is 50mm galvanised pipe?
A pipe with a 50mm nominal bore is close to 60mm across the outside. To match it, choose the Ø60mm tube in a 3mm or 3.5mm wall. If your existing tube measures 50mm across the outside, choose Ø50mm x 3mm.
What length does steel tube come in?
Our galvanised round tube is sold in 6m lengths, in 16 sizes from Ø20mm to Ø165mm.
Can you weld galvanised steel tube?
Yes. Remove the zinc coating around the weld area first, work with good ventilation and suitable protection because heated zinc gives off harmful fumes, and recoat the weld with a zinc rich paint when you are done.
Will galvanised steel tube rust?
The zinc coating protects the steel, and it does that well in most outdoor settings. Rust usually starts at cut ends, drilled holes and deep scratches, so seal those as you go.
Get the Right Size First Time
Browse the full galvanised round tube range and the rest of our galvanised steel sections online. If you are matching an existing tube, bring your measurements or an offcut to our Albany, Manukau or Christchurch branch and we will help you find the size that fits.
