The correct screwdriver bit is determined in two stages: identify the drive profile, then choose the size within that profile. A Phillips bit uses PH numbers, Pozidriv uses PZ, Torx uses T numbers, and hex bits use a metric or imperial measurement. The common 1/4-inch hex shank describes how a bit fits a holder; it does not describe the screw recess.
Before applying torque, insert the bit into the screw. It should sit fully down, feel stable and show almost no side-to-side movement. If it rattles, rocks or sits shallow, stop and try another profile or size. A bit that is too small concentrates force at the bottom of the recess, while an oversized bit cannot seat correctly and may cam out.
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How screwdriver bit sizing works
Bit markings are not one universal scale. Each drive family has its own naming system, and the number only has meaning within that family. PH2, PZ2 and T20 are not three versions of the same size: they describe different recess geometries and sizing systems.
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| Drive profile | Marking system | Where it is commonly encountered | Typical examples |
|---|---|---|---|
| Phillips | PH followed by a number | General machine, appliance and hardware screws | PH1, PH2, PH3 |
| Pozidriv | PZ followed by a number | Furniture, construction and general hardware | PZ1, PZ2 |
| Torx | T followed by a number | Devices, appliances, machinery and general fasteners | T10, T15, T20, T25 |
| Hex | Metric or imperial dimension | Machines, furniture, bicycles and other assemblies | 2, 3, 4, 5 or 6 mm |
| Specialty drives | Profile-specific name or size | Security, tamper-resistant or unusual fasteners | Security Torx, tri-wing, spanner and JIS |
The shank specification is separate from the tip size. Interchangeable driver systems commonly use a 1/4-inch hex shank, so a PH2 and a T20 may both fit the same bit holder even though they fit completely different screws.
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Use the recess, not a guess, to choose the bit
Step 1: Identify the profile
Look directly at the screw head under good light. A four-arm cross suggests Phillips or Pozidriv, but those profiles are not interchangeable. A six-lobed star is Torx. A six-sided socket is hex. A center pin, three-wing shape, two-hole spanner pattern or another unusual outline indicates a specialty drive.
Step 2: Start with the likely size
Use the common pairings as a starting point, not as a guarantee. PH2 and PZ2 are frequent general-purpose choices, while T20 and T25 appear often on larger general fasteners. Small electronics may use PH0, PH00 or smaller sizes, and precision kits extend to much smaller markings.
Step 3: Test the fit before turning
- Push the bit straight into the recess without rotating it.
- Check that the tip reaches the bottom and that its working faces contact the recess walls.
- Try a gentle side-to-side movement. Minimal play is correct; visible rocking or rattling is not.
- Apply hand pressure along the screw axis and turn slowly. If the bit climbs out, stop immediately and reassess the profile and size.
This test is more reliable than choosing by the screw’s outside diameter or by a familiar-looking label. Screw-head style and manufacturer tolerances can change the required size.
Phillips sizes: PH1, PH2 and PH3
Phillips bits are marked PH followed by a number. Within this family, a larger number generally indicates a larger recess. PH1 is common on smaller machine and electronics screws. PH2 is the broad general-purpose size found on many everyday fasteners. PH3 suits larger screw heads and heavier hardware.
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| Phillips marking | Typical use described in fitment guides | Selection note |
|---|---|---|
| PH1 | Smaller machine and electronics screws | Confirm that the cross recess is deep enough for PH1; do not force it into a smaller precision screw. |
| PH2 | General-purpose fasteners | A common first trial for a conventional Phillips screw, but it must still seat fully. |
| PH3 | Larger fasteners | Use only when the larger cross recess accepts it without rocking. |
A Phillips bit that feels loose is not made correct by pressing harder. Pressure can hide poor engagement for a moment, then cause the bit to slip and damage the recess when torque rises.
Pozidriv sizes and the PH2-versus-PZ2 mistake
Pozidriv also appears as a cross, but its geometry includes additional engagement features that distinguish it from Phillips. PZ1 and PZ2 are common markings, with PZ2 widely used in furniture and construction hardware.
PH2 and PZ2 are not interchangeable. A PH bit in a Pozidriv screw, or a PZ bit in a Phillips screw, makes contact in the wrong places. The reduced contact increases cam-out—the bit riding out of the recess—and can round or scar the screw head. If a cross-shaped recess has small secondary marks between the main arms, test a PZ bit rather than assuming it is Phillips.
How to verify PH or PZ in practice
- Try the profile that matches the fastener marking or packaging, if available.
- Seat the bit fully and compare stability with the other profile.
- Choose the one with broad, even contact and the least movement, then use moderate axial pressure while turning.
When the screw head is worn, the original profile may be difficult to identify. A fresh bit of the correct profile and size can reveal the geometry more clearly than visual inspection alone; do not use a loose fit as evidence that the two systems are equivalent.
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Torx sizes: T10 through T25 and beyond
Torx uses a six-lobed star and a T-number scale. T10 through T15 cover many small devices and appliances. T20 and T25 are frequent general-purpose sizes. The number belongs to the Torx family only, so a T20 cannot substitute for a Phillips or hex bit even if its tip appears similar in overall width.
| Torx range | Typical applications in the supplied fitment guidance | What to check |
|---|---|---|
| T10–T15 | Many small devices and appliances | Make sure every lobe reaches the bottom of the star recess. |
| T20–T25 | Frequent general-purpose sizes | Test adjacent sizes if the bit has noticeable play or refuses to seat. |
| Higher or lower numbers | Application-dependent | Use the actual recess; no universal screw-head rule guarantees the size. |
Security Torx, sometimes called tamper-resistant Torx, adds a center pin in the recess. A standard Torx bit with a solid center will not fit over that pin. Use a security Torx bit with the matching hollow center and the correct T-number.
Hex bit sizes: metric and imperial
Hex screws have a six-sided internal recess. Their size is a measurement rather than a PH, PZ or T number. Metric sizes such as 2, 3, 4, 5 and 6 mm cover many machine, furniture and bicycle applications. Imperial fasteners use inch-based dimensions, so a metric bit that looks close may still be loose enough to damage the recess.
Metric-versus-imperial check
- Read the fastener specification or assembly instructions when they are available.
- Test the exact metric and imperial candidates without forcing either one.
- Reject any bit that rocks, enters only partway or requires sideways pressure to stay engaged.
Because hex recesses are measured directly, a caliper, a known-good bit or a careful test fit is more useful than estimating from the screw’s external diameter. The recess must be matched, not merely approximated.
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Precision screws, JIS and other special cases
Precision electronics
Precision kits commonly include PH0000 through PH2, PZ000 through PZ1, very small metric hex sizes and Torx T2 through T6. These tiny screws have little tolerance for a mismatched tip. Use the smallest bit that fills the recess cleanly, keep the driver aligned with the screw, and avoid using a larger household bit just because it appears to touch the sides.
JIS screws
Japanese Industrial Standard (JIS) screws can resemble Phillips. iFixit describes JIS screws as having no cam-out option and 90-degree corners. A Phillips bit may appear to fit but engage differently, so a dedicated JIS bit is the appropriate choice when the fastener or equipment identifies that standard.
Tri-wing, spanner and other specialty drives
Tri-wing screws use a three-wing recess; spanner screws use two holes; other tamper-resistant designs may add pins or unusual lobes. A conventional Phillips, Pozidriv, Torx or hex bit is not a safe substitute. Identify the recess first and obtain the matching specialty bit, including any required center opening.
Choosing a practical bit set
A mixed 1/4-inch set is a sensible starter or replacement kit when you work on varied hardware. Look for coverage of Phillips, Pozidriv, Torx, hex and slotted profiles, with enough sizes to span both general-purpose and precision work. Individual size entries in mixed-bit catalogs show why a set should contain more than one profile and more than one number.
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| Set feature | Why it matters |
|---|---|
| Multiple profiles | Prevents treating PH, PZ, Torx and hex as interchangeable. |
| Adjacent sizes | Lets you test fit instead of forcing a nearly correct bit. |
| Precision range | Covers small PH, PZ, hex and T2–T6 fasteners in electronics and appliances. |
| 1/4-inch hex shank compatibility | Allows interchangeable bits to share a common holder, regardless of tip profile. |
| Specialty options when needed | Security Torx, tri-wing, spanner or JIS work requires dedicated geometry. |
Premium bit families are available in Phillips, Pozidriv and Torx formats, but brand level does not replace fit testing. A correctly marked bit that seats fully is more useful than an expensive bit in the wrong profile.
Diagnosing slipping, wobbling and damaged recesses
| Symptom | Likely cause | Action |
|---|---|---|
| Bit rattles or rocks before torque | Wrong size or profile | Try the adjacent size within the same profile, then verify whether the recess is actually PH, PZ, Torx or hex. |
| Bit sits shallow | Tip is too large, the recess is obstructed or the profile is wrong | Remove debris, inspect the recess and test a correctly shaped smaller bit. |
| Bit spins out under light pressure | Insufficient contact, often from PH/PZ confusion or an undersized bit | Stop turning, change profile or size and keep the driver aligned. |
| Recess edges are becoming rounded | Continued use of a loose or oversized/undersized bit | Stop before further damage; use a fresh, correctly fitting bit and steady axial pressure. |
| Standard Torx will not enter | Center pin indicates security Torx | Use the matching hollow-center security bit. |
Do not compensate for a poor fit by increasing force. The practical fit test is intended to prevent damage before torque is applied; once the recess is rounded, even the correct bit may no longer hold securely.
A quick decision checklist
- Read the shape: cross, star, six-sided socket or specialty pattern.
- Name the profile: PH, PZ, Torx, hex, JIS or another specialty drive.
- Select a starting size: PH1/PH2/PH3, PZ1/PZ2, T10–T25, or the measured hex dimension.
- Seat the bit fully: the tip should reach the bottom with minimal play.
- Compare adjacent sizes: choose the one with the most complete, stable engagement.
- Apply torque carefully: hold the driver in line with the screw and stop immediately if it climbs out.
The central rule is simple: profile comes before number. PH2 is not PZ2, common sizes are only starting points, and the correct bit is the one that matches the actual recess and remains stable before you turn.
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