Hinge Sizes in Inches and mm: What 4x3x3.0 Really Measures

Updated 2026-11-08 · RAYEN 雷茵

How is a hinge code built?

Take 4x3x3.0-4BB, written 4x3x3.0-4BB.

• 4 is the leaf height in inches.
• 3 is the width across both leaves laid open, in inches.
• 3.0 is the leaf thickness, and it is always in mm.
• 4BB is the bearing count: four ball bearings. DD means copper gaskets.
• A word after the code names the type: US standard, rounded, offset pivot, centre pivot, or single-pin or twin-pin security. On Chinese paperwork these words follow the code in Chinese.

So the code mixes two units. The first two figures are inches and the third is mm. A buyer who reads 4x3x3.0 as a 3-inch-thick leaf will never find it, and one who reads it as 4mm by 3mm will be confused. Write the millimetre size on your order as well.

Which inch sizes appear, and what are they in mm?

These are the figures written on our drawings. Use them as they are, and do not recalculate them.

• 2.5in = 63.5mm, the width of 3x2.5x2.5-2BB.
• 3in = 76.2mm.
• 3.5in = 89mm.
• 4in = 101.6mm.
• 4.5in = 114.3mm, found only on the US standard hinges such as 4.5x4.5x3.5-2BB US.
• 5in = 127mm.
• 6in = 152.4mm.
• 8in = 203.2mm, on 8x4x3.5-6BB and 8x4x3.5-4BB Single Security Stud.

Which codes are really metric?

This is where orders go wrong. Several hinges carry an inch-style code but a round metric leaf.

• Brass 4x3x3.0-4BB is 100 × 75mm, against 101.6 × 76.2mm in stainless.
• Brass 5x3x3.0-4BB is 125 × 75mm, against 127 × 76.2mm in stainless.
• 4x3x3.0-0BB is 100 × 70mm, and 4x3x3.3-2BB is 100 × 79mm.
• The pivot hinges are metric too. 4x3x2.5-2BB Centre Pivot is 100 × 75mm, 4x3x2.5-2BB Offset Pivot is 100 × 73mm, 5x3x2.5-2BB Offset Pivot is 125 × 78mm and 5x3.5x3.0-2BB Offset Pivot is 125 × 82mm.

Some codes are written in mm from the start: 100x75x3.0-0BB Pull, 100x89x3.0-1BB, 110x55x2.5-1BB and 140x55x2.5-1BB. There the figures are the leaf in mm.

What does the code not tell you?

The knuckle diameter. It runs from Ø10mm to Ø16.2mm across the range, and it changes with material. Brass 4x3x3.0-4BB Rounded keeps the 101.6 × 76.2mm leaf but has a Ø12.7mm knuckle, against Ø14.2mm in stainless. If you are replacing pins, or matching a hinge already on site, measure the knuckle too.

Every size, in mm

The chart sorts on height. Where a model comes in two materials, the cell gives both sizes. 5x3.5x3.5-1BB has no leaf size listed yet, so ask us for it.

How do you match an old hinge?

Measure the leaf with a caliper, closed and flat, then read the result against the chart.

• 127 × 76.2mm is an inch hinge: 5x3x3.0-4BB in stainless, or 5x3x3.0-4BB Rounded if the corners are round.
• 125 × 75mm is the brass 5x3x3.0-4BB.
• 125 × 78mm with a cut-out leaf is 5x3x2.5-2BB Offset Pivot.

Three hinges, one nominal size, and three different recesses. That is why a photo with a ruler beside the hinge saves a container.

Does thickness follow the inch size?

No. Thickness is chosen on its own: 2.5mm, 3mm, 3.3mm, 3.5mm or 4mm across the range. A 4 × 3in hinge comes at 2.5mm as 4x3x2.5-4BB and at 3mm as 4x3x3.0-4BB. A 5 × 3in hinge comes at 2.5mm as the offset pivot 5x3x2.5-2BB Offset Pivot, at 3mm, and at 4mm in brass as 5x3x4.0-4BB. A thicker leaf needs a deeper recess, so the third figure matters as much as the first two.

What should you write on the order?

• The full code, with its type word.
• The leaf size in mm, copied from the chart.
• Thickness and knuckle diameter.
• Material, with grade for stainless: 304 or 201.
• Finish and quantity.

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