Feet and Inches Calculator: Add, Subtract, and Convert

A feet and inches calculator helps you work with the mixed units used on U.S. building drawings. Use this tool to add dimension strings, subtract an opening from an overall length, divide a dimension into equal segments, or convert fractional inches to decimal feet and millimeters.

You can enter feet, inches, and fractions directly. For example, 12′-7½″ + 8′-3¾″ = 20′-11¼″. The calculator also shows decimal outputs so you can use the same measurement in a spreadsheet or a drawing workflow.

How to Use the Feet and Inches Calculator

1. Choose a calculation. Select addition, subtraction, equal division, or conversion.
2. Enter Length A and its input format. Use feet-and-inches notation, decimal feet, inches, or millimeters.
3. Enter Length B when needed. A and B may use different units. For division, enter the number of equal segments instead.
4. Choose the fraction display precision. This controls the architectural result, such as the nearest 1/16 inch.
5. Select Calculate. Read the fractional result and the decimal conversions together. Use Copy result to keep the calculation with your notes.

Accepted Input Examples

Use a period for decimal values and omit thousands separators. In feet-and-inches mode, an unmarked number is interpreted as inches: 12 means 12 inches, while 12' means 12 feet. A leading minus sign applies to the entire length. The input boxes accept one length each; choose subtraction from the menu instead of typing a full arithmetic expression.

Feet and Inches vs. Decimal Feet

12′-6″ is 12.5 feet, not 12.6 feet. The digits after a decimal point represent a fraction of a foot, not a number of inches. To convert the inches portion to feet, divide it by 12.

This distinction matters when moving a dimension from a plan into a spreadsheet. A value can look reasonable while representing a different length. Keep the unit visible beside the number, especially when sharing a calculation.

Worked Examples for Drawing Checks

1. Add Two Dimension Segments

Suppose two adjacent dimension segments are 12′-7½″ and 8′-3¾″. Convert each to total inches before adding:

• First segment: 12 × 12 + 7.5 = 151.5 inches.
• Second segment: 8 × 12 + 3.75 = 99.75 inches.
• Total: 151.5 + 99.75 = 251.25 inches = 20′-11¼″.

This is useful for checking whether a chain of dimensions agrees with an overall dimension. For more than two segments, use the unrounded total-inches result as the next input, then add the next segment.

2. Subtract an Opening from an Overall Length

An overall dimension is 16′-0″, and an opening is 3′-8½″. The remaining combined length is:

192 − 44.5 = 147.5 inches = 12′-3½″.

This gives the total length remaining outside the opening. It does not locate the opening or split the remaining length between its two sides; those depend on the dimension layout.

3. Divide a Length into Four Equal Segments

For a total dimension of 10′-3″, select equal division and enter 4:

123 inches ÷ 4 = 30.75 inches = 2′-6¾″ per segment.

Equal segments are not automatically equal clear gaps between posts. A clear-gap layout must first account for the widths of the posts and any end conditions. Likewise, a center-to-center layout needs a defined starting and ending centerline.

4. Convert a Metric Dimension

To convert 1,000 mm, select Convert A only, enter 1000, and choose Millimeters. The result is approximately 39.370079 inches, or 3′-3⅜″ when displayed to the nearest 1/16 inch.

The fractional display corresponds to 1,000.125 mm. That small difference is rounding, not a different conversion factor. Keep the original metric dimension when metric precision is required.

Fractional Inch to Decimal and Millimeter Chart

The chart covers sixteenth-inch increments within one inch. These decimal conversions are exact for the listed fractions; 1 inch equals exactly 25.4 mm.

Conversion Formulas

• Total inches = feet × 12 + inches.
• Decimal feet = total inches ÷ 12.
• Millimeters = total inches × 25.4.
• Total inches from mm = millimeters ÷ 25.4.
• Equal segment length = total length ÷ number of segments.

All calculations first convert inputs to inches. The tool then performs the selected operation and converts the result to the displayed units. It uses the international foot: exactly 0.3048 meters, or 304.8 millimeters.

Rounding: What the Display Means

The fraction setting changes only the architectural feet-and-inches display. Decimal feet, total inches, and millimeters are calculated from the result before fractional rounding and displayed to a maximum of six decimal places. Changing from 1/16 to 1/8 inch does not change the underlying calculation.

The rounding difference shows how far the displayed fractional dimension lies above or below the calculated length. At an exact half-step, the fraction display rounds away from zero. A finer display setting does not make an original field measurement more accurate.

Frequently Asked Questions

Can I enter fractions without changing them to decimals?

Yes. Enter 3/8, 7 1/2, or a full feet-and-inches dimension. Common fraction characters such as ½, ¼, ¾, ⅛, ⅜, ⅝, and ⅞ are also supported. A denominator of zero is rejected.

Why did the result become negative?

Subtracting a larger length from a smaller one produces a negative result. For example, 3 feet minus 4 feet equals −1 foot. The sign describes the arithmetic difference; it is not a physical negative cut length.

Does this calculate a steel member’s fabrication length?

It can add or subtract dimensions you supply, but it does not determine connection clearances, end setbacks, weld gaps, saw kerf, or other detailing allowances. Start with the approved drawing dimensions and apply the relevant allowances explicitly.

Can I use the calculator for area or volume?

This tool handles lengths only. Square feet and cubic feet require different calculations. For rectangular steel plate weight, use the separate plate calculator below.

Related Tables and Calculators

• W-Beam dimensions and properties table — look up a beam’s actual dimensions before checking a layout.
• HSS sizes and properties table — compare hollow-section dimensions in imperial and metric units.
• Steel plate weight calculator — enter plate length, width, thickness, and quantity.

Reference basis: NIST, SI Units—Length, and NIST Guide to the SI, Appendix B: Conversion Factors. Unit definitions: 1 inch = 25.4 mm exactly; 1 international foot = 0.3048 m exactly. This tool performs dimensional arithmetic and unit conversion; it does not evaluate structural capacity or code compliance.