COAL Cartridge Overall Length Calculator

Enter your case length, bullet length, seating depth, and headspace to calculate cartridge overall length (COAL), protrusion, and SAAMI fit.

Created by Luis GonzalezLast updated: May 9, 2026

How to Use This Calculator

1
Enter Case Length
Input the measured length of your cartridge case in inches.
2
Specify Bullet Length
Enter the total length of your projectile from base to tip in inches, measured with calipers.
3
Input Seating Depth
Provide the depth in inches to which the bullet is seated into the case neck.
4
Enter Headspace Length
Input the headspace length in inches, which is the distance from the breech face to the chamber shoulder (for bottleneck cartridges) or case mouth (for straight-wall cases).
5
Review Your Results
The calculator will display the Cartridge Overall Length (COAL), bullet protrusion, seating depth percentage, and base-to-ogive length.

Example Calculation

A reloader is assembling .308 Winchester cartridges with a case length of 2.015 in, a 1.200 in bullet, a seating depth of 0.390 in, and a headspace length of 0.125 in.

Case Length (in)

2.015

Bullet Length (in)

1.200

Seating Depth (in)

0.390

Headspace Length (in)

0.125

Results

2.8250 in

Tips

Measure Consistently

Always use the same calipers and technique when measuring components. Small variations in measurement can lead to significant differences in COAL, affecting pressure and accuracy.

Check Magazine Fit

Before finalizing your COAL, ensure the assembled cartridge fits and feeds reliably from your firearm's magazine. An overly long cartridge may not cycle properly.

Monitor for Pressure Signs

When developing new loads, start with a conservative COAL and watch for signs of excessive pressure (e.g., flattened primers, ejector marks) as you adjust. Small changes in seating depth can dramatically alter chamber pressure.

Precision Reloading: Calculating Cartridge Overall Length (COAL)

The COAL Cartridge Overall Length Calculator provides essential measurements for ammunition reloaders, including the precise cartridge overall length, bullet protrusion, seating depth percentage, and base-to-ogive length. This tool is critical for ensuring both safety and accuracy in hand-loaded ammunition, allowing reloaders to check their work against SAAMI compliance instantly. Precise control over COAL is paramount for optimizing bullet jump, managing chamber pressure, and achieving consistent performance from your firearm.

Precision in Ammunition Reloading for Optimal Performance

Precise cartridge dimensions are fundamental to the reliability, safety, and accuracy of firearms. The Cartridge Overall Length (COAL) is a critical specification, as even minor variations can profoundly affect chamber pressure, bullet jump (the distance a bullet travels before engaging the rifling), and the ability of a round to feed smoothly from a magazine, especially in semi-automatic firearms. Adhering to SAAMI (Sporting Arms and Ammunition Manufacturers' Institute) standards for maximum COAL is essential to prevent dangerous overpressure conditions and ensure safe interchangeability of ammunition across different firearms, making meticulous measurement a cornerstone of responsible reloading.

The Mathematics of Cartridge Assembly

The calculation for Cartridge Overall Length (COAL) is a simple sum and subtraction based on the individual components of a cartridge. Other derived measurements provide further insight into the bullet's seating.

Cartridge Overall Length (COAL) = Case Length + Bullet Length - Seating Depth
Bullet Protrusion = Bullet Length - Seating Depth
Base-to-Ogive Length = COAL - Headspace Length
Seating Depth Percentage = (Seating Depth / Bullet Length) × 100

Here, Case Length is the brass casing, Bullet Length is the projectile, and Seating Depth is how far the bullet is inserted into the case. Headspace Length is used for an approximate base-to-ogive measurement.

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Assembling a .308 Winchester Cartridge

A reloader is preparing .308 Winchester ammunition. The components measure: case length 2.015 in, bullet length 1.200 in, seating depth 0.390 in, and headspace length 0.125 in.

Enter Case Length: Input "2.015".
Enter Bullet Length: Input "1.200".
Enter Seating Depth: Input "0.390".
Enter Headspace Length: Input "0.125".
Calculate Cartridge Overall Length (COAL): 2.015 + 1.200 - 0.390 = 2.8250 in.
Calculate Bullet Protrusion: 1.200 - 0.390 = 0.8100 in.
Calculate Base-to-Ogive Length: 2.8250 - 0.125 = 2.7000 in.
Calculate Seating Depth Percentage: (0.390 / 1.200) × 100 = 32.50%.

The resulting cartridge has an overall length of 2.8250 inches, which the reloader must verify against SAAMI maximums and their firearm's chamber.

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The Development of Cartridge Standardization

The journey of ammunition standardization began in earnest with the advent of metallic cartridges in the mid-19th century, replacing earlier, less consistent loading methods. As firearms manufacturing industrialized, the need for safe and interchangeable ammunition became paramount. Early efforts were often proprietary, leading to a confusing array of cartridges. The establishment of organizations like the Sporting Arms and Ammunition Manufacturers' Institute (SAAMI) in the United States in 1926 marked a pivotal moment. SAAMI provided a common framework for industry standards, including critical dimensions like Cartridge Overall Length (COAL), chamber specifications, and pressure limits. This formalized approach ensured that ammunition produced by one manufacturer would safely and reliably function in firearms from another, laying the groundwork for the modern, highly regulated firearms and ammunition industry.

Frequently Asked Questions

What is Cartridge Overall Length (COAL) in reloading?

Cartridge Overall Length (COAL) is the total length of an assembled cartridge from the base of the case to the tip of the bullet, measured in inches or millimeters. It is a critical dimension in ammunition reloading, directly impacting chamber pressure, bullet jump to the rifling, and the cartridge's ability to feed reliably from a magazine, making precise measurement essential for safety and accuracy.

Why is COAL important for firearm accuracy?

COAL significantly influences firearm accuracy by affecting the 'bullet jump,' which is the distance a bullet travels before engaging the barrel's rifling. A consistent and optimal bullet jump, often between 0.010 and 0.030 inches from the lands, can improve projectile stability and reduce variations in pressure, leading to tighter groups and enhanced precision, especially in precision shooting disciplines.

What is 'bullet jump'?

Bullet jump, also known as freebore, is the distance a bullet travels from the point it exits the cartridge case until it engages the rifling in the barrel. Optimizing this distance through precise COAL adjustments can significantly impact accuracy, as too much or too little jump can lead to inconsistent pressures and bullet flight, affecting group sizes at the target.

What are SAAMI standards for COAL?

SAAMI (Sporting Arms and Ammunition Manufacturers' Institute) establishes voluntary industry standards for cartridge and chamber dimensions, including maximum Cartridge Overall Length (COAL), to ensure safety and interchangeability across firearms. Adhering to SAAMI maximums prevents overpressure issues and ensures reliable feeding, though reloaders often develop loads with COALs slightly different from SAAMI specs for optimized accuracy in specific firearms.