20mm is about the width of a large adult thumbnail. It sits in a practical middle ground between too small to see clearly without looking closely and large enough to hold between two fingers without effort. In metric terms, it is exactly 2 centimeters. In imperial terms, it converts to approximately 0.787 inches, which is just under 13/16 of an inch.
It is not a tiny measurement. A grain of rice is roughly 5 to 7mm long. A pencil eraser is around 10 to 12mm wide. At 20mm, you are dealing with something noticeably larger than those references and easy to perceive without tools.
Quick Reference: 10 Things That Are 20mm
| Thing | 20mm Measurement | Type |
| 20mm Autocannon Barrel Bore | 20mm bore diameter | Exact |
| M10 Standard Flat Washer | 20mm outer diameter | Standard |
| 20mm Open-Ended Wrench | 20mm jaw opening | Standard |
| Round Ceramic Disc Magnet | 20mm diameter | Approximate |
| Standard Split Key Ring | 20mm outer diameter | Approximate |
| Medium Padlock Shackle Clearance | About 20mm clearance | Approximate |
| Standard Lipstick Case | About 20mm outer diameter | Approximate |
| Stack of 11 US Quarters | 19.25mm total thickness | Close match |
| Men’s Size 10 Wedding Band | About 19.8mm inner diameter | Close match |
| Size 32L Suit Jacket Button | 20.32mm diameter | Close match |
The Objects in Detail
20mm Autocannon Barrel Bore
The 20mm designation in military autocannons refers specifically to the bore diameter of the barrel, meaning the internal diameter of the tube through which the projectile travels. This is a standardized, internationally recognized caliber used across multiple cannon families, including the M61 Vulcan used on many US military aircraft and the Oerlikon 20mm series. The measurement is exact by specification.
What makes this comparison striking is that a bore of 20mm represents the threshold where a weapon is generally reclassified from a heavy machine gun to a cannon in most military doctrine. At exactly the size of two stacked centimeter marks on a ruler, it is a caliber that has shaped aircraft armament standards since the Second World War.
M10 Standard Flat Washer (Form A)
An M10 flat washer in Form A specification has an inner diameter of approximately 10.5mm to accommodate an M10 bolt, but its outer diameter is defined at 20mm under ISO standards. This outer edge is the 20mm dimension and is what you would measure if you laid the washer on a flat surface and measured across its widest point.
This is a useful everyday reference because M10 hardware is common in furniture assembly, automotive work, and general construction. If you have a standard M10 washer nearby, the outer rim gives you a reliable 20mm circle to reference.
20mm Open-Ended Wrench Opening
A 20mm open-ended spanner or wrench has a jaw opening of exactly 20mm, sized to fit the flats of an M12 hex bolt head or nut (which measures 20mm across its parallel faces). This is a standardized dimension in both metric tool sets and ISO hardware specifications. The wrench itself may be larger, but the opening is the 20mm reference.
In a practical workshop context, a 20mm wrench opening gives you a clear sense of the measurement as a gripping width rather than a length. Hold two fingers tightly together: a large index and middle finger pressed flat span close to this width, though human anatomy varies too much to use as a reliable substitute for a tool.
Read also: How Big Is 16mm? 10 Real Things That Measure 16 Millimeters
Round Ceramic Disc Magnet
Ceramic ferrite disc magnets are sold in a range of diameters, and 20mm is one of the most widely available sizes for hobby, craft, and light industrial use. The 20mm figure refers to the disc’s outer diameter as listed by the manufacturer. Actual dimensions can vary slightly due to sintering tolerances in the ceramic manufacturing process, so treat this as nominal rather than exact without measuring.
At 20mm across, these magnets are about the size of a large shirt button. They are commonly used in cabinetry, craft projects, and sensor assemblies, which makes them a practical hands-on reference if you have one available.
Standard Split Key Ring
A standard split key ring listed as 20mm refers to its outer diameter. The actual usable inner diameter, which is the opening through which you thread a key, is smaller because the ring’s wire adds thickness on both sides. Wire gauge varies, but most common split rings use steel wire around 1.2 to 1.5mm thick, leaving an inner diameter of roughly 17 to 17.5mm.
This distinction matters if you are trying to use the key ring as a size reference: the outer edge gives 20mm, but the ring itself is not a solid 20mm disc. It is a circular loop, and what you measure depends on where you measure it.
Medium Padlock Shackle Clearance
On a medium-sized padlock, the shackle clearance describes the usable gap between the top of the lock body and the closed shackle arc. This is the space available to thread the shackle through a hasp or chain link. For many medium padlocks in the 40 to 50mm body size range, this clearance is approximately 20mm, though it varies by manufacturer and model.
This is not a standardized measurement in the way that a bolt specification would be. It is better understood as a common approximate dimension within a product category. If you are evaluating whether a padlock will fit a particular hasp, always check the manufacturer’s stated clearance rather than assuming 20mm applies.
Standard Lipstick Case
The bullet of a standard full-size lipstick, meaning the cosmetic product column that extends from the case, is typically around 12 to 15mm in diameter. However, the outer diameter of the lipstick case itself, the cylindrical tube you hold, commonly falls in the 19 to 21mm range for standard-format products. The 20mm measurement here refers to the external case diameter, not the lipstick bullet inside.
This is an approximate measurement that varies across brands and product lines. Slim-format and travel-size lipsticks run narrower, while some luxury formats run slightly wider. A standard drug store lipstick is a reasonable field reference, but the 20mm figure describes a typical range, not a fixed specification.
Read also: How Big Is 18mm? 12 Real Things That Measure 18 Millimeters
Stack of 11 US Quarters
This is the most mathematically precise comparison in the list. The US quarter has a standardized thickness of exactly 1.75mm as specified by the United States Mint. Eleven quarters stacked directly on top of one another produce a total height of 11 × 1.75mm, which equals exactly 19.25mm. Twelve quarters would measure 21mm. So eleven gives the closest match at 19.25mm, approximately 0.75mm under the 20mm target.
If you want to reach 20mm more precisely using coins, you could combine denominations. A US dime is 1.35mm thick. Ten quarters and one dime yields 17.5mm plus 1.35mm, totaling 18.85mm. The eleven-quarter stack remains the simplest practical approximation with common pocket change.
Men’s Size 10 Wedding Band
A US ring size 10 has an inner diameter of approximately 19.8mm, which is rounded to 20mm in many sizing charts and product descriptions. This inner diameter is the measurement that determines fit. It is the diameter of the circular opening that slides over the finger, not the outer diameter of the band itself.
Ring sizing in the United States uses a scale where each full size represents a change of approximately 0.4mm in diameter. Size 9 is about 19.4mm and size 11 is about 20.2mm. So a size 10 ring falls almost exactly at 20mm inner diameter, making it one of the more accurate anatomical references for this measurement if you happen to wear that size.
Size 32L Suit Jacket Button
Button sizes in tailoring are measured in lignes, a unit from the French button industry where one ligne equals approximately 0.635mm. A button marked 32L has a diameter of 32 × 0.635mm, which equals 20.32mm. This rounds naturally to 20mm and makes the 32L the standard button size for suit jacket fronts in most ready-to-wear and bespoke tailoring.
The specific dimension matters in tailoring because buttonholes are cut to match the button diameter precisely, with a small addition for ease. A 32L button at just over 20mm requires a buttonhole of approximately 21 to 22mm in length. This is why button size notations appear in alteration instructions when replacing jacket buttons.
Read more: How Big Is 19 mm? 10 Real Objects That Show the Size
Putting 20mm in Scale
To place 20mm in a useful range of familiar sizes:
20mm sits just under the diameter of a US quarter (24.26mm), noticeably larger than an AA battery’s diameter (14.5mm), and about double the width of a standard pen clip. It is a size you can span comfortably between a thumb and index finger and see clearly with the naked eye, but it is small enough that most people would not estimate it accurately without a reference nearby.
Vera loves exploring the size and dimensions of everyday objects. She shares practical, visual guides to help readers understand measurements clearly. With a focus on accuracy and usefulness, Vera creates content that informs, engages, and supports learning for all.
