Estimate shotgun bore diameter in inches, millimeters, and thousandths from a gauge number, convert a measured bore to its theoretical gauge equivalent, or compare measurements with the same formula’s reference. These are dimensional estimates, not manufacturer specifications.
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Gauge to Caliber Formula
The following formulas are used to calculate the approximate bore diameter from a shotgun gauge.
Variables:
- d(in) is the bore diameter in inches
- d(mm) is the bore diameter in millimeters
- G is the shotgun gauge number
To calculate gauge to caliber, divide 1.67 by the cube root of the gauge number to estimate the bore diameter in inches. If you need the answer in millimeters, multiply the inch value by 25.4.
What is Gauge to Caliber?
Gauge to caliber conversion is the process of turning a shotgun gauge number into an approximate bore diameter. Unlike caliber, which is a direct measurement of diameter, gauge is based on how many round lead balls of the same diameter as the bore would equal one pound. Because of that definition, a higher gauge number actually means a smaller bore diameter.
This relationship is approximated here with d(in) = 1.67 / ∛(gauge); the inverse is G = (1.67 / d(in))³. For example, a 12-gauge shotgun has an approximate bore diameter of 0.729 inches, while a 20-gauge shotgun has an approximate bore diameter of about 0.615 inches. This makes gauge-to-caliber conversion useful for comparison, reference, and educational purposes.
How to Calculate Gauge to Caliber?
The following steps outline how to calculate the approximate caliber from a shotgun gauge.
- First, determine the shotgun gauge number (G).
- Next, take the cube root of the gauge number.
- Then divide 1.67 by that cube root to find the bore diameter in inches.
- If needed, multiply the inch value by 25.4 to convert the result to millimeters.
- Finally, check your answer with the converter above.
Gauge to Caliber Conversion Table
The following table lists rounded common nominal bore references in inches and millimeters. The widget consistently uses the approximate 1.67 / ∛G model, so its extra digits may differ slightly from these rounded references. .410 bore denotes 0.410 inches and is not a gauge number.
| Gauge | Bore Diameter (inches) | Bore Diameter (mm) |
|---|---|---|
| 10 gauge | 0.775 in | 19.7 mm |
| 12 gauge | 0.729 in | 18.5 mm |
| 16 gauge | 0.662 in | 16.8 mm |
| 20 gauge | 0.615 in | 15.6 mm |
| 24 gauge | 0.580 in | 14.7 mm |
| 28 gauge | 0.550 in | 14.0 mm |
| 32 gauge | 0.526 in | 13.4 mm |
These values are approximate references, not manufacturing tolerances or an ammunition-compatibility test. Use manufacturer specifications for an actual firearm. The important trend is that bore diameter decreases as the gauge number increases.
Example Problem :
Use the following variable as an example problem to test your knowledge.
Shotgun Gauge (G) = 12
Cube Root of Gauge = ∛12 ≈ 2.289
Bore Diameter = 1.67 / 2.289 ≈ 0.729 inches
Bore Diameter in Millimeters = 0.729 × 25.4 ≈ 18.5 mm
