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The 31 Critical ACT Math Formulas You MUST Know

Posted by Courtney Montgomery | May 26, 2015 12:38:41 PM

ACT Math



The two biggest challenges of the ACT math test are the time crunch--the math test has 60 questions in 60 minutes!--and the fact that the test doesn’t provide you with any formulas. All the formulas and math knowledge for the ACT comes from what you’ve learned and memorized.

In this complete list of critical formulas you'll need on the ACT, I'll lay out every formula you MUST have memorized before test day, as well as explanations for how to use them and what they mean. I'll also show you which formulas you should prioritize memorizing (the ones that are needed for multiple questions) and which ones you should memorize only when you've got everything else nailed down tight. 


Already Feeling Overwhelmed?

Does the prospect of memorizing a bunch of formulas make you want to run for the hills? We've all been there, but don't throw in the towel just yet! The good news about the ACT is that it is designed to make anyone be able to succeed. Many of you will already be familiar with most of these formulas from your math classes.

The formulas that show up on the test the most will also be most familiar to you. Formulas that are only needed for one or two questions on the test will be least familiar to you. For example, the equation of a circle and logarithm formulas only ever show up as one question on most ACT math tests. If you’re going for every point, go ahead and memorize them. But if you feel overwhelmed with formula lists, don’t worry about it--it’s only one question.

So let’s look at all the formulas you absolutely must know before test day (as well as one or two that you can figure out yourself instead of memorizing yet another formula).



Linear Equations & Functions

There will be at least 5-6 questions on linear equations and functions on every ACT test, so this is a very important section to know.



Slope is the measure of how a line changes. It’s expressed as: the change along the y-axis/the change along the x-axis, or $\rise/\run$.

    • Given two points, $A(x_1,y_1)$, $B(x_2,y_2)$, find the slope of the line that connects them:

$$(y_2 - y_1)/(x_2 - x_1)$$


Slope-intercept form

  • A linear equation is written as $y=mx+b$
    • m is the slope and b is the y-intercept (the point of the line that crosses the y-axis)
    • A line that passes through the origin (y-axis at 0), is written as $y=mx$
    • If you get an equation that is NOT written this way (i.e. $mx−y=b$), re-write it into $y=mx+b$


Midpoint formula

  • Given two points, $A(x_1,y_1)$, $B(x_2,y_2)$, find the midpoint of the line that connects them:

$$((x_1 + x_2)/2, (y_1 + y_2)/2)$$

Good to know

Distance formula

  • Find the distance between the two points

$$√{(x_2 - x_1)^2 + (y_2 - y_1)^2}$$

  • You don’t actually need this formula, as you can simply graph your points and then create a right triangle from them. The distance will be the hypotenuse, which you can find via the pythagorean theorem



There will usually only be 1 question on the test involving logarithms. If you’re worried about having to memorize too many formulas, don’t worry about logs unless you’re trying for a perfect score.

$log_bx$  asks “to what power does b have to be raised to result in x?”

  • Most of the time on the ACT, you’ll just need to know how to re-write logs

$$log_bx=y => b^y=x$$


$$log_b{x/y} = log_bx - log_by$$


Statistics and Probability


The average is the same thing as the mean

  • Find the average/mean of a set of terms (numbers)

$$\Mean = {\sum\of\the\terms}/{\the\number(\amount)\of\different\terms}$$

  • Find the average speed

$$\Speed = {\total\distance}/{\total\time}$$



May the odds be ever in your favor. 



Probability is a representation of the odds of something happening. A probability of 1 is guaranteed to happen. A probability of 0 will never happen.


  • Probability of two, mutually exclusive, outcomes both happening is


  • e.g., Event A has a probability of $1/4$ and event B has a probability of $1/8$. The probability of both events happening is: $1/4 * 1/8 = 1/32$. There is a 1 in 32 chance of both events A and event B happening.



The possible amount of different combinations of a number of different elements

  • A “combination” means the order of the elements doesn’t matter (i.e. a fish entree and a diet soda is the same thing as a diet soda and a fish entree)
    • Possible combinations = number of element A * number of element B * number of element C….
    • e.g. In a cafeteria, there are 3 different dessert options, 2 different entree options, and 4 drink options. How many different lunch combinations are possible, using one drink, one, dessert, and one entree?
      • The total combinations possible = 3 * 2 * 4 = 24



  • Find x percent of a given number n


  • Find out what percent a number n is of another number m


  • Find out what number n is x percent of



The ACT is a marathon. Remember to take a break sometimes and enjoy the good things in life. Puppies make everything better. 







  • l is the length of the rectangle
  • w is the width of the rectangle





Rectangular solid



$$\Volume = lwh$$

  • h is the height of the figure



An easy way to get the area of a parallelogram is to drop down two right angles for heights and transform it into a rectangle.

  • Then solve for h using the pythagorean theorem



  • (This is the same as a rectangle’s lw. In this case the height is the equivalent of the width)





$$\Area = {1/2}bh$$

  • b is the length of the base of triangle (the edge of one side)
  • h is the height of the triangle
    • The height is the same as a side of the 90 degree angle in a right triangle. For non-right triangles, the height will drop down through the interior of the triangle, as shown in the diagram.


Pythagorean theorem

$$a^2 + b^2 = c^2$$

  • In a right triangle, the two smaller sides (a and b) are each squared. Their sum is the equal to the square of the hypotenuse (c, longest side of the triangle)




Properties of special right triangle: isosceles triangle

  • An isosceles triangle has two sides that are equal in length and two equal angles opposite those sides
  • An isosceles right triangle always has a 90 degree angle and two 45 degree angles.
  • The side lengths are determined by the formula: x, x, x√2, with the hypotenuse (side opposite 90 degrees) having a length of one of the smaller sides * √2.
    • E.g., An isosceles right triangle may have side lengths of 12, 12, and 12√2


Properties of special right triangle: 30, 60, 90 degree triangle

  • A 30, 60, 90 triangle describes the degree measures of its three angles
  • The side lengths are determined by the formula: x, x√3, and 2x.
    • The side opposite 30 degrees is the smallest, with a measurement of x
    • The side opposite 60 degrees is the middle length, with a measurement of x√3
    • The side opposite 90 degree is the hypotenuse, with a length of 2x
    • For example, a 30-60-90 triangle may have side lengths of 5, 5√3, and 10.




  • Take the average of the length of the parallel sides and multiply that by the height

$$\Area = [(\parallel\side\a + \parallel\side\b)/2]h$$

  • Often, you are given enough information to drop down two 90 angles to make a rectangle and two right triangles. You’ll need this for the height anyway, so you can simply find the areas of each triangle and add it to the area of the rectangle, if you would rather not memorize the trapezoid formula
  • Trapezoids and the need for a trapezoid formula will be at most one question on the test. Keep this as a minimum priority if you're feeling overwhelmed. 






  • π is a constant that can, for the purposes of the ACT, be written as 3.14 (or 3.14159)
    • Especially useful to know if you don’t have a calculator that has a π feature or if you're not using a calculator on the test.
  • r is the radius of the circle (any line drawn from the center point straight to the edge of the circle)


Area of an arc

  • Given a radius and a degree measure of an arc from the center, find the circumference of the arc
  • Use the formula for the area multiplied by the angle of the arc divided by the total angle measure of the circle

$$Area\of\an\arc = (πr^2)(\degree\measure\of\center\of\arc/360)$$






  • d is the diameter of the circle. It is a line that bisects the circle through the midpoint and touches two ends of the circle on opposite sides. It is twice the radius.


Circumference of an arc

  • Given a radius and a degree measure of an arc from the center, find the circumference of the arc
  • Use the formula for the circumference multiplied by the angle of the arc divided by the total angle measure of the circle (360)

$$\Circumference\of\an\arc = (2πr)(\degree\measure\center\of\arc/360)$$

    • Example: A 60 degree arc has $1/6$ of the total circle's circumference because $60/360 = 1/6$

An alternative to memorizing the “formulas” for arcs is to just stop and think about arc circumferences and arc areas logically.

    • If you know the formulas for the area/circumference of a circle and you know how many degrees are in a circle, put the two together.
      • If the arc spans 90 degrees of the circle, it must be $1/4$th the total area/circumference of the circle, because $360/90 = 4$. 
      • If the arc is at a 45 degree angle, then it is $1/8$th the circle, because $360/45 = 8$.
    • The concept is exactly the same as the formula, but it may help you to think of it this way instead of as a “formula” to memorize.


Equation of a circle

  • Useful to get a quick point on the ACT, but don’t worry about memorizing it if you feel overwhelmed; it will only ever be worth one point.
  • Given a radius and a center point of a circle $(h, k)$

$$(x - h)^2 + (y - k)^2 = r^2$$







Almost all the trigonometry on the ACT can be boiled down to a few basic concepts


Sine, cosine, and tangent are graph functions

  • The sine, cosine, or tangent of an angle (theta, written as Θ) is found using the sides of a triangle according to the mnemonic device SOH, CAH, TOA


Sine - SOH

$$\Sine‌ Θ = \opposite/\hypotenuse$$

    • Opposite = the side of the triangle directly opposite the angle Θ
    • Hypotenuse = the longest side of the triangle
  • Sometimes the ACT will make you manipulate this equation by giving you the sine and the hypotenuse, but not the measure of the opposite side. Manipulate it as you would any algebraic equation:
    • $Sine Θ = \opposite/hypotenuse => \hypotenuse * \sine Θ = \opposite$


Cosine - CAH

$$\Cosine Θ = \adjacent/\hypotenuse$$

    • Adjacent = the side of the triangle nearest the angle Θ (that creates the angle) that is not the hypotenuse
    • Hypotenuse = the longest side of the triangle


Tangent - TOA

$$\Tangent‌ Θ = \opposite/\adjacent$$

    • Opposite = the side of the triangle directly opposite the angle Θ
    • Adjacent = the side of the triangle nearest the angle Θ (that creates the angle) that is not the hypotenuse


Cosecant, Secant, Cotangent

  • Cosecant is the reciprocal of sine
    • $\Cosecant‌ Θ = \hypotenuse/\opposite$
  • Secant is the reciprocal of cosine
    • $\Secant‌ Θ = \hypotenuse/\adjacent$
  • Cotangent is the reciprocal of tangent
    • $\Cotangent‌ Θ = \adjacent/\opposite$


Useful formulas to know
$$\Sin^2Θ + \Cos^2Θ = 1$$

$${\Sin Θ}/{\Cos Θ} = \Tan Θ$$



Hurray! You've memorized your formulas. Now treat yo' self.


But Keep in Mind

Though these are all the formulas you should memorize to do well on the ACT math section, this list by no means covers all aspects of the mathematical knowledge you’ll need on the exam. For example, you’ will also need to know your exponent rules, how to FOIL, and how to solve for absolute values. To learn more about the general mathematical topics covered by the test, see our article on what's actually tested on the ACT math section.


What's Next?

Now that you know the critical formulas for the ACT, it might be time to check out our article on How to an Perfect Score on the ACT Math by a 36 ACT-Scorer. 

Don't know where to start? Look no further than our article on what is considered a good, bad, or excellent ACT score. 

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Courtney Montgomery
About the Author

Courtney scored in the 99th percentile on the SAT in high school and went on to graduate from Stanford University with a degree in Cultural and Social Anthropology. She is passionate about bringing education and the tools to succeed to students from all backgrounds and walks of life, as she believes open education is one of the great societal equalizers. She has years of tutoring experience and writes creative works in her free time.

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