# How To Calculate The Grade Of A Slope

In elementary school mathematics, when students learn to graph simple linear functions, they are introduced to the concept of a **slope**.

A linear function is just one with a graph represented by a straight line of some sort, with its placement and direction in relation to the *x*- and *y*-axes depending on the function's properties.

A linear equation has the form

\(y=mx+b\)

Where *y* is the dependent variable, *m* is the slope, and *b* is a quantity called the *y*-intercept, the point the line crosses on the *y*-axis.

But you may have also heard of a mathematical construct called a **grade**, or a percent grade. Muddled, ambiguous terms like "slope ratio" and "grade of slope" don't help.

Are slopes and grades related? They are indeed, and both are indispensable in mathematics and engineering.

## What is Slope?

In everyday terms, a slope is a steady, sustained climb or descent. That's what it means in mathematics as well, but in a more formal way. *The slope of a line is the change in vertical (y) distance per one-unit change in horizontal (x) distance.*

For example, if a point in a coordinate system moves 11 units in the positive *x*-direction and four units in the negative *y*-direction, the slope is (–4)/(11) = –0.364. The minus sign means the line angles "downhill" in relation to the horizontal *x*-axis.

A horizontal line such as the function *y* = 5, in which there is no vertical change throughout, has a slope of 0. **A vertical line,** such as *x* = −3**, has an undefined slope** as there is no horizontal change and dividing by zero is not permitted in mathematics.

## The Point-Slope Formula

The point-slope formula is helpful for determining the equation of a line when either two points or one point and the slope are known. It has the form

\(y − y_0 = m(x − x_0)\)

If you were given coordinates (12, −7) and told that the graph of the function had a slope of 1.25, you could determine the general equation:

\((y − (−7)) = 1.25(x − 12)\)

\((y + 7) = 1.25x −15\)

\(y = 1.25x − 22\)

## Percent Grade

Grade, or **percent grade**, is just the slope expressed as a percentage. It is often used in real-life situations involving the construction of roads, the very steepest of which have surprisingly low slope values.

For example, the Pennsylvania Turnpike in the Eastern U.S. has a maximum slope of 0.03, meaning it rises or falls no more 3 feet for every 100 horizontal feet traveled over any segment. The percent grade in this instance is 100 × 0.03 = 3 percent.

In trigonometry, *y*/*x*, or **"rise over run,"** is also the tangent of the angle formed by the ascending or descending line and the horizontal. This means that the inverse tangent (tan ^{−1} or arctan on a calculator) of the slope equals this angle.

•In the grueling Tour de France, a three-week race through the mountains of Western Europe featuring the best male cyclists in the world, grades that reach 13 percent are considered extraordinarily fierce.

## Slope Distance Calculator

If you know the slope of a line, you can calculate horizontal distance traveled as a function of vertical distance, or the other way around. Say you know you're walking up a 4 percent grade. If you walk for 30 minutes and your horizontal position changes at a rate of 4 miles per hour, how much elevation have you gained?

4 mph for 30 min (1/2 hr) is 2 miles, and if the percent grade is 4, the slope is 4/100 = 0.04. Since slope is rise over run and in this case the "run" is 2 miles, the vertical gain can be found as follows:

\(\begin{aligned}

0.04 &= \frac{y}{2 \;\text{miles}}\)

\(y &= 0.04×2 \

&= 0.08 \;\text{miles, or about} \ &0.08 \;\text{mi}×5,280 \;\text{ft/mi} = 422 \;\text{ft}

\end{aligned}\)

### Cite This Article

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Beck, Kevin. "How To Calculate The Grade Of A Slope" *sciencing.com*, https://www.sciencing.com/calculate-grade-slope-5135843/. 21 December 2020.

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Beck, Kevin. How To Calculate The Grade Of A Slope last modified March 24, 2022. https://www.sciencing.com/calculate-grade-slope-5135843/