Free Feet & Inches Calculator — Construction Math Made Easy

How to Use the Feet & Inches Calculator

How to Add and Subtract Feet, Inches, and Fractions

A feet-inches-fractions calculator performs arithmetic on construction measurements expressed in the format carpenters and contractors use every day — feet, inches, and fractional inches. Instead of converting measurements to decimals, doing the math, and converting back, this calculator accepts inputs like 5' 3-1/2" and 2' 6-3/4" and returns the exact result — 7' 10-1/4" — in the same format you read on a tape measure.

Construction math uses fractions rather than decimals because tape measures are marked in fractional increments: halves, quarters, eighths, and sixteenths. A measurement of 3/16" is easy to find on a tape, whereas its decimal equivalent (0.1875") is not marked anywhere. By keeping all arithmetic in fractions, every result maps directly to a tape measure marking — no rounding, no ambiguity, no accumulated error across a series of cuts and layouts.

Understanding Construction Measurement Formats

This calculator accepts every common construction measurement format. You can enter feet-inches-fractions like 5' 3-1/2", standalone inches like 63.5", decimal feet like 5.292ft, or metric values like 1613mm, 161.3cm, or1.613m. You can also enter bare fractions such as3/4" or mixed numbers like 7-3/4".

When you type a bare number without any unit mark, it is interpreted as decimal inches. This keeps the most common use case fast — just type a number and go. If you need to enter feet, add the foot mark (') after the number. Metric inputs are fully supported as well: append mm, cm, or m to any number and the calculator converts it to feet-inches-fractions automatically. Mix imperial and metric inputs freely within the same calculation — the conversion is handled behind the scenes.

Multiplying and Dividing Measurements

Multiplication and division use a scalar quantity (a plain number), not a second measurement. This makes physical sense: you can multiply a length by a count, but multiplying inches by inches gives square inches — a different unit entirely. Enter the measurement first, select the × or ÷ operator, then type a plain number.

For example, 5' 3" × 4 = 21' — useful when you need four identical stud lengths or want to calculate the total material for repeated spacing. 22' 6" ÷ 3 = 7' 6" divides a board into three equal sections. These operations keep the result in the same feet-inches-fractions format, ready to mark directly on your material.

Using the Running Tape

The running tape works like the paper tape on a printing calculator. Every operation you perform is recorded as a line showing the operand, the operator, and the running total. This gives you a complete audit trail of every step in a multi-part calculation — essential when adding up a series of wall lengths for a perimeter or totaling cut-list measurements for a material order.

Scroll back through the tape at any time to double-check your entries. If you spot an error partway through a long calculation, you can identify exactly which entry was wrong without starting over. The tape also serves as documentation you can reference later on the job site.

Memory Registers

Three independent memory slots — M1, M2, andM3 — let you store intermediate values while you work on a separate calculation. Tap Store to save the current result into a register, Recall to bring a saved value back into the display, andClear to reset a register.

A typical use case: you are calculating the perimeter of a room with walls of different lengths. Store the sum of the first two walls in M1, calculate the remaining walls separately, then recall M1 and add them together. Memory registers prevent you from losing intermediate results when a calculation branches into multiple steps.

Pro Tips

• Always double-check critical measurements by reviewing the running tape — a single miskeyed digit in a long addition chain can throw off your entire material order.
• Use memory registers for complex multi-step calculations, such as computing a room perimeter when opposite walls have slightly different lengths. Store each pair of walls separately, then add the totals.
• When entering a plain number for multiply or divide, do not add foot or inch marks — just type the number. The calculator treats it as a scalar quantity, not a measurement.
• Mix imperial and metric inputs freely within a single calculation. Enter a wall length in feet-inches and a tile size in millimeters — the calculator converts automatically and shows the result in all formats.
• Use the precision selector to match your tape measure markings. Most standard tapes are marked to 1/16", so selecting 1/16" precision ensures every result lands on an actual tape mark.
• For quick unit conversions, enter a measurement in one format and read the result in another — type 1000mm and see the feet-inches-fractions equivalent instantly, or type 3' 4-1/2" and read the metric conversion.