Screwdriver Size Chart: Pick the Right Screwdriver

What does a 1/4-inch screwdriver at 6.35 mm mean?

On the default — a 1/4-inch screwdriver — the blade width is 6.35 mm and the class is standard. The width is the width of the flat blade that sits in the screw slot, and 1/4 in is the standard screwdriver for a household — the general-purpose size that fits most screws, strong enough to turn a stuck screw without twisting the way a 1/8-in precision driver does. The millimetres are the inches times 25.4: 0.25 × 25.4 = 6.35 mm. The classes are CLOSED at both ends: precision 1/8–3/16 in, standard 1/4–3/8 in, heavy 7/16–5/8 in (screwdriver BLADE WIDTH). The 7/32-in width between precision and standard matches NO standard class — the classes have a gap, they are not continuous — and widths of 5/8 in and above, and below 1/8 in, match NO standard class either.

Three things about the model are worth stating plainly.

First, the conversion is 0.25 × 25.4 = 6.35 mm. Second, the meaning is a 1/4-inch screwdriver for most household screws — the general-purpose size that fits most screws, strong enough to turn a stuck screw without twisting the way a 1/8-in precision driver does. And third, the millimetre figure is displayed to two decimals, half-up, and a figure that displays as 6.35 mm can sit a hair below the line — 1/4 in gives 6.3500 mm, which displays as 6.35 mm — and one that displays as 9.53 mm can sit a hair below it — 3/8 in gives 9.5250 mm, which displays as 9.53 mm. The page prints the unrounded value so the rounding does not mislead you.

The worked example, line by line

Setup: screwdriver 1/4 inch wide.

Line 1 — the millimetres. 0.25 × 25.4 = 6.35 mm (unrounded 6.3500 mm).

Line 2 — the class. 1/4 inch sits in the middle of the 1/4–3/8 in standard class, so the class is standard.

Line 3 — the check. At the 1/4-in row the width is 6.35 mm, above 6.00 mm; and at the 7/32-in row it is 5.56 mm — below 6.00 mm.

Every screwdriver width in the table, checked

The table runs from 1/8 to 5/8 inches. Every width in the table sits inside one of the three classes except the 7/32-in width, which sits in the gap between precision and standard and matches no standard class: 1/8–3/16 in are precision screwdrivers, 1/4–3/8 in are standard screwdrivers and 7/16–5/8 in are heavy screwdrivers. The classes are closed at both ends — 3/16 in is a precision screwdriver, 3/8 in is a standard screwdriver and 5/8 in is a heavy screwdriver. Widths between the classes (the 7/32-in width between precision and standard) match NO standard class, and widths of 5/8 in and above, and below 1/8 in, match NO standard class either. The table is a synthesis of published slotted-driver sizing guidance for household use — a 1/4-in blade for general screws is the published standard, and the width roughly doubles as the screw grows. Sizing varies by screw head, material and torque — size up one row for rusted or painted screws, and always check the fastener maker's own table for the exact product.

1/8 in: 1/8 in = 3.18 mm; precision — 1/8 inch is the bottom of the table, a precision screwdriver.

5/32 in: 5/32 in = 3.97 mm; precision — 5/32 inch is a precision screwdriver, one row above the bottom of the table.

3/16 in: 3/16 in = 4.76 mm; precision — 3/16 inch is a precision screwdriver, the top of the precision class.

1/4 in: 1/4 in = 6.35 mm; standard — 1/4 inch is a standard screwdriver, the bottom of the standard class.

9/32 in: 9/32 in = 7.14 mm; standard — 9/32 inch is a standard screwdriver, one row above the bottom of the standard class.

5/16 in: 5/16 in = 7.94 mm; standard — 5/16 inch is a standard screwdriver, the middle of the standard class.

Screwdriver widths — 1/8 in, 1/4 in, 5/8 in: 1/8 in: 3.18 mm — precision; 1/4 in: 6.35 mm — standard; 5/8 in: 15.88 mm — heavy — one width from each class: the bottom of the table, the household standard and the top of the table.

The reverses, worked backwards

Smallest table row whose width reaches 6.00 mm: At the displayed 1/4-in row the figures are 1/4 in = 6.35 mm (unrounded 6.3500 mm); standard. At the 7/32-in row the width is 5.56 mm (unrounded 5.5563 mm), below 6.00 mm — so the 1/4-in row is the smallest table row whose width reaches 6.00 mm.

Largest table row whose width STAYS at or below 6.00 mm: At the displayed 7/32-in row the figures are 7/32 in = 5.56 mm (unrounded 5.5563 mm); matches no standard class. At the 1/4-in row the width is 6.35 mm (unrounded 6.3500 mm), above 6.00 mm — so the 7/32-in row is the largest table row whose width stays at or below 6.00 mm.

Smallest table row that needs a standard screwdriver: At the displayed 1/4-in row the figures are 1/4 in = 6.35 mm (unrounded 6.3500 mm); standard. At the 7/32-in row the width is 5.56 mm, no class — so the 1/4-in row is the smallest table row that needs a standard screwdriver.

Smallest table row whose width reaches 10.00 mm: At the displayed 7/16-in row the figures are 7/16 in = 11.11 mm (unrounded 11.1125 mm); heavy. At the 3/8-in row the width is 9.53 mm (unrounded 9.5250 mm), below 10.00 mm — so the 7/16-in row is the smallest table row whose width reaches 10.00 mm.

Largest table row whose width STAYS at or below 10.00 mm: At the displayed 3/8-in row the figures are 3/8 in = 9.53 mm (unrounded 9.5250 mm); standard. At the 7/16-in row the width is 11.11 mm (unrounded 11.1125 mm), above 10.00 mm — so the 3/8-in row is the largest table row whose width stays at or below 10.00 mm.

Smallest table row that needs a heavy screwdriver: At the displayed 7/16-in row the figures are 7/16 in = 11.11 mm (unrounded 11.1125 mm); heavy. At the 3/8-in row the width is 9.53 mm (unrounded 9.5250 mm), a standard screwdriver — so the 7/16-in row is the smallest table row that needs a heavy screwdriver.

A warning about the neighbours: every neighbour above sits one table row from the star, and every neighbour millimetre figure is a hair below or above the target even though it displays as the target — a one-row move changes the millimetres by more than a millimetre, but the unrounded figure still sits a hair from the round displayed figure, so the displayed two-decimal figure cannot tell the star from its neighbour. The page prints the unrounded four-decimal value in full so the rounding does not mislead you: 6.3500, 5.5563, 7.1438, 4.7625, 11.1125, 9.5250, 12.7000, 7.9375, 15.8750, 14.2875, 3.9688, 3.1750.

What the defaults assume

The defaults are a screwdriver 1/4 inch wide — 1/4 in = 6.35 mm; standard. With 1/8 in, 1/8 in = 3.18 mm; precision. With 5/32 in, 5/32 in = 3.97 mm; precision. With 3/16 in, 3/16 in = 4.76 mm; precision. With 1/4 in, 1/4 in = 6.35 mm; standard. With 9/32 in, 9/32 in = 7.14 mm; standard. With 5/16 in, 5/16 in = 7.94 mm; standard. With Screwdriver widths — 1/8 in, 1/4 in, 5/8 in, 1/8 in: 3.18 mm — precision; 1/4 in: 6.35 mm — standard; 5/8 in: 15.88 mm — heavy. With Width 0 in, 0 in = 0.00 mm; matches no standard class — no width at all, and no matching class. With Width -0.5 in, 0 in = 0.00 mm; matches no standard class — a negative width is floored to 0.00 mm, never a negative number. With Width 1/4 in, 1/4 in = 6.35 mm (unrounded 6.3500 mm); standard — the millimetre figure displays as 6.35 mm. With Width 3/8 in, 3/8 in = 9.53 mm (unrounded 9.5250 mm); standard — the millimetre figure displays as 9.53 mm. With Width 5/8 in, 5/8 in = 15.88 mm (unrounded 15.8750 mm); heavy — the top of the table. With Width 0.21875 in, 7/32 in = 5.56 mm; matches no standard class — in the gap between the precision and standard classes, so no standard class. With Width 0.75 in, 0.75 in = 15.88 mm; matches no standard class — past the heavy class, so no standard class. With Width 0.1 in, 0.1 in = 3.18 mm; matches no standard class — below the precision class, so no standard class. With Width 5/32 in, 5/32 in = 3.97 mm (unrounded 3.9688 mm); precision — the millimetre figure displays as 3.97 mm. With Width 5/16 in, 5/16 in = 7.94 mm (unrounded 7.9375 mm); standard — the millimetre figure displays as 7.94 mm. With Width 9/16 in, 9/16 in = 14.29 mm (unrounded 14.2875 mm); heavy — the millimetre figure displays as 14.29 mm. With Width 0.30 in, 0.3 in = 7.94 mm; standard — between rows, so it sizes up to the 5/16-in row.

A note on the rounding: the millimetre figure is displayed to two decimals, half-up. With a 1/4-in screwdriver the unrounded figure is 6.3500 mm — it displays as 6.35 mm. With a 3/8-in screwdriver the unrounded figure is 9.5250 mm — it displays as 9.53 mm. With a 5/32-in screwdriver the unrounded figure is 3.9688 mm — it displays as 3.97 mm. With a 5/16-in screwdriver the unrounded figure is 7.9375 mm — it displays as 7.94 mm. With a 9/16-in screwdriver the unrounded figure is 14.2875 mm — it displays as 14.29 mm. The page prints the unrounded value so the rounding does not mislead you.

Output · formula · meaning

Who uses this

• The homeowner tightening a loose screw: they learn a 1/4-inch screwdriver is 6.35 mm — standard — and they check the 1/4-in row to see the smallest row whose width reaches 6.00 mm.
• The electronics hobbyist opening a gadget: they learn below 3/16 in the blade is a precision driver for electronics that twists under real torque — and they check the 1/4-in row to see the smallest row that needs a standard screwdriver.
• The DIYer driving a big lag screw: they learn above 7/16 in the driver needs a heavy handle and the screw needs a pilot hole — and they check the 7/16-in row to see the smallest row whose width reaches 10.00 mm.
• The student converting inches to millimetres: they learn the millimetres are the inches times 25.4 — and they check the 7/16-in row to see the smallest row that needs a heavy screwdriver.

Accuracy & limitations

The math is first-principles: the millimetres are the width in inches times 25.4, and the class comes from the closed bands — precision 1/8–3/16 in, standard 1/4–3/8 in, heavy 7/16–5/8 in (screwdriver BLADE WIDTH) — all in inches and millimetres, the millimetre figure displayed to two decimals half-up. There is no division in this calculator — nothing is ever undefined. A negative width is floored to 0.00 mm, never a negative number. With a 1/4-in screwdriver the unrounded figure is 6.3500 mm — it displays as 6.35 mm. With a 3/8-in screwdriver the unrounded figure is 9.5250 mm — it displays as 9.53 mm. With a 5/32-in screwdriver the unrounded figure is 3.9688 mm — it displays as 3.97 mm. With a 5/16-in screwdriver the unrounded figure is 7.9375 mm — it displays as 7.94 mm. With a 9/16-in screwdriver the unrounded figure is 14.2875 mm — it displays as 14.29 mm. 1/4 in is the standard screwdriver for a household — the general-purpose size that fits most screws, strong enough to turn a stuck screw without twisting the way a 1/8-in precision driver does; below 3/16 in the blade is a precision driver for electronics that twists under real torque; above 7/16 in the driver needs a heavy handle and the screw needs a pilot hole. 1/8 in is the precision floor — below it a jeweller's driver does the job; 5/8 in is the practical ceiling — above it the driver is a pry bar's job, sized by the fastener maker, not a chart. This is why the standard zone runs 1/4–3/8 in while the precision zone stops at 3/16 in — the standard driver is the household's driver, and 1/4 in is its middle. What the model cannot do is pick the screwdriver for you: it says nothing about the screw head, the material or the torque — a 1/4-in blade that turns a painted screw can slip and chew the slot, and a rusted screw needs a bigger blade than the chart says. Size up one row for rusted or painted screws, and always check the fastener maker's own table for the exact product. Use it to check the size before you buy — then check the allen-key-size, screw-length and bolt-size charts before you set the rest of the toolbox.

No screwdriver-size page of any kind exists on the site — this is the first. The closest page is the allen-key size chart (HEX keys — the L-shaped keys for hex bolts, not screwdrivers). This one is the SCREWDRIVER blade width's own 12-row width → millimetres reference (1/8–5/8 in, inches and millimetres) with its three classes (precision 1/8–3/16 in, standard 1/4–3/8 in, heavy 7/16–5/8 in) and class/gap logic.

Frequently asked questions