Two Systems, One Screw
Screws are sized in two completely different languages. The metric system names a screw by its diameter in millimetres — an M5 screw is 5 mm across its threads. The imperial system uses gauge numbers like #8 or #10, a legacy of 19th-century wire-gauge practice where the number only indirectly describes the diameter. If you have ever stood in a hardware aisle holding a metric bolt up to an imperial nut, you already know why a translator matters.
What 'M' Numbers Actually Mean
A metric designation like M6 × 1.0 × 40 packs three facts into a few characters: the thread diameter is 6 mm, the thread pitch (distance between threads) is 1.0 mm, and the screw is 40 mm long. The 'M' simply stands for metric. For the common coarse-thread screws in this guide, the standard pitches are:
| Size | Diameter | Coarse pitch | Tap drill |
|---|---|---|---|
| M3 | 3.0 mm | 0.5 mm | 2.5 mm |
| M4 | 4.0 mm | 0.7 mm | 3.3 mm |
| M5 | 5.0 mm | 0.8 mm | 4.2 mm |
| M6 | 6.0 mm | 1.0 mm | 5.0 mm |
| M8 | 8.0 mm | 1.25 mm | 6.8 mm |
| M10 | 10.0 mm | 1.5 mm | 8.5 mm |
The tap drill column is the money column: it is the drill bit you use before cutting threads with a tap, computed as diameter minus pitch. For ordinary wood screws (which cut their own threads), the pilot hole is slightly different — see below.
Decoding Imperial Gauge Numbers
Imperial screw gauges follow a simple formula most people never learn: diameter in inches = 0.060 + 0.013 × gauge number. So a #8 screw is 0.060 + 0.104 = 0.164 inches, or 4.17 mm. The most common wood-screw gauges convert as follows:
| Gauge | Diameter (in) | Diameter (mm) | Closest metric |
|---|---|---|---|
| #4 | 0.112 in | 2.84 mm | M3 |
| #6 | 0.138 in | 3.51 mm | M3.5 |
| #8 | 0.164 in | 4.17 mm | M4 |
| #10 | 0.190 in | 4.83 mm | M5 |
| #12 | 0.216 in | 5.49 mm | M5–M6 |
| #14 | 0.242 in | 6.15 mm | M6 |
'Closest metric' is approximate — a #10 at 4.83 mm is not an M5 at 5.0 mm, and the two are not interchangeable in threaded holes. They are close enough for clearance holes and rough substitution in wood, which is exactly what the converter above tells you.
Pilot Holes: The Difference Between Clean and Split
A pilot hole is a pre-drilled guide that lets the screw's threads bite without splitting the wood. Size it wrong and you get one of two failures: too small and the wood splits or the screw snaps; too large and the threads have nothing to grip, so the joint pulls apart under load.
The rule of thumb for softwood: drill the pilot to roughly the screw's root diameter (the shank under the threads), about 70–80 percent of the outer diameter. For hardwood like oak, go slightly larger — around 85 percent — and always drill a clearance hole through the top piece so the screw slides freely and pulls the joint tight. The ScrewSize wizard gives you a recommended pilot drill for each size; when drilling into end grain or near board edges, err toward the larger end and add a dab of wax on the threads.
Head Types: Choosing the Right One
- Flat (countersunk): sits flush with or below the surface. The standard choice for woodworking and anywhere a smooth finish matters.
- Pan: a low, rounded dome that sits on top of the surface. Good general-purpose head with a neat look.
- Round: a taller dome than pan; traditional look for visible fixings and thin materials.
- Oval: a compromise — decorative like round, but with a countersunk base that sits nearly flush.
- Hex / hex washer: driven with a wrench or nut driver for maximum torque. The choice for structural and outdoor work like decking ledger boards.
Drive Types Matter More Than You Think
The recess in the head — Phillips, Pozidriv, Torx, Robertson (square), or hex — decides how much torque you can apply before the driver cams out and chews the head. For power driving, Torx (star) and Robertson are dramatically better than Phillips, which was designed to cam out on assembly lines. If you are buying screws for a big project, choose the drive first and the head second; your wrists will thank you.
Length, Materials, and Coatings
Screw length is measured differently by head type: flat-head screws are measured overall (tip to top of head), while pan, round, and hex heads are measured from under the head to the tip. As a rule, the screw should penetrate the receiving piece by at least the thickness of the piece being fastened — the classic 'two-thirds in, one-third through' guideline.
Material choice is about environment: zinc-plated steel for dry indoor use, hot-dip galvanized or stainless steel for outdoors and pressure-treated lumber (the copper in modern treated wood corrodes ordinary zinc coatings). Brass screws are soft and beautiful for hinges and hardware but snap easily — always drill a full pilot hole and drive them by hand.
Five Mistakes Everyone Makes Once
- Driving a #8 screw into hardwood with no pilot hole and splitting the board inches from the finish line.
- Using Phillips screws with a worn bit, camming out, and stripping the head beyond rescue.
- Measuring screw length from the wrong end and buying screws a head-height too short.
- Mixing metric and imperial ('close enough') in a threaded hole and destroying the threads.
- Skipping the clearance hole in the top board, so the screw threads lock both pieces apart and the joint never closes.
Keep this converter bookmarked, drill the pilot the chart recommends, and your fasteners will disappear into the work the way they should — holding everything together and drawing no attention to themselves.
Wood Screws vs. Machine Screws vs. Sheet-Metal Screws
Three screw families cover nearly all fastening, and mixing them up is a classic beginner error:
- Wood screws have a tapered body, a sharp gimlet point, and deep, widely spaced threads that cut their own path into wood. They are sold by gauge (#4–#14) and almost never need a nut.
- Machine screws have a uniform-diameter shank with precise, evenly spaced threads designed to mate with a tapped hole or a nut. They follow metric (M3–M10) or UNC/UNF standards, and the pilot hole must be drilled and tapped to the exact thread.
- Sheet-metal screws (self-tappers) have sharp threads along the full shank for biting into thin metal or plastic without a tapped hole. They are sized by gauge like wood screws but with finer threads.
The converter above covers wood-screw gauges and metric machine-screw sizes — the two systems you will meet most. If a screw needs a nut, it is a machine screw; if it bites straight into the material, it is a wood or sheet-metal screw.
Reading a Screw Box Like a Pro
A typical box label such as '#8 × 1-1/4 in, flat head, Phillips, zinc' tells you everything: gauge (#8 = 4.17 mm diameter), length (1-1/4 in), head style, drive type, and finish. Metric boxes read like 'M5 × 30, pan head, Torx, A2 stainless': 5 mm diameter, 30 mm long, A2 grade stainless steel. Learn to parse these and you will never again buy the wrong box because the picture looked right. Two details people miss: thread count on imperial machine screws (e.g. 1/4'-20 means 20 threads per inch — coarse; 1/4'-28 is fine) and length conventions (flat heads measured overall, everything else from under the head).
Worked Example: Screws for a Wall-Mounted Bookshelf
You are fixing a 19 mm (¾ in) oak shelf standard to a softwood wall stud. The rule from above: the screw must penetrate the receiving piece by at least the thickness of the piece being fastened — so you need 19 mm through the oak plus at least 19 mm into the stud, roughly 40–50 mm of thread engagement minimum. A #10 × 2½ in (63 mm) screw fits: 19 mm passes through the oak, leaving about 44 mm buried in the stud, with margin for the countersunk head.
Pilot hole: #10 measures 4.83 mm across the threads, and softwood wants 70–80% of that — roughly 3.4–3.9 mm. A 9/64 in (3.57 mm) bit is ideal; 1/8 in (3.18 mm) is slightly tight and risks snapping the screw in dense studs. Drill a clearance hole through the oak (about 5 mm) so the threads pull the oak tight against the wall instead of jacking it away. Choose a flat head in Torx drive, zinc-plated for indoors — and drive by hand for the last quarter-turn to feel the joint close.
Specialty Fasteners Worth Knowing
When ordinary wood screws are the wrong tool, reach for the specialists. Pocket-hole screws have a flat washer head that seats in angled pocket holes for strong, hidden joints in cabinetry. Concrete screws cut their own threads into pre-drilled masonry and need the exact bit size printed on the box. Structural screws replace lag bolts with no pre-drilling in most cases and carry published shear ratings — the modern choice for deck framing. Drywall screws, with their bugle heads and black phosphate coating, are for drywall only — brittle and rust-prone anywhere else.