fx-991CW Style Calculator Online
The modes students use most on an fx-991CW, in your browser: calculate, solve equations with step-by-step working, run statistics, integrate and differentiate, and work with complex numbers, binary and hex. This is an independent tool, not made by Casio.
History
Keyboard
Type numbers and + - * / ^ directly. Enter calculates, Backspace deletes, Esc clears. ← → move the cursor, f makes a fraction.
What this calculator can do
| Mode | What it solves |
|---|---|
| Calculate | Everyday scientific math: trig, logs, powers and roots, factorials, nCr and nPr, fractions, memory and history |
| Equation | Quadratic, cubic and quartic equations, including complex roots, and systems of 2 or 3 linear equations, with exact answers and step-by-step working |
| Statistics | Mean, sum, standard deviation (population and sample), quartiles, median and range; linear regression with r for x–y data |
| Calculus | Definite integrals, derivatives at a point and summations |
| Complex | Arithmetic with a + bi numbers, modulus, argument, polar form and conjugate |
| Base-N | Conversion between decimal, hex, binary and octal, plus AND, OR, XOR and NOT |
| Table | A table of f(x) values over a range, up to 200 rows |
How to solve a quadratic or cubic equation
Open the Equation tab, pick the equation type and enter the coefficients. For 2x² − 4x − 6 = 0, enter a = 2, b = −4 and c = −6, then press Solve to get x₁ = 3 and x₂ = −1. For quadratics you also get the discriminant and the vertex, which tells you the minimum or maximum point of the parabola. When the discriminant is negative, the roots are shown as complex numbers such as −1 + 2i.
Below the answer, a step-by-step solution shows the working the way a teacher would write it: the discriminant, the quadratic formula with your numbers substituted, and the simplified radical. Answers also appear in exact form, such as (3 + √3)/2, not only as decimals. Cubic and quartic equations are factored with the rational root theorem and synthetic division whenever they have a rational root. Systems of equations are solved step by step with Cramer’s rule, and fractions stay exact, so 3x + 2y = 7 and 5x − 4y = 1 give x = 15/11 and y = 16/11.
The steps come from fixed math rules, not from AI, so the same problem always gets the same correct working.
For a system like 2x + y = 5 and x − y = 1, choose “2 equations, 2 unknowns” and fill in each row as a, b, c: 2, 1, 5 and 1, −1, 1. The answer is x = 2, y = 1.
Mean, standard deviation and regression
Paste your numbers into the Statistics tab, separated by commas, spaces or new lines. You get both kinds of standard deviation: σx (population) divides by n, and sx (sample) divides by n − 1. Use sx when your data is a sample from a larger group, which is the usual case in science labs and AP Statistics. Quartiles follow the same method as most handheld calculators: the median splits the data, and Q1 and Q3 are the medians of the lower and upper halves.
For two-variable data, put one x, y pair on each line. The result is the least-squares line y = a + bx, the correlation coefficient r, and r².
Integrals and derivatives
Enter the function using x as the variable, for example x^2 + sin(x) or e^(-x^2). Integrals use adaptive Simpson’s rule and derivatives use a high-order central difference. Both give about 10 correct significant digits for smooth functions. Trig functions default to radians here, which is the convention in calculus; switch to degrees only if your problem asks for it.
Complex numbers and Base-N
Type complex numbers the way you would write them: 3+4i, -2i, 5 or 1.5-0.5i. Results show both rectangular form and polar form r∠θ. In Base-N, negative numbers use 32-bit two’s complement, the same as the Base-N mode on hardware calculators, so −1 in hexadecimal is FFFFFFFF.
How it differs from a real fx-991CW
This page is not an emulator of Casio’s calculator and does not copy its layout or software. It covers the most-used features in a form built for the web. A physical fx-991CW also has matrix, vector, distribution, spreadsheet and inequality features, and can show many answers in exact form, such as √2⁄2. This tool shows most answers as decimals. In the United States, Casio sells closely related models under the fx-115 name.
Handheld calculators are what you can take into a classroom exam; online tools usually are not allowed. Use this page for homework, checking your work and learning the steps.
Frequently asked questions
Is this the official Casio fx-991CW?
No. It is an independent free tool with similar modes. Casio and fx-991CW are trademarks of Casio Computer Co., Ltd.
Does it show the steps?
Yes, for equations. Quadratics show the discriminant and the quadratic formula; cubics and quartics show rational-root factoring; systems show Cramer’s rule. Steps appear when the coefficients are whole numbers or decimals.
Can it solve equations with complex roots?
Yes. Quadratic, cubic and quartic equations show complex roots in a + bi form.
Which standard deviation should I use?
Use the sample standard deviation (sx) when your data is a sample of a larger population. Use the population standard deviation (σx) when you have data for every member of the group.
Why does my integral give an error?
The function is undefined somewhere in the interval, for example 1/x on an interval that includes 0. Split the interval, or check the function for division by zero or the square root of a negative number.
Last updated September 27, 2026.