What size Torx or hex bit do I need?

Unlike Pozidriv, where three sizes cover nearly everything, Torx and hex run across a long series and the fastener never tells you which. There is no measurement you can take with a household ruler that gives the answer directly, so the practical method is systematic trial with the right acceptance test.

Step by step

1. Understand what a Torx number means.T followed by a number, from very small electronics sizes upwards. The number is a size designation for the six-lobed star, not a dimension in millimetres, so you cannot convert it — you match it.
2. Understand what a hex size means.Hexagon socket fasteners are described by the across-flats measurement of the socket, in millimetres for metric and fractions of an inch for imperial. That one you can measure, with a small rule or a vernier.
3. Try bits in ascending order.Start smaller than you think and work up. The correct bit slides in fully and will not rock at all. The moment one goes in with any perceptible wobble, the next size up is the one you want.
4. Reject any bit that wobbles.A Torx bit one size down engages on the tips of the lobes rather than the flanks. It will turn a loose fastener once and round out a tight one immediately. Wobble is the test that matters.
5. Watch for metric and imperial hex on the same object.Bicycles, motorcycles, machinery and imported flat-pack mix them. A 5mm key in a 3/16 inch socket feels almost right and will round it. If a key nearly fits, try the other system before forcing it.
6. Recognise the security variants.A Torx with a pin in the centre of the recess needs a tamper-resistant bit with a hole down the middle. Appliances, white goods and public fittings use them, and a set is inexpensive.
7. Cover the household range with one set.A driver bit set spanning the common Torx sizes and a hex key set in both metric and imperial covers flat-pack, appliances, bikes, garden machinery and decking screws. Buy the set rather than individual sizes as you meet them.
8. Look for the size on the packaging first.Decking screws, structural screws and flat-pack fittings almost always state the driver size on the box, and many include the bit. Reading it takes seconds and beats trial and error.

Tips

• Torx is often called star drive, and TX in some manufacturers' catalogues. They are the same standard.
• Torx Plus is a related but different geometry with a slightly squarer lobe. A standard Torx bit fits loosely and will damage it under load.
• If the head is already slightly rounded, a Torx bit one size up will not fit — the answer is a screw extractor, not a bigger bit.

Common mistakes

• Forcing a bit that nearly fits — Torx works by bearing on the flanks of the lobes. A loose bit bears on the tips, which are thin, and it rounds the recess in a single turn.
• Assuming all hex is metric — Bicycles and anything American frequently is not, and the two series are close enough to feel right and round the socket.

Questions people ask