Feet and Inches Calculator - Add Lengths and Fractions

Lengths

Add, subtract, multiply and divide feet, inches and fractions as a tape measure reads them, exactly. Keep a measurement list with totals and a cut list.

Enter a length

Add, subtract, multiply or divide

Add it to the list

Choose the mark

The arithmetic is exact, and only the figures shown are rounded. The calculator does not check a measurement: measure twice before you cut, and confirm quantities with your supplier.

Formulas and sources

Each formula this calculator uses, where it comes from, and what it leaves out. "How it works" shows the same formula with your own numbers.

Feet and inches

For a length

Length = Feet × 12 + Inches
Length
the exact answer, in inches

The inches may carry a fraction of any size, and may be more than 12. Every size is kept as an exact fraction of an inch, so nothing is rounded until the answer is shown. Decimal feet are read as well: Handbook 44 allows a tape graduated in tenths or hundredths of a foot [5].

Source: Uses 12 in = 1 ft from [3]. The foot is the international foot, 0.3048 m exactly [2]; the U.S. survey foot, which NIST calls obsolete [4], is not offered.

The answer shown

For a length

Shown = Length, rounded to the chosen mark Gap = Shown − Length
Shown
the length at the precision you chose
Length
the exact answer, in inches

The mark is 1/2, 1/4, 1/8, 1/16, 1/32 or 1/64 in, 1/16 unless you change it. Nearest takes the closest mark, and a length exactly halfway goes up; Up and Down always round that way. A gap above zero means the shown length is longer than the exact one. The drawing form writes feet, a hyphen and inches, keeping a zero inch: 3'-0".

Source: The marks are binary submultiples of the inch, the subdivisions Handbook 44 allows on a linear measure [5]. 1/16 is the precision one drafting standard sets for dimensions, and the drawing form is how it writes them [6]. The rounding is standard arithmetic.

Feet and millimeters

For a length

Feet = Length ÷ 12 Millimeters = Length × 25.4

Both exact. Read the other way, a size typed in millimeters, centimeters or meters becomes inches exactly: 1 mm = 5/127 in.

Source: Uses 12 in = 1 ft from [3] and 1 in = 25.4 mm, printed as exact in [1].

Equal pieces

With equal pieces

Piece = (Length − (Pieces − 1) × Kerf) ÷ Pieces, rounded down
Piece
each equal piece, rounded down so all of them fit
Length
the exact answer, in inches

There is one cut fewer than the pieces, and each takes the kerf. The piece is rounded down to the chosen mark, never to the nearest, because a piece rounded up leaves the last one short.

Source: Standard arithmetic; no single publisher. The kerf is your own figure.

A plain number

For a plain number

Answer = a size ÷ a size of the same kind: the units cancel

How many of one size fit in another: 10' ÷ 2' 6" is 4. A length divided by a plain number is a length again, and is worked as one.

Source: Standard arithmetic; no single publisher.

Areas

For an area

Square feet = Square inches ÷ 144 Square meters = Square inches × 0.00064516
Square feet
two lengths multiplied, in square feet

Source: Uses 144 in² = 1 ft² from [3] and the square inch, 6.4516 E-04 m², printed as exact in [1].

Volumes

For a volume

Cubic feet = Cubic inches ÷ 1,728 Cubic yards = Cubic feet ÷ 27
Cubic feet
three lengths multiplied, in cubic feet

Source: Uses 1,728 in³ = 1 ft³ and 27 ft³ = 1 yd³ from [3].

Between two marks: the Feet Plus Millimeters example

Between two marks: the Feet Plus Millimeters exampleOne inch of a tape from 9' 5" to 9' 6", marked in sixteenths. The exact length, 9' 5 91/127", falls between the 11/16 and 3/4 marks. The mark shown is 9' 5 11/16", 0.029 in shorter than exact.9' 5"9' 6"Exact 9' 5 91/127"Shown 9' 5 11/16"0.029 in shorter than exact
8 ft + 450 mm is 9' 5 91/127" exactly, between the 11/16 and 3/4 marks. The calculator shows the nearest sixteenth, 9' 5 11/16", and says it is 0.029 in shorter than exact; the exact length stays beside it, so you know which way to mark.

Sources

  1. National Institute of Standards and Technology. NIST Guide to the SI, Appendix B.9: Factors for units listed by kind of quantity or field of science, length and area: the inch, 2.54 E-02 m, the foot, 3.048 E-01 m, and the square inch, 6.4516 E-04 m², all exact. Special Publication 811 (2008), web page updated August 18, 2025.Our automated tests check this calculator against 2 cases from it.
  2. National Institute of Standards and Technology. NIST Guide to the SI, Appendix B: Conversion Factors, B.6, U.S. survey foot and mile (the international foot, 0.3048 m exactly). Special Publication 811 (2008), web page updated August 18, 2025.Our automated tests check this calculator against 1 case from it.
  3. National Institute of Standards and Technology. NIST Handbook 44, Appendix C: General Tables of Units of Measurement, p. C-5 (12 in = 1 ft) and p. C-6 (144 in² = 1 ft², 1,728 in³ = 1 ft³, 27 ft³ = 1 yd³). 2026 edition.Our automated tests check this calculator against 3 cases from it.
  4. National Institute of Standards and Technology. The U.S. survey foot. Undated page, read November 2, 2026.
  5. National Institute of Standards and Technology. NIST Handbook 44, Section 5.52: Linear Measures, S.1 Units (binary submultiples of the inch; tapes in tenths or hundredths of a foot). 2026 edition.Our automated tests check this calculator against 2 cases from it.
  6. Illinois State University, Facilities Management. CAD Standards Manual, primary units: architectural lengths to a precision of 1/16 in (p. 13). Undated PDF, read November 2, 2026.Our automated tests check this calculator against 2 cases from it.

What this calculator does not cover

  • Stairs, rafters, roof pitch and right triangles: each has rules of its own.
  • Board feet of lumber, hardwood quarters, nominal sizes and the hardwood tally: the Board Foot Calculator does that.
  • Converting one length to another unit: the Unit Converter does that.
  • Room areas with waste, whole boxes and a price per area: the Square Footage Calculator does that.
  • Checking a measurement: the answer is only as good as the sizes typed.

Sources checked on .

Help

Quick Start Guide

  1. Enter a length: In the boxes, a row a length: feet, inches and a fraction, with + or − to add it or take it off. Or choose Typed Line and write it as on paper, 5' 3 1/2" or 5 ft 3 1/2 in; the line under the box says how it was read.
  2. Add, subtract, multiply or divide: Another Length adds a row; Multiply or Divide adds one that multiplies or divides the sum above by a number. The typed line also takes millimeters and areas. The answer shows as you type.
  3. Add it to the list: Add to List keeps the length or sum on the measurement list and empties the boxes for the next one. With nothing entered, the answer card shows the list's total.
  4. Choose the mark: Round to 1/2 through 1/64 of an inch, to the nearest mark, up or down. The arithmetic stays exact either way.
  5. Cut equal pieces: Type a number of pieces and the saw kerf to see the length of each, rounded down so every piece fits.

Frequently Asked Questions

Add the feet and the inches apart, then turn every 12 inches into a foot: 5' 9" + 3' 7" is 8 feet and 16 inches, or 9' 4". Fractions are added first and carried into the inches the same way. Type 5' 9" + 3' 7" and the calculator does the carrying for you.

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