Conversion Table for Imperial to Metric Screw Diameters
When a screw is marked 1/4-20 or 3/8-16 and you need to know which wrench or metric drill bit to use, guessing gets expensive. This table brings together the exact equivalences between imperial and metric screw diameters, with the values in millimeters you need to order the right replacement or choose the correct tool without trial and error.
Imperial to metric diameter conversion table
This screw diameter conversion table covers the most common sizes in general hardware, furniture, electronics, and automotive. The values in millimeters are the exact diameter of the screw shaft, not the head:
- #4 (0.112″) = 2.84 mm
- #6 (0.138″) = 3.50 mm
- #8 (0.164″) = 4.17 mm
- #10 (0.190″) = 4.83 mm
- #12 (0.216″) = 5.49 mm
- 1/4″ = 6.35 mm
- 5/16″ = 7.94 mm
- 3/8″ = 9.53 mm
- 7/16″ = 11.11 mm
- 1/2″ = 12.70 mm
- 9/16″ = 14.29 mm
- 5/8″ = 15.88 mm
- 3/4″ = 19.05 mm
- 7/8″ = 22.23 mm
- 1″ = 25.40 mm
Notice the pattern: every inch equals exactly 25.4 mm, so any fraction can be calculated by multiplying the numerator by 25.4 and dividing by the denominator. This is the basis of any well-made screw diameter inches to mm table you’ll find.
Screw equivalences by numbered gauge (#) to millimeters
Small screws in the imperial system aren’t numbered in fractions of an inch, but in gauges (#4, #6, #8…). This confuses a lot of people when looking for screw inch to mm equivalence tables in fine electronics hardware or furniture imported from the US.
- #0 = 1.52 mm
- #1 = 1.85 mm
- #2 = 2.18 mm
- #3 = 2.51 mm
- #4 = 2.84 mm
- #5 = 3.18 mm
- #6 = 3.50 mm
Quick formula if a gauge is missing from the list: diameter in inches = 0.060 + (gauge number × 0.013). Multiply the result by 25.4 to get it in millimeters.
How to read imperial screw measurements tables without getting confused
An imperial screw is usually marked as 1/4-20 UNC. The first number (1/4) is the diameter in inches; the second (20) is the threads per inch, meaning how many threads there are in one inch of thread length. Don’t confuse the two: the diameter is what you’re looking for in this table, the thread pitch is a separate piece of data.
To fully understand this difference between systems, it’s worth reviewing the differences between inch and metric screws, since it also affects thread pitch and not just diameter.
Difference between nominal diameter and actual diameter
The nominal diameter is the one printed on the label or catalog (for example, 1/4″). The actual diameter of the shaft can vary by a few tenths depending on manufacturing tolerance and whether the screw has full or partial threading. For precision work, always measure with calipers before relying solely on the label.
If you need to confirm a specific measurement of a loose screw without a label, the most reliable method is to measure the shaft and check how to find the equivalence of a screw from inches to millimeters step by step.
Screw diameters for wood, metal, and drilling
Wood screws in the imperial system are measured by gauge, not by fractions of an inch, although their length is measured in inches. Machine screws and sheet metal screws usually use fractions. This distinction matters because a #8 wood screw and a 5/32″ machine screw have similar diameters but aren’t interchangeable due to their thread type.
- #6 wood ≈ 3.50 mm
- #8 wood ≈ 4.17 mm
- #10 wood ≈ 4.83 mm
- #12 wood ≈ 5.49 mm
How to convert any diameter not listed in the table
If the screw you’re holding doesn’t appear in any table because it’s an uncommon fraction (like 11/32″ or 13/16″), the manual calculation is simple: divide the numerator by the denominator to get the decimal value in inches, and multiply that result by 25.4.
To avoid doing the math by hand every time, the fastest way is to convert inches to millimeters with the calculator, enter the fraction and get the exact result instantly, without rounding that could complicate the fit later.
Common mistakes when comparing imperial and metric diameters
The most common mistake is assuming that an M6 screw (6 mm) is the same as a 1/4″ (6.35 mm) because they “look alike.” The 0.35 mm difference is enough to prevent the thread from fitting properly into a nut from the other system, or to leave the fit loose and get damaged with use.
- M6 (6.00 mm) is not the same as 1/4″ (6.35 mm)
- M8 (8.00 mm) is not the same as 5/16″ (7.94 mm)
- M10 (10.00 mm) is not the same as 3/8″ (9.53 mm)
- M12 (12.00 mm) is not the same as 1/2″ (12.70 mm)
Working with these mixed systems is common when importing parts or repairing machinery manufactured in the US, and knowing how to tell them apart avoids buying incompatible hardware.
Practical uses of the table in hardware stores and maintenance
At a hardware store counter, this table solves three daily situations: identifying a screw a customer brings in with no visible marking, replacing a broken imperial screw with a metric equivalent when there’s no stock of the original, and calculating which drill bit to use before pre-drilling. In all three cases, the key data point is always the actual diameter in millimeters.
For drill bits specifically, the conversion logic is the same as here, but with a different tolerance depending on the material being drilled.
Frequently asked questions
How many millimeters is a 1/4 inch screw?
A 1/4″ screw has a diameter of exactly 6.35 mm. It’s calculated by multiplying 0.25, the decimal value of 1/4, by 25.4, the number of millimeters in one inch.
Are an M6 screw and a 1/4 inch screw interchangeable?
Not exactly. The M6 measures 6.00 mm and the 1/4″ measures 6.35 mm, a difference of 0.35 mm that can prevent the thread from fitting properly into a nut or hole designed for the other system.
How are numbered gauge screws like the #8 measured?
Numbered gauges use a fixed formula: 0.060 inches plus the gauge number multiplied by 0.013 inches. A #8, for example, gives 0.164 inches, which is equivalent to 4.17 mm.
Why doesn’t my imperial screw fit into a similar metric nut?
Because even if the nominal diameter is similar, the thread pitch (the distance between threads) is different in each system. Even if you force the screw in, the thread can get damaged and become unusable for future adjustments.
What tolerance margin is acceptable when replacing an imperial screw with a metric one?
Generally, a difference of up to 0.1 mm is usually tolerable in non-critical applications. Larger differences, like the 0.35 mm between 1/4″ and M6, already affect the fit and are not recommended unless you use a nut or thread specifically designed to absorb that clearance.