How to Identify a Screw Without Calipers (5 DIY Methods)

Reviewed by the ScrewBoltSize Editorial Team

You pulled a screw out of a cabinet hinge, or a furniture leg fell off and took the bolt with it. You need a replacement, and you don't own a caliper. Here's the good news: you can narrow any screw down to a handful of candidates — and often to the exact size — using things you already have at home.

These five methods range from "works on any smartphone" to "takes 90 seconds at the hardware store." Start with Method 1 if you're at your desk; go straight to Method 4 if you're already at the store.

Method 1: The On-Screen Ruler (Phone or Computer)

This site's identifier tool includes a calibration ruler that displays on your screen at actual size. Hold your screw up to the screen and read off the diameter and length directly. It works on any phone, tablet, or computer with a known screen resolution.

To use it: open the identifier, tap "Calibrate ruler," and the on-screen ruler appears. Hold the screw parallel to the ruler with the shaft at the ruler edge. Read the diameter where the thread crests line up. You'll get within ±0.5 mm on most phones — good enough to distinguish M5 from M6 and #8 from #10.

Accuracy tip: lay the screw flat on a hard surface next to the screen rather than holding it up in mid-air. Parallax from a slightly angled hold can add 0.5–1 mm of apparent error.

Method 2: Steel Ruler and Thread Count

A standard steel ruler gives you three numbers: shaft diameter, screw length, and thread pitch. Together, those three numbers identify almost every common fastener.

Measuring Diameter

Lay the screw horizontally and hold the ruler perpendicular to the shaft so the ruler edge crosses the threads. Read across the outer thread crests — not the valley between threads, not the bare shank. If it reads 6 mm, you have an M6 or a close imperial equivalent. If it reads just under 6.4 mm, you likely have a 1/4" imperial screw.

Counting Thread Pitch

Place the screw on a ruler with the threads visible. Count the number of thread peaks that fall in exactly one inch. That count is the threads per inch (TPI). Common coarse-thread counts: #8 = 32 TPI, 1/4-20 = 20 TPI, 5/16-18 = 18 TPI.

For metric screws, count 10 threads and measure the total span in mm. Divide by 10. If 10 threads span 10 mm, pitch = 1.0 mm (standard M6 coarse). If 10 threads span 12.5 mm, pitch = 1.25 mm (standard M8 coarse).

Method 3: Compare to Coins and Cards

US coins are standardized to a fraction of a millimeter and make surprisingly good diameter references. A credit card is 0.76 mm thick. Stacking them against the screw shaft gives you a rough diameter bracket without any measuring tool.

The technique: hold a stack of coins flat against the screw shaft at a right angle. If 4 pennies (≈ 6.08 mm) just barely fit alongside the screw threads, you're looking at an M6. If 5 pennies (≈ 7.6 mm) fit loosely but 4 are too short, you're probably at M8 (8 mm). This narrows you to two or three candidates to confirm at the store.

Method 4: Hardware Store Bolt Board

This is the most reliable no-tool method. Bring your sample screw to the hardware store. Near the fastener bins, almost every store has a bolt board — a pegboard with labeled holes of known thread sizes. Thread your screw into each hole from the back. When it threads in smoothly for more than 3–4 turns without resistance or wobble, you have your size.

What to watch for: a screw that "almost" threads in but needs force is the wrong thread pitch — don't force it. A screw that wobbles before catching is oversized. A screw that drops in freely without catching is undersized. Only a smooth, hand-tight thread is a match.

Some stores also have depth gauges at the bolt board. Even if they don't, you can hold the screw against the bin label to confirm length.

Method 5: Thread Into a Known Nut or Fitting

If you have a pile of spare nuts at home — from past projects, an appliance, or a furniture kit — you can use them as go/no-go gauges. A nut is a thread reference. If your mystery screw threads smoothly into a nut you already know the size of, the screw matches that nut's thread specification.

Most people have a handful of known sizes at home without realizing it:

• IKEA furniture hardware — typically M6 cam lock bolts and M5 screws throughout their range
• PC case screws — almost always #6-32 (the standard motherboard/drive size)
• Bicycle fenders and racks — typically M5 × 0.8
• Standard US light switches and outlet covers — #6-32 machine screws
• Garden hose fittings — 3/4" GHT (garden hose thread, not a bolt thread, but useful as a size reference)

Thread your mystery screw into these known sizes. If it catches in the IKEA cam lock nut, it's M6. If it threads into a PC case standoff, it's #6-32.

Putting the Methods Together: A Decision Flow

Here's how to combine the methods efficiently depending on your situation:

At home, have your phone? → Method 1 first (on-screen ruler), then Method 2 (ruler + thread count) to confirm pitch. Enter the estimates into the identifier tool to get a designation.

At home, have a steel ruler? → Method 2 for diameter and TPI. If you have spare hardware, try Method 5 as a go/no-go check.

About to go to the hardware store? → Skip everything and use Method 4 at the bolt board. It's faster and more accurate than any at-home method.

Dealing with an unusual metric screw (from machinery, a bike, a European appliance)? → Method 2 for diameter, count 10 threads for pitch, enter into the identifier. Most metric screws are one of seven standard sizes — the identifier narrows it quickly.

Common Sizes and Their Diameter Reference

When These Methods Aren't Enough

The five methods above will identify the vast majority of household and workshop screws. They won't cover everything.

If you're dealing with fine-thread variants (e.g., 1/4-28 vs 1/4-20), a ruler thread-count is borderline. Twenty threads per inch versus 28 threads per inch is hard to distinguish without more precision than a household ruler provides. A $20 digital caliper will measure pitch to 0.01 mm and remove all ambiguity.

Similarly, identifying fasteners for structural or high-torque applications — brake caliper bolts, suspension components, load-bearing lag bolts — should always be confirmed with a proper tool or manufacturer spec sheet. The bolt board method is fine for furniture and household hardware; it's not a substitute for torque-spec verification in safety-critical contexts.