UK Rebar Sizes: Stop Googling the Same Chart Every Time

Look, we get it. You’re in the middle of estimating an order or checking a bar bending schedule and you need the weight of an H16 bar right now. Not after wading through a 200-page BS standard or firing up a creaking spreadsheet.

Here’s every standard UK rebar size, weight, and cross-sectional area in one place — plus the formula so you can work it out yourself for any diameter that isn’t on the list.

Standard UK Rebar Sizes (BS4449)

In the UK, reinforcing bar is manufactured to BS4449 and specified in bending schedules to BS8666. The bars come in standard stock lengths of 6 m or 12 m and are designated by their nominal diameter in millimetres, prefixed with H (high-yield deformed bar, Grade B500B).

The sizes you’ll encounter on UK construction and fabrication projects:

Quick sanity check: H16 is one of the most commonly misquoted. It’s 1.579 kg/m — not 1.5, not 1.6. Use the formula below to verify any size.

How to Calculate Rebar Weight Per Metre

You don’t need a fancy calculator for this. The formula for the weight per metre of any circular steel bar is derived from its volume and the density of steel (7,850 kg/m³):

Weight (kg/m) = (π ÷ 4) × d² × 7,850 ÷ 1,000,000

For day-to-day use, this simplifies to the standard industry approximation:

Weight (kg/m) = d² ÷ 162

Where d is the nominal diameter in millimetres. That’s it. Square the diameter, divide by 162.

Worked Examples

H10 bar: 10² ÷ 162 = 100 ÷ 162 ≈ 0.617 kg/m (table shows 0.616 kg/m — see note below)

H20 bar: 20² ÷ 162 = 400 ÷ 162 ≈ 2.469 kg/m (table shows 2.466 kg/m — see note below)

H32 bar: 32² ÷ 162 = 1,024 ÷ 162 ≈ 6.321 kg/m (table shows 6.313 kg/m — see note below)

A note on rounding: The d²/162 shortcut is an approximation. The BS standard table values are derived from the exact formula using π/4 × d² × 7,850 ÷ 1,000,000 (equivalent to d² × 0.006165). The discrepancy is less than 0.1% — irrelevant for estimating, but if you need certified accuracy for mill certificate reconciliation or payment purposes, use the Metal Weight Calculator or the exact formula.

Cross-Sectional Area of Reinforcing Bars

The cross-sectional area of a rebar determines how much load it can carry — this is what your structural engineer cares about. It’s also what you need when spacing bars to achieve a target reinforcement area per metre width of slab or wall.

Area (mm²) = (π ÷ 4) × d²
           = 0.7854 × d²

Common area calculations by size

Total Weight for a Bundle or Order

Once you have the weight per metre, total weight is simple:

Total Weight (kg) = Weight per Metre (kg/m) × Total Length (m)

If you’ve got 250 bars of H16, each 6 m long:

Total length = 250 × 6 = 1,500 m
Total weight = 1,500 × 1.579 = 2,368.5 kg ≈ 2.37 tonnes

For anything more complex — mixed sizes, multiple cut lengths, or a full bar bending schedule — the Metal Weight Calculator will handle it faster and without the risk of a fat-finger mistake.

Rebar Area Per Metre Width (for Structural Calculations)

When detailing slabs, walls, or beams, you often need to achieve a target area of steel per metre width. To find the area provided by bars at a given spacing:

Area per metre width (mm²/m) = (Bar Area × 1,000) ÷ Spacing (mm)

Common spacing and area table (mm²/m)

Standard Stock Lengths and Cutting Considerations

UK rebar comes in two standard stock lengths:

• 6 metre lengths — standard for most fabricators and smaller cut lengths
• 12 metre lengths — common for longer members, lower cost per tonne, less waste on longer cuts

Kerf and saw losses

Every cut loses material. A typical bandsaw or abrasive disc removes 3–8 mm per cut. On an H32 bar cut 50 times, that’s up to 400 mm of material you’re not getting paid for. Factor it in.

Minimising cutting waste

The biggest lever you have on scrap rates is cutting pattern optimisation. Randomly assigning lengths to bars typically yields 5–8% scrap. Planned nesting of cut lists typically brings this below 2.5% — that’s the difference between writing off a significant chunk of your steel budget or not.

The GoSmarter Scrap Rate Calculator gives you an instant read on waste for a given bar length and cut length. For full cutting optimisation across an entire order set, the Rebar Optimiser applies genetic algorithms to find the most material-efficient cutting patterns.

BS4449: The Standard Behind the Sizes

BS4449 is the British Standard for steel for the reinforcement of concrete. The current version (BS4449:2005+A3:2016) specifies three strength grades:

• Grade B500A — weldable reinforcing steel, characteristic yield strength 500 MPa, ductility class A
• Grade B500B — the most common grade in UK construction, class B ductility
• Grade B500C — high ductility, used where earthquake or impact loading applies

The H prefix (High yield) is the UK trade designation for deformed bars manufactured to this standard. You’ll also see T used interchangeably on older drawings.

For bending schedules and shape codes, BS8666:2020 is the relevant standard — it defines the shape codes (00–99) and the bending dimension tolerances used in bar bending schedules.

Why Your Spreadsheet Gets This Wrong

Here’s the thing most people miss: the d²/162 formula gives you the nominal weight per metre. It doesn’t account for:

• Rib geometry — deformed bars are heavier than smooth bars of the same nominal diameter (typically +2–5%)
• Tolerances — BS4449 allows a ±4.5% tolerance on weight per metre for individual bars
• Mill certification — actual weight may differ from nominal; for payment and compliance purposes, certified weights from the mill certificate take precedence

If you’re doing material accounting, cost tracking, or sustainability reporting, using nominal weights introduces systematic error. Our Steel Emissions Calculator and Scrap Rate Calculator use certified weights where available, so your numbers actually stack up.

Quick Reference: Most Common Sizes in UK Rebar Fabrication

If you’re specifying or ordering and need the numbers in your head:

• H10 — 0.616 kg/m, widely used for lightly loaded slabs and links
• H12 — 0.888 kg/m, roughly 1 kg/m (easy to estimate)
• H16 — 1.579 kg/m, probably the single most common UK specification bar
• H20 — 2.466 kg/m, standard for beams and columns
• H25 — 3.854 kg/m, heavy structural elements
• H32 — 6.313 kg/m, large columns, transfer slabs, heavy civil

Other Free Tools for MEtals Calculations

Doing the maths manually gets old fast. These tools do it for you — no login, no subscription, no corporate faff:

• Metal Weight Calculator — calculate the weight of any rebar size and length combination instantly
• Scrap Rate Calculator — find out how much many your losing and could save by reducing your scrap rates
• Material Yield Planner — calculate yield from 2D cuts from and see exactly how much usable material you get

For production-scale optimisation — where you’ve got hundreds of cut lengths across thousands of bars — the Rebar Optimiser is where the real savings are.

Related Reading

• Smart Cuts, Less Scrap: A 1D Cutting Stock Problem — how mathematical optimisation reduces rebar waste
• GoSmarter Launches Rebar Optimiser — cutting scrap with genetic algorithms