Impact Driver Bit Sizes Chart
A bit one size too small is the most common reason a screw strips. These charts match the bit to the screw so the first drive is the only drive.
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All common impact driver bit sizes share a 1/4-inch hex shank; what changes is the tip. For most wood screws, #6 to #9 take a PH2 bit, SQ1 or SQ2 square, and T15 to T25 Torx; #10 to #14 take PH2 or PH3, SQ2 or SQ3, and T25 to T30. Most deck screws use T20 or T25. Structural screws often need T30 or T40. Always check the screw box, which usually names the exact bit.
On this page
How impact driver bit sizes work
Every impact driver bit has two size labels. The shank is the part that goes into the tool, and for impact drivers it is always a 1/4-inch hex. The tip is the part that goes into the screw, and that is where the size codes come in: PH2, SQ2, T25 and so on. The letters name the drive type and the number names the size within it.
Tip sizes are not measured in inches. A T25 is not 25 of anything you can measure with a tape. It is a standard code, so you match the code on the bit to the code on the screw box. If you are unsure which drive type to use at all, read Phillips vs square vs Torx first.
Bit size chart by screw gauge
Screw gauge is the number on the box, such as #8 or #10. Bigger number, thicker screw. This chart shows the bit most screws of each gauge take. Screw makers vary, especially on Torx, so treat it as a starting point and confirm with the box.
| Screw gauge | Phillips | Square | Torx / star | Typical use |
|---|---|---|---|---|
| #4 | PH1 | SQ0 or SQ1 | T10 | Small hinges, cover plates |
| #6 | PH2 | SQ1 | T15 | Drywall, light trim, hardware |
| #7 | PH2 | SQ1 | T15 or T20 | Trim, pocket screws |
| #8 | PH2 | SQ2 | T20 or T25 | General wood screws, cabinets |
| #9 | PH2 | SQ2 | T25 | Deck screws |
| #10 | PH2 or PH3 | SQ2 | T25 | Deck, fence and framing screws |
| #12 | PH3 | SQ3 | T27 or T30 | Heavy construction screws |
| #14 | PH3 | SQ3 | T30 | Heavy timber and ledger screws |
The overlap is real. A #10 screw from one brand may take a PH2 and another a PH3. If the bit rocks in the recess or sits shallow, it is the wrong size. Moving up one size usually fixes it.
Hex key and nut driver sizes
Two other families use real measurements. Hex key (Allen) bits are sized in inches or millimeters across the flats, such as 5/32 inch or 4 mm, and show up on furniture bolts and machinery. Inch and metric hex bits are close but not interchangeable, so a near-fit will round the socket head. Nut drivers are sized by the hex head of the screw: 1/4 inch for small self-drillers, 5/16 inch for the most common #10 and #12 self-drillers, and 3/8 inch for larger ones. Use impact-rated nut drivers with a magnet so the screw stays on.
PH2 bit: the size you will use most
If you own one bit, it is a PH2. It fits nearly every drywall screw, most general-purpose wood screws, many hinges and much of the hardware around a house. It is also the bit that wears out fastest, because Phillips recesses push the bit out under torque. Buy PH2 power bits in multi-packs and replace them as soon as the tip edges look rounded.
Two common mix-ups:
- PH1 in a PH2 screw. It turns, then strips the screw. Small screws in cover plates are PH1; most others are PH2.
- Phillips in Pozidriv. European and flat-pack screws often use PZ2, marked by small lines between the cross arms. Use a PZ2 bit.
Deck and structural screw sizes
| Screw type | Common bit | Notes |
|---|---|---|
| Wood deck screws (#8 to #10) | T20 or T25 | Many boxes include the bit |
| Composite deck screws | T15 or T20 | Small heads; check the brand's spec |
| Combo-drive construction screws | SQ2 or PH2 | Square grips better |
| Structural wood screws | T30, T40 or larger | Use the bit the maker specifies |
| Hex-head self-drillers | 1/4 or 5/16 in nut driver | 5/16 in most common |
| Lag screws | 3/8 to 3/4 in socket | Depends on lag diameter |
Structural screws are engineered fasteners with published installation instructions. Use the exact bit the manufacturer lists; a near-fit bit can strip a recess on a screw that is expensive to remove. The deck method is in how to drive deck screws. For hex-head lags, you need a socket and an impact driver socket adapter or a hex-shank socket.
T25 bit size and the Torx range
T25 is the workhorse of the star-drive world. It fits the majority of #9 and #10 exterior wood screws, many deck screws and plenty of cabinet and construction screws. T20 is next most common, used on #8 screws and some deck screws. T15 shows up on trim and composite deck screws.
Torx sizes sit close together, so a T20 slides into a T25 recess and feels almost right. It is not. The loose bit spins and rounds the lobes. Push the bit in by hand before driving: the right size seats fully with almost no rotational play.
Shank and bit length: insert bits vs power bits
| Bit format | Length | Locks in collet? | When to use |
|---|---|---|---|
| Insert bit | about 1 in | No; needs a holder | Large mixed sets, odd sizes |
| Power bit | 2 in | Yes | Everyday driving; most stable |
| Power bit | 3 to 3-1/2 in | Yes | Recessed screws, extra reach |
| Power bit | 6 in | Yes | Deep reach, between framing; more wobble |
Insert bits are short and have no groove for the collet to lock onto, so they need a magnetic bit holder. Power bits have a groove near the shank end that the collet locks into. Longer bits reach further but wobble more. How the collet grips each format is explained in impact driver chucks and hex collets.
Getting the size right every time
- Read the box. Most screw boxes print the bit size, and many include a bit.
- Test-fit by hand. The bit should seat to full depth with almost no wobble.
- Size up if it rocks. A loose fit means too small, never just right.
- Replace worn tips. A worn bit acts like a smaller size.
- Push hard and start slow. More on technique in how to stop stripping screws.
The full kit, from starter bits to holders, is in the impact driver bits guide, and set picks are in the best impact driver bit sets.
Mixed impact bit sets on Amazon T25 power bits
Frequently asked questions
What size is a standard Phillips bit?
PH2. It fits most screws from about #6 to #10, including drywall screws and most general wood screws. PH1 is for small screws like those in electrical cover plates and small hinges, and PH3 for larger screws, usually #12 and up.
Is T25 the same as 1/4 inch?
No. Torx sizes are not inch or millimeter measurements. T25 is a size code for the star tip, measuring a little under 4.5 mm point to point. Do not confuse it with the 1/4-inch hex shank that all impact bits share.
What bit size for a 5/16 hex head screw?
A 5/16-inch nut driver. Most hex-head self-drilling metal screws in #10 and #12 sizes use a 5/16-inch head, while larger ones and some lags use 3/8 inch. Magnetic impact nut drivers hold the screw on the tip.
Why does my T25 bit feel loose in the screw?
The recess may actually be T27 or T30, the bit may be worn, or the screw may be an off-brand star drive with loose tolerances. Try the next size up, and use the bit that comes in the screw box when there is one.
Are 1/4-inch hex shanks universal across brands?
Yes. Any 1/4-inch hex-shank bit fits any brand's impact driver collet. Power bits need the locking groove near the shank end to lock in place, while short insert bits need a bit holder.