Screw Size Checker — Match a Screw or Bolt on Your Screen
Lay a screw on the real-size drawing and read its US or metric size, thread and wrench size.
Lay a screw on the drawing and pick sizes until the width and threads line up.
Not calibrated yet, so the drawing may not be real size. Sizes are a guide. To be certain, try a nut or a bolt of known size.
How the screw size checker works
A screw size is two things: the diameter across the threads, and the thread itself (threads per inch for US sizes, pitch in millimeters for metric). This tool draws a bolt at real size on your screen with true thread spacing. Lay your screw or bolt on the drawing and step through the sizes. When the outline matches the outside of your threads and the drawn threads line up with yours along the whole length, you have the size. Measured it with calipers instead? The thread identifier works out the thread from caliper measurements.
Related guides: Screw and Bolt Size Chart: US and Metric · Thread Pitch Chart: UNC, UNF and Metric Threads
How to use it
- Tap Calibrate and match a bank card first. Without calibration the drawing is not real size.
- Choose US or Metric, then lay the screw flat on the drawing with its head on the dashed head outline.
- Tap sizes (or use − and +) until the outline matches the outside of the threads.
- Check the threads: if they drift out of line after a few turns, try Fine or the other system.
Good to know
- US number sizes follow a formula: diameter = 0.060 in + 0.013 in × the number, so a #8 is 0.164 in (4.17 mm).
- From 1/4 inch up, the US size is the diameter: a 1/4-20 bolt is 1/4 inch across with 20 threads per inch.
- Metric sizes are the diameter in millimeters: M6 is 6 mm across, with a 1.0 mm pitch unless a finer one is marked.
- Some US and metric sizes are almost the same width, such as 5/16 in (7.94 mm) and M8. Their threads never match, so compare the threads too.
- Full tables with drill and wrench sizes: screw and bolt size chart and thread pitch chart.
Three ways to calibrate
Auto-detect your device
The ruler recognizes about 130 phones, tablets, laptops and monitors, including iPhones, Galaxy phones, Pixels and MacBooks.
Enter your screen size
Enter your screen's diagonal size (for example, 15.6" for a laptop), and the ruler works out the scale from your screen resolution.
Calibrate with a bank card
Hold any bank card against the outline on screen and move the slider until they match. Bank cards are the same size everywhere: 85.60 × 53.98 mm.
How accurate is the ruler?
How accurate the ruler is depends on how well it knows your screen's pixel density, meaning how many pixels fit in an inch. There are three ways to set it, from most to least accurate:
Bank card (±0.5%): Bank and credit cards are made to one worldwide size, 85.60 × 53.98 mm (ISO/IEC 7810 ID-1). Because every card is exactly the same, matching one on screen gives the most accurate result.
Auto-detect (±1%): The ruler looks up your device's published screen size and resolution. Screens of the same model can differ very slightly, so this is a little less exact. It covers about 130 devices.
Screen size (±2%): You type in your screen size, and the ruler works out the rest from your resolution. Advertised sizes are rounded (a "15.6-inch" screen may really be 15.59 inches), so this is the least exact method. It's a good fallback when you don't have a card.
Real-life objects to check your measurements
Use these everyday objects to confirm the ruler is calibrated correctly.
Bank or credit card: 85.60 × 53.98 mm (3.37 × 2.13 in)
A4 paper: 210 × 297 mm (8.27 × 11.69 in)
US Letter paper: 8.5 × 11 in (215.9 × 279.4 mm)
US quarter: 24.26 mm (0.955 in) across
1 euro coin: 23.25 mm (0.915 in) across
AA battery: about 50.5 mm long and 14.5 mm wide