If you’ve seen a bar bending schedule or a structural drawing with callouts like “T12” or “H16” and weren’t sure what the number meant, you’re not alone. It’s simpler than it looks once you know what the letter and number are actually telling you.
What the letter means
“T” stands for Type 2 deformed high yield reinforcing bar — the ribbed steel bar used in almost all UK reinforced concrete work today, manufactured to BS 4449 / BS EN 10080. You’ll sometimes still see the older “H” prefix (high yield) on older drawings or from some engineers out of habit — T and H mean the same thing on a modern UK bar bending schedule. Both refer to grade B500 steel, just described two different ways.
What the number means
The number after the letter is simply the bar’s nominal diameter in millimetres. T8 is an 8mm diameter bar, T16 is 16mm, T25 is 25mm, and so on. UK stock sizes run T8, T10, T12, T16, T20, T25, T32 and T40 — each step up roughly doubles the bar’s cross-sectional area and strength, which is why a structural drawing might specify T12 for a light domestic slab but T25 or T32 for a heavily loaded beam or column.
Why the size matters for your order
The diameter determines the bar’s cross-sectional area, its weight per metre, and how much load it can carry — all of which your engineer has already calculated into the drawing. Getting the size wrong isn’t a small mistake: an undersized bar can leave a structural element unable to carry its design load, and it’s one of the most common reasons a building control inspector rejects reinforcement on site.
Where to get exact specs
We’ve got a dedicated specification page for every standard diameter, with exact weights, lengths and typical uses: T8, T10, T12, T16, T20, T25, T32 and T40, or see the full range on our reinforcing bar page.
This guide is intended as general background only. The reinforcement your project actually needs — bar size, mesh type, cover, spacing, lap lengths — is set by your structural engineer’s drawings or your building control surveyor, not by a website. Always work to the specification you’ve been given, and if you don’t have one, ask your engineer before you order.
Need reinforcement for your project? Get a fast, competitive quote or call 0207 856 0141 — we deliver rebar and mesh across London and the South East.
