To add or subtract fractions, turn them into improper fractions over a common denominator, then simplify. 2 3/4 + 1 5/8 = 22/8 + 13/8 = 35/8 = 4 3/8, or 4.375. To multiply, multiply the tops and the bottoms; to divide, turn the second fraction upside down and multiply. Tape-measure inches work the same way: 4 3/8″ is 111.125 mm.
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Answer
4 3/8
As an improper fraction
35/8
As a decimal
4.375
If the numbers are inches
111.125 mm
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Common denominator = LCM of 4 and 8 = 8: 11/4 = 22/8, 13/8 = 13/8
22/8 + 13/8 = 35/8
Lowest terms: 35/8 = 4 3/8 = 4.375
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Generated with myconstructioncalculator.com. Estimates for planning and ordering only; verify against drawings and specifications.
Formula
Mixed to improper: w n/d = (w × d + n) / dAdd or subtract: a/b ± c/d = (a × d ± c × b) / (b × d), using the LCM of b and dMultiply: a/b × c/d = (a × c) / (b × d)Divide: a/b ÷ c/d = a/b × d/cSimplify: divide top and bottom by their GCF
Worked Example
2 3/4 + 1 5/8:
Improper fractions: 2 3/4 = 11/4, 1 5/8 = 13/8
Common denominator = LCM of 4 and 8 = 8: 11/4 = 22/8
22/8 + 13/8 = 35/8
35 ÷ 8 = 4 remainder 3, so the answer is 4 3/8 = 4.375 (111.125 mm if inches)
Inch Fractions as Decimals and mm
Fraction
Decimal inches
mm
1/16″
0.0625
1.5875
1/8″
0.125
3.175
3/16″
0.1875
4.7625
1/4″
0.25
6.35
5/16″
0.3125
7.9375
3/8″
0.375
9.525
7/16″
0.4375
11.1125
1/2″
0.5
12.7
5/8″
0.625
15.875
3/4″
0.75
19.05
7/8″
0.875
22.225
Frequently Asked Questions
How do you add fractions with different denominators?
Find the least common denominator (the LCM of the bottom numbers), rewrite each fraction over it and add the tops. For 3/4 + 5/8 the common denominator is 8, so 6/8 + 5/8 = 11/8 = 1 3/8.
How do you add mixed numbers?
Turn each one into an improper fraction: whole number × denominator + numerator. 2 3/4 = 11/4 and 1 5/8 = 13/8. Then 22/8 + 13/8 = 35/8 = 4 3/8.
How do you divide fractions?
Multiply by the second fraction turned upside down. 2/3 ÷ 4/9 = 2/3 × 9/4 = 18/12 = 3/2 = 1 1/2.
How do you simplify a fraction?
Divide the top and the bottom by their greatest common factor. For 12/16 the GCF is 4, so 12/16 = 3/4.
How do I add tape-measure measurements?
Add the whole inches and the fractions separately, then carry. 5 7/8″ + 3 3/4″ = 8 in + 7/8 + 6/8 = 8 + 13/8 = 9 5/8″, which is 244.5 mm.
What is 3/8 as a decimal?
0.375. Divide the top by the bottom: 3 ÷ 8 = 0.375. In millimetres, 3/8″ is 9.525 mm.
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