Screwdriver bits explained: Phillips, Torx, square and hex

Kelvin 07 spinner with its bits
Kelvin Tools

Most stripped screws are not the screw's fault. They are the result of the wrong bit, or the right shape in the wrong size. Four families cover the overwhelming majority of what you meet indoors.

Phillips (PH)

The cross. Designed to cam out, meaning the bit climbs out of the head under high torque, which protected early assembly lines from over-driving. Sizes run PH0 (electronics) through PH1, PH2 (the most common house screw) and PH3. If the bit wobbles in the head, go up a size.

Flathead (slotted)

The single slot. Still common on electrical cover plates, older furniture and anything decorative. The bit has to fill the slot in both width and thickness, or it will slip and chew the head.

Torx (T)

The six-point star. It resists cam-out, carries more torque, and has taken over in appliances, electronics, bicycles and cars. T10 to T25 are the household sizes; T15 and T20 show up constantly on bikes and furniture hardware. The Kelvin PakRatchet ships with T15 and T20 for exactly that reason.

Hex / Allen

The six-sided socket. The flat-pack standard, and the fastener on most bike components. Metric sizes (2 to 6 mm) dominate furniture; bikes mix metric sizes too. The stamped L-key in the furniture bag is a hex bit without a handle, which is why it hurts.

Square (Robertson)

A Canadian invention and the default screw in Canadian construction and decking. Square #2 (the red one) is the common size. If you are in Canada, a square bit gets used far more than you expect.

What the Kelvins carry

  • Kelvin 17: thirteen bits across Phillips, Torx, flathead and Allen.
  • Kelvin 23: sixteen bits.
  • Kelvin 36 and 36 SL: twenty-six double-sided CR-V bits, including Torx and a knife or box-cutter bit.
  • PakRatchet: four 25 mm bits (Square #2, Phillips #2, T15, T20) plus seven sockets.
  • Kelvin 07 spinner: two flat, two Phillips, two Torx on magnetic heads.

Three habits that save screw heads

  1. Push harder than you turn. Most cam-out is from too little downward pressure.
  2. Match the size, not just the shape. A PH1 in a PH2 head is the classic strip.
  3. Start by hand. Set the screw a turn or two before you lean on it, especially in particleboard.

Compare the Kelvin range

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