Feet and Inches Calculator

A feet and inches calculator handles the arithmetic behind most layout errors: fractions with different denominators, inches that roll over into feet, and divisions that land between the marks on a tape. Type measurements the way you read them, such as 12' 6 3/8", and the answer comes back in the same form, rounded to the nearest sixteenth unless you pick another mark.

Neighboring tools cover the follow-up questions: the Square Footage Calculator adds up several rooms, the Linear Feet Calculator totals boards, trim and fencing, the cm to Feet Converter handles heights in centimeters, and the Unit Converter covers every other unit.

How feet-and-inches math works

Feet, inches and fractions are three units stacked into one number, so the reliable method is to flatten everything into inches, work with ordinary decimals and convert back at the end:

The 16 in the last line is the precision: a tape graduated in sixteenths uses 16, a finer rule 32 or 64, and rough framing can live with 8. Rounding happens once, on the final answer, so a long chain of additions does not drift the way it does when each piece is rounded first.

Worked example 1: adding two tape measurements

A wall has a 12' 6 3/8" section and a 4' 9 1/2" return. By hand, add each unit separately and carry the overflow:

• Fractions: 3/8 + 1/2 = 3/8 + 4/8 = 7/8, no carry.
• Inches: 6 + 9 = 15 inches, which is 1 foot and 3 inches. Carry the foot.
• Feet: 12 + 4 + 1 = 17 feet.
• Result: 17' 3 7/8", the same as 207.875 inches, 17.3229 decimal feet or 5.28 meters.

The calculator opens with this example. Add lines for more walls, and switch any line to minus to take a length away.

Worked example 2: cutting a board, kerf included

You need a 2' 7 13/16" piece from an 8-foot board. Subtracting fractions needs borrowing: write 8' as 7' 11 16/16". Then 16/16 − 13/16 = 3/16, 11 − 7 = 4 inches and 7 − 2 = 5 feet, leaving 5' 4 3/16" (64.1875 inches).

That assumes the saw removes nothing. A blade with a 1/8-inch kerf turns that much wood into sawdust, so enter it as a third line set to minus: 8' − 2' 7 13/16" − 1/8" = 5' 4 1/16". When the offcut becomes another finished piece, that sixteenth matters.

Dividing a length: the result at every tape precision

Division rarely lands on a mark. Splitting 17' 3 7/8" into three equal pieces gives 207.875 ÷ 3 = 69.2917 inches. The table shows how that one length rounds at each common precision and how far each mark sits from the exact value:

1/8 and 1/4 give the same mark here, and the 1/32 mark misses by half as much as the 1/16 mark but in the opposite direction. Three pieces cut at 5' 9 5/16" come out 1/16 inch longer than the board, so let the last piece take up the difference.

Conversion charts: inches, decimal feet, fractions and millimeters

Plans and laser measures often use decimal feet, while tapes use inches and fractions. To turn feet into inches, multiply by 12; to turn inches into feet, divide by 12. The first chart lists each inch of a foot as decimal feet and metric:

Example: 5' 7 1/2" is 5 × 12 + 7.5 = 67.5 inches, or 5.625 feet, or 171.45 cm. The second chart gives every sixteenth of an inch as a decimal and in millimeters. Because 1 inch is exactly 25.4 mm, these metric values are exact:

For finer work, 1/32 inch is 0.03125 in (0.79375 mm) and 1/64 inch is 0.015625 in (0.396875 mm).

Reading a tape measure, and on-center layout marks

Between two inch numbers, the longest line is the half inch, then the quarters, the eighths, and the shortest lines are sixteenths. Note the last whole inch, count sixteenths past it and reduce: 9 inches plus 6 short marks is 9 6/16", or 9 3/8". Many tapes highlight every 16 inches for studs and joists, and some add small black diamonds every 19.2 inches, the spacing that divides 8 feet into five equal spaces for joists or trusses.

Those spacings let 4 × 8 sheets of plywood, OSB or drywall break on a framing member: 96 inches holds six 16-inch, five 19.2-inch or four 24-inch spaces. The marks below are measured from the end of the wall and rounded to the nearest sixteenth:

The 19.2-inch column shows why measuring from one end beats stepping the tape forward: 19.2 inches rounds to 19 3/16", and five of those add up to 95 15/16" instead of 96. Measured from the start, the fifth mark lands exactly on 8' 0".

Equal spacing for studs, balusters and shelves

The Equal spacing mode finds the number of spaces and the size of each. Without a piece width it simply divides: a 10' 5" wall (125 inches) with spaces no larger than 16 inches needs 125 ÷ 16 = 7.8, rounded up to 8 spaces of 15 5/8". When the pieces have a width of their own, such as balusters or pickets, they sit between the spaces:

For a railing with 6' (72 inches) between posts, 1 1/2" balusters and a 3 7/8" maximum gap, the calculator returns 14 spaces, 13 balusters and gaps of exactly 3 3/4", with the balusters 5 1/4" on center. Codes based on the International Residential Code do not allow guard openings that a 4-inch sphere can pass through (Baltimore's adopted code blocks a sphere "4 inches (102mm) or more in diameter"), which is why the maximum is entered just under 4 inches.

Square feet from feet and inches

Area needs decimal feet. Divide the inches by 12 and multiply the two sides: a room 12' 6" by 10' 9" is 12.5 × 10.75 = 134.375 square feet, or 19,350 square inches divided by 144. The Area mode also gives square yards (14.93), square meters (12.48) and the perimeter (46' 6") for baseboard or crown molding.

How to use this calculator

1. Pick a mode: Add / subtract, Multiply / divide, Equal spacing, Convert or Area.
2. Type the measurements as you would write them: 12' 6 3/8", 12ft 6-3/8in, 150.375", 12.5 ft, 381 cm or 2.4 m. Each box shows how it read your entry, so a typo shows up at once.
3. Choose the rounding, from 1/2 inch for rough framing to 1/64 inch for cabinet work. The default, 1/16, matches most tapes.
4. Read the result: feet and inches in large type, inches only in the badge, and decimal feet, inches and metric in the tiles, followed by the steps and the answer at every precision.

Who this calculator is for

Carpenters and remodelers use it for cut lists and wall totals where one wrong carry ruins a board. DIYers building shelves, decks or railings use the spacing mode instead of trial and error. Flooring and trim estimators turn tape readings into decimal feet for pricing, students check mixed-unit homework, and anyone converting a height such as 5' 10" gets the exact metric figure (177.8 cm).

Key terms

• Foot (ft, '): 12 inches, defined since 1959 as exactly 0.3048 meter.
• Inch (in, "): exactly 2.54 centimeters, or 25.4 millimeters.
• Precision: the smallest fraction a result is rounded to, such as 1/16 inch.
• Decimal feet: a length written as one decimal number of feet, such as 17.3229 ft.
• On center (o.c.): the distance from the center of one stud, joist or baluster to the center of the next.
• Kerf: the width of material a saw blade removes in one cut.
• U.S. survey foot: an older foot of 1200/3937 meter, about two parts per million longer, retired after December 31, 2022.

What changes the result, and tips for accurate marks

• Precision: the same division gives 5' 9 1/4" at 1/8 inch and 5' 9 5/16" at 1/16 inch. Pick the finest mark you can actually cut to.
• Where you round: rounding once at the end keeps a long sum exact; rounding each piece first shifts the total.
• Kerf: every cut removes material, so subtract the blade width when the leftover matters.
• Starting point: hook the tape at the same reference end for every layout mark.
• Burn an inch: when the hook is unreliable, measure from the 1-inch line and subtract 1 inch from every reading.
• Mark with a V: a small V with its point on the line is easier to cut to than a dash.

Limitations and related calculators

The calculator works on straight lengths. It does not handle angles, miters, arcs or rafters, and it does not know actual lumber sizes, which are smaller than their names. Equal spacing treats the ends of the run as fixed and does not shift the layout onto existing framing. For related jobs, the Fraction Calculator works with pure fractions, the Decimal to Fraction Calculator converts any decimal, the Board Feet Calculator prices lumber, the Stair Calculator handles rise and run, and the Feet to Meters Converter covers quick metric conversions.

Sources

Tape arithmetic is exact and needs no source. The unit definitions and the code rule mentioned above come from these primary documents:

• National Institute of Standards and Technology, NIST Handbook 44 (2026), Appendix C: General Tables of Units of Measurement: 1 foot = 0.3048 meter and 1 inch = 2.54 centimeters, both exact.
• National Institute of Standards and Technology, U.S. Survey Foot: the 1959 international foot and the retirement of the survey foot on January 1, 2023.
• Federal Register, Vol. 85, No. 193, October 5, 2020, Deprecation of the United States Survey Foot (85 FR 62698).
• Baltimore City Building Codes, Part X International Residential Code, Section 312.1.3, Opening limitations: one adopted example of the 4-inch sphere rule. Check the code your jurisdiction enforces.