Mild Steel Round Bar Sizes Explained: Diameter, Length & Applications
9 Jan 2026
When it comes to buying mild steel round bar, size really does matter. Knowing what you need can make all the difference, whether you’re fabricating machinery, repairing kit or building something from the ground up.
But with so many sizes and grades out there, it can be tricky to know where to start. That’s where we come in. At The Metal Store, we make metal buying simple, cutting it to the exact size you need and delivering it straight to your door. Read on as we make mild steel round bar sizes simple.
Article in Brief
Mild steel round bar is a strong, versatile material used in fabrication, construction and engineering.
The two main size variables are diameter and length, which affect how the bar performs in different projects.
Other key factors like tolerance, weight, finish and grade also impact precision and performance.
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What is a mild steel round bar?
Mild steel round bar is a solid, circular length of steel used everywhere from workshop repairs to large-scale construction. It’s known for being easy to cut, drill, weld and machine, which is why it’s a favourite across trades.
At The Metal Store, you’ll find a wide range of mild steel round bars available in both black (hot-rolled) and bright (cold-drawn) finishes. Whatever the project, there’s a size and style to suit.
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The main sizing variables
Diameter
The first number to check when ordering is diameter. It’s the width of the bar, measured in millimetres (mm). Typical diameters range from 6mm right up to 100mm+, depending on what you’re building.
- Smaller diameters (6mm-20mm): Ideal for light fabrication, model making, supports and repair jobs.
- Mid-range (25mm-50mm): Great for shafts, frames and general manufacturing.
- Larger diameters (above 50mm): Perfect for heavy-duty engineering or load-bearing applications.
Remember: The bigger the diameter, the heavier and stronger the bar in bending and load-bearing applications, but also the trickier it is to cut and handle.
If you’re unsure what size suits your job, just ask. Our team are happy to help you choose the right round bar first time, saving you time (and waste) on site.
Length
Mild steel round bar usually comes in standard lengths of 3 metres or 6 metres, but you don’t have to stick to that.
At The Metal Store, we offer unlimited free cutting, so you only pay for what you need. No offcuts. No excess. Just the right size, ready to go.
You can also select your delivery date and get next-day delivery on over 3,000 products, meaning no downtime waiting around for stock.
In practice, diameter and length are the primary sizing considerations across UK fabrication and engineering, with finish, tolerance and grade refined based on how the bar will be used.
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Other key factors to think about
Tolerances
If you’re working to tight specifications, precision matters. Round bars are produced to strict tolerances set out in BS EN 10060, which governs dimensions, roundness and straightness.
For most fabrication and general use, standard tolerance is more than enough. But for precision engineering or CNC machining, it’s worth checking tolerance grades before you order.
Weight
Knowing the weight per metre helps when calculating load limits, delivery weights or project costs.
Here’s a quick guide to give you an idea:
Diameter (mm) | Approx. weight per metre (kg)
10mm | 0.62kg
20mm | 2.47kg
40mm | 9.86kg
60mm | 22.2kg
80mm | 39.4kg
10mm | 0.62kg
20mm | 2.47kg
40mm | 9.86kg
60mm | 22.2kg
80mm | 39.4kg
Weights shown are approximate and based on standard mild steel density.
Finish
Mild steel round bar is available in two main finishes:
- Black (hot rolled): Slightly rough surface, perfect for welding, fabrication and general structural work.
- Bright (cold drawn): Smooth, shiny and more precise. Ideal for machining, turning or applications where appearance matters.
Both can be painted, powder-coated or galvanised if you need extra corrosion protection or a cleaner look.
Grades
The two most common mild steel grades for round bar are:
- EN1A: Easy to machine, ideal for turned parts, threads and general engineering.
- EN3B: Stronger and more versatile, suitable for welding and structural use.
Both grades are highly workable. That’s why mild steel is known as the all-rounder of the metal world.
Applications
You’ll find mild steel round bar used just about everywhere:
- Construction: Rebar, supports, braces and fixings.
- Manufacturing: Shafts, axles, machine parts.
- Automotive: Steering components, brackets and tooling.
- General fabrication: Frames, gates, jigs and fixtures.
- DIY & repairs: Small projects, workshop use or hobby engineering.
If it needs to be tough, round and ready to work, mild steel round bar fits the bill.
Stock and customisation
We stock a huge range of diameters and lengths, ready to go from our UK warehouses.
And thanks to unlimited free cutting, you’ll only ever get the size you actually need. No waste, no cutting charges and no surprises at checkout.
Need it fast? Choose TMS Express for next-day delivery on thousands of products or pick your own delivery date to suit your schedule.
Why buy from The Metal Store?
We’ve been making metal simple since 2011. Whatever the size or scale of your project, you’ll get unlimited free cutting and fast delivery within 1-3 days.
That’s why tradespeople, makers and big businesses alike trust The Metal Store for their steel supplies.
Browse our full range of mild steel round bars or give us a call on 01274 875 479 to talk to our team.
FAQs
How do I measure mild steel round bar size?
Measure across the centre of the bar. That’s your diameter. Most round bars are specified by diameter in millimetres (mm), not by radius or circumference.
What’s the difference between diameter and thickness?
Thickness applies to sheet or hollow products like tube. For solid bar, we only talk about diameter. That’s the full width across the circular section.
What size round bar should I use for my project?
It depends on the job. Smaller diameters (6-20mm) work well for lightweight frames or decorative use, while larger sizes (50mm+) are best for load-bearing or structural applications.
What tolerances apply to mild steel round bar?
Mild steel round bars are produced to BS EN 10060, which sets limits on diameter and straightness. If you need exact precision for machining, bright bar offers a tighter tolerance than black bar.
Does bar length affect strength or performance?
Not directly. Strength comes mainly from the diameter and grade. But longer bars can flex or bend more easily under load, so cutting to a shorter length can improve rigidity.
What’s the difference between black and bright bar?
Black bar is hot-rolled and rougher, better for fabrication. Bright bar is cold-drawn and smoother, ideal for precision machining.
