Understanding Screw Bit Sizes: Navigating The World Of Screwdriver Bits

Choosing the right screwdriver bit size is essential for any DIY enthusiast or professional. The wrong size can lead to stripped screws, damaged bits, and ruined work surfaces. Screwdriver bits come in a variety of shapes and sizes, and selecting the right one depends on the specific screw type and the surface being driven into.

The most common screwdriver bit types include flat head, Phillips, Torx, Robertson (or square recess), and hex key. Flat head screwdrivers are the most basic model, with a chisel-shaped blade that fits into slotted screws. Phillips head screws are the closest to a universal screw, found in electronics and most home hardware. Torx screwdrivers have a six-pointed star shape, providing greater torque and resistance to camming out. Robertson screwdrivers have a square head and are popular in Canada due to their 'stick fit' into the screw head. Hex keys, also known as Allen wrenches, are L-shaped with hexagonal ends and are measured across the flat parts of the hex.

When choosing a screwdriver bit, it's important to consider the size of the screw head and the amount of torque required. Bits that are too small or too big will not seat properly and can lead to stripped screws. Additionally, the material and quality of the bit are important factors, as ordinary bits are harder and more brittle, while impact-rated bits are softer, more flexible, and less likely to damage screws.

Flat Head/Slotted Screwdriver Sizes

Flat-head screwdrivers, also known as slotted screwdrivers, are one of the most basic tools ever designed. They are called flat-head because the flat tip of the driver fits snugly into the slot of a screw to tighten or loosen it. Flat-head screwdrivers come in a variety of sizes and lengths, with each serving a particular purpose.

The two measurements to note for each screwdriver are the length of the shaft and the width of the head. Flat-head screwdrivers range in size from mere millimetres to bigger fractions of an inch. The length of the shaft determines how far the screwdriver can reach into tight spaces. Longer shafts are useful for automotive work, as screws are often placed far back in an engine block. However, if you can reach the screw without a long shaft, it is better to use a tool with a smaller shaft. Shorter shafts, or "stubby" screwdrivers, are useful for close-up work, but they have a stubby handle which can be difficult to grip. Screwdrivers with longer shafts have longer handles, which allow for a better grip.

Examples of flat-head screwdriver sizes in inches include 3/32 by 1 1/2, 3/16 by 1 1/2, 1/8 by 4, 3/16 by 4, and 1/4 by 4. The first number is the width of the driver head, and the second number is the shaft length. Flat-head screwdrivers are also available in miniature sizes measured in millimetres, which are useful for precision screws found in glasses, watches, and small electrical components.

Phillips Screwdriver Sizes

Phillips head screwdrivers are sized using numbers, typically ranging from #0000 to #5, with #0 to #4 being the most common sizes for basic household projects. The size of the Phillips head screwdriver is indicated by a number and a "PH" prefix, for example, PH0, PH1, PH2, and so on.

The #0 Phillips head screwdriver, also known as a "mini" Phillips head, has a 2.5 mm tip and is used for small appliances and home electronics. The #1 Phillips head driver has a 3 mm tip and is commonly used for small projects like crafting kits and jewellery boxes.

The #2 Phillips head screwdriver is the most popular size for household projects and repairs. It has a 3.5 mm tip and is typically used with wood screws that come in sizes 6 and 8. This size is also suitable for light construction work.

The #3 Phillips head driver has a 5 mm tip and is often used in construction and building projects, fitting screw sizes 10, 12, 14, and 16.

The #4 Phillips head screwdriver has a 6 mm tip and is used for larger screw sizes 18, 20, and 24.

There are also smaller Phillips head screwdrivers available, such as the #0000 (1 mm tip), #000 (1.5 mm tip), and #00 (2 mm tip). These sizes are typically used for precision work and are not commonly found in standard toolsets.

When choosing a Phillips head screwdriver, it is important to select the correct size to prevent stripped screws, damaged bits, and spoiled work surfaces. The chosen bit should fill the screw head entirely, ensuring a proper fit.

Robertson/Square Screwdriver Sizes

Robertson screwdriver bits, also known as square bits, are commonly used in Canada and are becoming more common in other countries. They are named after their inventor, Peter Lymburner Robertson, who created them in 1906 or 1908 after cutting his hand with a slotted screwdriver. Robertson screws have a square-shaped socket in the screw head and a corresponding square protrusion on the tool, with both the tool and socket featuring a slight taper. This taper makes it easier to insert the tool and helps keep the screw on the tool without the user needing to hold it there. The screw will "cling" to the driver, allowing for one-handed use and reducing slippage.

When purchasing Robertson bits, it is important to distinguish between true Robertson bits and square bits, as using the wrong type can lead to damaged tools and stripped screws. True Robertson bits have a taper, while square bits do not. Some bits are labelled with an "R" designation, such as "R1" or "R2", which may indicate a Robertson taper, but this is not always clear. One way to ensure compatibility is to buy Robertson-branded screws and screwdrivers or bits. Additionally, some manufacturers produce bits that are specifically designed for Robertson screws, such as the C1 & C2 from Klein and the ECX #1 & #2 from Milwaukee.

Robertson bits are typically available in 1”, 2”, 3”, 4”, 6”, and 12” lengths. When choosing a Robertson bit, it is important to select the correct size for the screw head. Choosing a bit that is too big or too small can result in a stripped screw.

Overall, Robertson screwdriver bits offer several advantages over other types of bits, including their ability to be used one-handed, reduced slippage, and ease of insertion. However, it is important to distinguish between true Robertson bits and square bits to avoid damage and ensure a proper fit.

Hex Key Screwdriver Sizes

Hex keys, also known as hex wrenches or Allen keys, are simple drivers for bolts or screws with heads that have internal hexagonal recesses (sockets). They are formed from a single piece of hard hexagonal steel rod, with blunt ends that fit snugly into similarly shaped screw sockets. The rods are bent to 90º, forming two arms of unequal length that resemble an "L".

Hex keys are designated with a socket size and are manufactured with tight tolerances, so they are commonly sold in kits that include a variety of sizes. The key length typically increases with size but not necessarily proportionally.

Standard Metric Hex Key Sizes

• 0.7 mm, 0.9 mm, 1.3 mm
• 1.5 to 5 mm in 0.5 mm increments
• 5 mm to 19 mm in 1 mm increments
• 21 mm, 22 mm, 23 mm, 24 mm, 27 mm, 29 mm, 30 mm, 32 mm, and 36 mm

US Customary Hex Key Sizes

• 0.028"-3/8"
• 0.7 mm-10 mm

When choosing a hex key screwdriver, it is important to select the correct size that matches your screw head. Opting for a high-quality hex key that fits your screw head perfectly will help you avoid stripped screws, damaged bits, and spoiled work surfaces. Additionally, consider the material and finish of the hex key. Chrome vanadium steel, for example, is a common material for hex keys due to its strength and durability.

Torx Screwdriver Sizes

Torx screwdrivers are commonly used for automobiles, appliances, computers, and electronics. The head of a Torx screwdriver is a six-pointed star, and its larger surface area makes it the least likely type of screwdriver to slip.

Torx screwdrivers are available in a range of sizes. The size of a Torx screwdriver is determined by the distance (diameter) between the diagonal points of the Torx head. The following are some of the most commonly used Torx screwdriver sizes:

• T8: 2.31mm diameter
• T10: 2.74mm diameter
• T15: 3.27mm diameter
• T20: 3.86mm diameter
• T25: 4.43mm diameter
• T27: 4.99mm diameter
• T30: 5.52mm diameter

It is important to match the size of the Torx screwdriver to the screw or bolt socket head to avoid stripped screws, damaged bits, and spoiled work surfaces.

Frequently asked questions