Kelly Pipe Decimal Calculator: Sizes & Decimals

The Kelly Pipe Decimal Calculator converts between fractional drill pipe sizes (like 4-1/2″) and their decimal inch equivalents, and it can also estimate the weight per foot of steel drill pipe based on outside diameter and wall thickness. You enter the pipe’s outside diameter and wall thickness in inches, and the calculator returns the decimal equivalent of any fractional size, plus the cross-sectional area and approximate weight per foot.

Formula

The calculator uses two basic formulas. First, for the decimal equivalent of a fractional size, the fraction is simply converted to a decimal: numerator divided by denominator. For example, 4-1/2 inches = 4 + 1/2 = 4.5 inches. Second, the cross-sectional area of the pipe wall is calculated as:

Area = (π/4) × (OD² − ID²)

where OD is the outside diameter and ID is the inside diameter (OD − 2 × wall thickness). The weight per foot is then estimated by multiplying the cross-sectional area by the density of steel (about 0.2836 lb/in³), which gives approximately 3.402 lb per inch² of cross-section per foot.

Worked Example

Using the default values of OD = 4.5 in and wall thickness = 0.362 in:

1. Inside diameter = 4.5 − 2×0.362 = 4.5 − 0.724 = 3.776 in
2. Area = (π/4) × (4.5² − 3.776²) = 0.7854 × (20.25 − 14.258) = 0.7854 × 5.992 = 4.706 in²
3. Weight per foot = 4.706 × 3.402 = 16.0 lb/ft

The calculator shows the fractional size as 4-1/2 (since 4.5 = 4 + 1/2), the area as 4.71 in², and the weight as 16.0 lb/ft.

Why Use a Decimal Equivalent?

Drill pipe sizes are often given in fractions (e.g., 3-1/2″, 5″), but many engineering calculations—such as tensile strength, burst pressure, or fluid displacement—require decimal inches. Using a decimal equivalent avoids mistakes when plugging numbers into formulas or comparing with API specifications that list decimals.

For example, a 2-7/8″ pipe is 2.875 inches in decimal form. If you accidentally use 2.7 or 2.8, your calculations will be off. A quick decimal conversion ensures accuracy.

How to Choose the Right Wall Thickness

The wall thickness of drill pipe is a key factor in determining its strength and weight. Common API wall thicknesses for drill pipe range from about 0.250″ to 0.500″ depending on the size and grade. Heavier wall thickness increases weight per foot and burst resistance but reduces the inside diameter, which affects fluid flow and tool clearance.

When choosing a wall thickness, consider the depth of the well, the mud weight, and the tensile loads. A thicker wall adds strength but also adds weight, which increases the load on the derrick and the cost of the pipe.

Common Mistakes and Misconceptions

• Confusing OD and ID: Always use the outside diameter in the formula, not the nominal size. For example, a 2-3/8″ pipe may have an actual OD of 2.375″.
• Forgetting to subtract the wall thickness twice: The inside diameter is OD minus 2×wall thickness, not minus wall thickness.
• Using the wrong density: The weight formula assumes steel. If you are working with aluminum drill pipe, the density is about 0.098 lb/in³, so the weight would be about 1.176 lb per inch² of cross-section per foot.
• Rounding too early: Always keep full precision in intermediate steps to avoid rounding errors in the final result.

Limitations of the Formula

The weight per foot formula is an approximation that assumes a solid steel pipe with no tool joints, no internal upset, and no coatings. Actual drill pipe has thicker ends (tool joints) that add weight, and the pipe may be internally or externally upset. The formula gives the weight of the pipe body only, not the entire joint.

For exact weights, refer to the API specification or the manufacturer’s data sheet. The formula is useful for quick estimates during planning or field calculations.

Frequently Asked Questions

What is a decimal equivalent of a fraction?

A decimal equivalent is simply the fraction expressed as a decimal number. For example, 1/2 = 0.5, 3/4 = 0.75, and 5-1/2 = 5.5. This is useful in engineering because most formulas and tables use decimals rather than fractions.

How do I convert a fraction to a decimal?

Divide the numerator by the denominator. For a mixed number like 4-1/2, first convert it to an improper fraction (9/2) and then divide 9 by 2 to get 4.5. Alternatively, just add the whole number to the decimal of the fraction: 4 + 0.5 = 4.5.

What is the weight of a 4-1/2″ drill pipe?

The weight depends on the wall thickness. For a common 4-1/2″ pipe with a wall thickness of 0.362″, the weight per foot is about 16.0 lb/ft. For a thinner wall of 0.250″, the weight would be about 11.3 lb/ft. Check the API specifications for the exact weight of the specific grade and wall.

Is the weight per foot the same as the total weight of the pipe?

No, the weight per foot is only the pipe body. The total weight of a joint includes the tool joints (the thicker ends) and any other attachments. For a full joint, you need to add the weight of the tool joints, which can add several hundred pounds to a 30-foot joint.

Can I use this calculator for aluminum drill pipe?

Yes, but you would need to change the density factor. The formula uses a factor of 3.402 for steel. For aluminum, use 1.176 (0.098 lb/in³ × 12). The cross-sectional area calculation remains the same.

Why does the calculator show a fractional size as a decimal?

The calculator displays the fractional size as a decimal because that is what you entered. If you enter 4.5, it shows “4-1/2� as a convenience. But the underlying value is decimal, which is what you need for calculations.

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in od | Outside diameter | in | 4.5 | 0
in wt | Wall thickness | in | 0.362 | 0
out frac | Fractional size | - | frac(od) | 0
out area | Cross-section area | in² | (pi/4)*(od^2 - (od-2*wt)^2) | 2
out weight | Weight per foot | lb/ft | area * 3.402 | 1