In short
Most trade math comes down to five skills: converting fractions to decimals and back, using the Pythagorean theorem (the 3-4-5 rule) to square layouts, calculating offsets with angle multipliers, figuring slope and grade, and working out area and volume for materials like concrete. With those, plus Ohm's law for electrical work, you can handle almost everything a first-year apprentice runs into.
Fractions and decimals
Tape measures read in sixteenths. Calculators read in decimals. To convert, divide: 3/8 = 0.375. To go back, multiply the decimal by 16 and round: 0.6875 x 16 = 11, so 11/16.
| Fraction | Decimal |
|---|---|
| 1/16 | 0.0625 |
| 1/8 | 0.125 |
| 1/4 | 0.25 |
| 3/8 | 0.375 |
| 1/2 | 0.5 |
| 5/8 | 0.625 |
| 3/4 | 0.75 |
| 7/8 | 0.875 |
Squaring with 3-4-5
A triangle with sides of 3, 4 and 5 always has a perfect right angle. Measure 3 feet along one line and 4 feet along the other; when the diagonal between those marks reads exactly 5 feet, the corner is square. Scale up for bigger layouts: 6-8-10 or 9-12-15.
Offsets
To jog a pipe or conduit around an obstacle, the length of the angled piece (the travel) is the offset times a multiplier:
| Angle | Multiplier |
|---|---|
| 22.5° | 2.613 |
| 30° | 2.0 |
| 45° | 1.414 |
| 60° | 1.155 |
Example: a 10 inch offset with 45 degree fittings needs about 10 x 1.414 = 14.1 inches of travel, measured center to center.
Slope
Drain pipes are sized and sloped by code. A common slope for smaller drain lines is 1/4 inch per foot. Over 20 feet, that is 20 x 0.25 = 5 inches of drop. Check the code your area uses.
Concrete and volume
Concrete is ordered in cubic yards. Multiply length x width x depth in feet, then divide by 27. A 10 by 10 foot slab, 4 inches (0.333 feet) thick: 10 x 10 x 0.333 = 33.3 cubic feet, divided by 27 = about 1.23 cubic yards. Order a little extra.
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Straight answers
Is there a lot of math in the trades?
Every day, but it is practical math: fractions, measurements, angles and a few formulas.
What math do I need for an electrical apprenticeship?
Algebra for the entry test, plus fractions, Ohm's law and some trigonometry for conduit bending.
How do I get better at trade math fast?
Practice converting fractions, do offset and slope problems by hand, and take timed practice tests.