How to Convert a Drill Bit Size from Inches to Millimeters
You buy a drill bit at a US store or find a set labeled in inches, but your drill, your spare bits, or your project use millimeters. Without a clear conversion, you end up drilling a hole too big or too small for the screw or anchor you need. Here’s the exact method to convert drill bit inches to mm without errors.
The formula to convert drill bit inches to mm
One inch equals exactly 25.4 millimeters. To convert a drill bit size from inches to mm, multiply the value in inches by 25.4. If the bit comes in a fraction (like 1/4″), first convert the fraction to decimal by dividing the numerator by the denominator, then multiply by 25.4.
- Decimal inches × 25.4 = millimeters
- Fraction → decimal → × 25.4 = millimeters
1/4 inch drill bit in mm: the most searched example
1/4 inch is 0.25 inches. Multiplying 0.25 × 25.4 gives you exactly 6.35 mm. So a 1/4″ drill bit measures 6.35 mm in diameter. It’s one of the most common sizes in DIY, both for wood and light metal.
Drill bit size in mm equivalence: common fraction table
These are the exact conversions of the fractions of an inch that appear most often in drill bit sets:
- 1/16″ = 1.59 mm
- 1/8″ = 3.18 mm
- 3/16″ = 4.76 mm
- 1/4″ = 6.35 mm
- 5/16″ = 7.94 mm
- 3/8″ = 9.53 mm
- 1/2″ = 12.70 mm
Save this table, but remember that many imperial drill bit sets include finer sizes, such as 1/32″ or 5/64″, which also convert using the same formula.
How to read the marking on a drill bit in inches
The shank or the drill bit case usually has the size engraved as a fraction, for example 1/4″ or 3/8″. If you don’t see the inch symbol (“), pay attention to the context: if the set comes from the US or UK, it’s almost certainly in fractional inches, not mm.
To find out if the same screw bought abroad works with your material, it’s also worth checking how to identify if a screw bought in the US works for your project, because the drill bit diameter depends directly on that thread.
Use a calculator instead of calculating by hand
Multiplying fractions and decimals by hand opens the door to error, especially if you’re in the middle of a project with your hands busy. The fastest and safest way is using an inches to millimeters calculator: you enter the fraction or decimal and get the exact result in seconds, without rounding that later complicates the fit.
Why the exact drill bit size matters so much
A poorly converted drill bit ruins the hole: if you drill with a bit slightly larger than needed, the screw or anchor won’t grip well; if it’s smaller, the screw gets stuck or the wood splits. In metal, the margin for error is even smaller because the material doesn’t give way like wood does.
This is especially noticeable when installing screws, where it’s worth checking how to know the equivalence of a screw in inches to millimeters before choosing the final drill bit.
Difference between imperial and metric drill bits
Metric drill bits use round numbers like 3 mm, 4 mm, 5 mm, or 6 mm. Imperial ones use fractions of an inch like 1/8″, 3/16″, or 1/4″. When converting, they almost never match exactly with a round metric number: for example, 1/4″ is 6.35 mm, not 6 mm. That 0.35 mm difference may seem minimal, but in precision hardware it determines whether the fit is correct or ends up loose.
How to choose the right drill bit for a screw or anchor
The practical rule in DIY: for expansion anchors, the drill bit must match exactly the diameter of the anchor (in mm). For self-tapping screws in wood, the bit is usually somewhat smaller than the screw diameter, so the thread grips the fiber well. Always convert the package measurement to mm first before deciding.
- Wall anchor: drill bit = exact diameter of the anchor in mm
- Screw in soft wood: drill bit slightly smaller than the screw diameter
- Screw in metal: drill bit according to thread table, never by eye
Common mistakes when converting drill bit size
The most frequent mistake is rounding too soon: converting 1/4″ to “about 6 mm” instead of exactly 6.35 mm. Another common error is confusing the diameter with the length of the drill bit, two completely different measurements that sometimes appear together on the packaging. And finally, mixing systems: using a metric drill bit for an imperial screw without first checking the actual equivalence.
Frequently asked questions
How do you convert a drill bit from inches to mm?
You multiply the value in inches by 25.4, which is the exact equivalent of one inch in millimeters. If the measurement comes as a fraction, first divide the numerator by the denominator to get the decimal, then multiply by 25.4.
How many mm is a 1/4 inch drill bit?
A 1/4 inch drill bit equals exactly 6.35 millimeters, the result of multiplying 0.25 by 25.4.
Why don’t imperial drill bits match round numbers in mm?
Because fractions of an inch are based on divisions of 25.4 mm, a number that rarely produces whole results when converted. That’s why 1/4 inch gives 6.35 mm and not exactly 6 mm.
Can I use a metric drill bit instead of an imperial one?
Yes, as long as you first convert the imperial measurement to millimeters and choose the closest metric drill bit according to the type of anchor or screw you’re going to use, taking into account whether it needs an exact or slightly smaller fit.
What happens if I use a drill bit slightly larger than needed?
The screw or anchor won’t grip the material properly, ending up loose or unstable. In load-bearing structures, this can compromise the fixing over time.