Adding Feet and Inch Fractions

Adding measurements is easier when feet, whole inches and fractions stay in their own columns. Give unlike fractions a common denominator, carry a full inch when needed, then convert every 12 inches to one foot. The method works on paper as well as on a calculator.

Keep the units separate

Write each measurement as feet, whole inches and a fractional inch. Convert the fractions to a shared denominator before adding them. OpenStax explains why equivalent fractions let you add like-sized parts [1]; NIST lists 12 inches as one foot [2].

1. Add the fractions after making their denominators match.
2. Turn any whole inch in the fractional sum into an inch and add it to the whole-inch column.
3. Add the feet. If the inches total 12 or more, carry each full 12 inches into the feet column.
4. Simplify the remaining fraction and label every unit.

Example: 2 ft 8¾ in + 1 ft 5⅞ in

Suppose two lengths are measured as 2 feet 8¾ inches and 1 foot 5⅞ inches. This original example shows both types of carry.

1. Use eighths: ¾ inch = 6/8 inch. Then 6/8 + 7/8 = 13/8 inches = 1⅝ inches.
2. Add the whole inches and the carried inch: 8 + 5 + 1 = 14 inches, with ⅝ inch remaining.
3. Add feet: 2 + 1 = 3 feet. Since 14 inches = 1 foot 2 inches, carry one more foot.

Result: 4 ft 2⅝ in.

For a single cut, use a full-size mark and verify the measurement on the actual tape before cutting. The calculated sum is a length; it does not include saw kerf, joinery allowances or trimming unless those are added separately. See how saw kerf affects a cut list.

Check by converting everything to inches

Convert the first measurement to 32¾ inches and the second to 17⅞ inches. In eighths, these are 262/8 inches and 143/8 inches. The sum is 405/8 inches, or 50⅝ inches. Because 48 inches equals four feet, the result is again 4 ft 2⅝ in.

Choose a denominator appropriate to the inputs. A tape marked in sixteenths can express ⅛ as 2/16 and ¾ as 12/16. Do not round intermediate results unless the job requires it and you have chosen the acceptable precision.

Three mistakes to avoid

• Adding denominators: ⅜ + ¼ is ⅜ + 2/8 = ⅝, not 4/12.
• Forgetting the inch carry: 7/8 + 6/8 = 13/8, or 1⅝ inches; the extra inch belongs in the inch column.
• Treating decimal feet as feet and inches: 4.5 feet means 4 feet 6 inches, not 4 feet 5 inches. The digits after the decimal describe a fraction of a foot.

Try two shorter examples

Without a foot carry: 1 ft 4⅜ in + 2 ft 3¼ in = 3 ft 7⅝ in, because ⅜ + ¼ = ⅝.

Exactly one foot of inches: 7½ in + 4½ in = 12 in = 1 ft. There are no inches left over.

Before ordering or cutting, compare the answer with your plan and the material you actually have. The cut-list guide explains how to record dimensions, quantity and allowances.

Sources and scope

1. OpenStax, Prealgebra 2e: Add and Subtract Fractions with Different Denominators. Supports equivalent fractions and common denominators; the woodworking examples here are independently calculated.
2. NIST Handbook 133, Appendix E, U.S. units of length. Supports the conversion of 12 inches to one foot; it does not prescribe a woodworking procedure.

Sources checked November 7, 2026. All worked measurements are original examples.