Slopes, Ratios, and Percentages — Construction Math

Slope as percentage

Slope can be expressed as rise per run, as a ratio, or as a percentage. A 1/4 inch per foot drainage slope is about 2 percent. ADA-compliant ramps are limited to 1:12, which is about 8.3 percent. Pick the unit your customer or inspector uses and stick with it.

Pythagorean theorem

For right triangles, a squared plus b squared equals c squared. The 3-4-5 triangle is the contractor's friend for squaring foundations. Measure 3 feet along one side, 4 feet along the other, and the diagonal should be exactly 5 feet if the corner is square. Scale up to 6-8-10 or 9-12-15 for larger work.

Ratios and proportions

A 3:1 mortar mix means 3 parts sand to 1 part cement. A 1:2:4 concrete mix is cement to sand to gravel. When scaling up, multiply each part by the same factor. When ordering, calculate the bag count by the dry volume of the recipe.