What This Online Scientific Calculator Is
This is a free scientific calculator that runs in your browser and follows the layout of the Casio fx-991ES PLUS (2nd edition): the same key rows, the same SHIFT and ALPHA labels, and a natural display that shows fractions, roots and integrals the way a textbook does. Nothing to install, no sign-up. It is an independent tool and is not affiliated with Casio.
It is built for the person who has a handheld at home, or an exam on Monday, and wants to practise on a screen. If you only need to add two numbers, any calculator will do. If you need to see √3/2 instead of 0.866025…, solve a quadratic, or check an integral, this is the right tool.
How to Use It in Five Minutes
Three ideas cover the whole keypad. Each key has a main label on the key itself. The small yellow text above it is reached with SHIFT. The small pink text is reached with ALPHA, and it is mostly variable names (A to F, X, Y, M). Press SHIFT or ALPHA once and the next key uses that label.
Then learn four keys. MODE opens the work areas (calculate, equation, matrix, vector, statistics, base-n and more). SETUP (SHIFT + MODE) sets degrees or radians and fraction or decimal output. S⇔D flips an answer between exact and decimal. Ans brings back the last result, so you never retype long numbers.
Try it now: type sin(60) in Deg mode and you get √3/2. Type 1/3 + 1/6 and you get 1/2. If either looks wrong, check the angle chip on the screen first.
What Each Mode Does
| Mode | Use it for |
|---|---|
| Calculate | Everyday arithmetic, fractions, powers, roots, trig, logs, integrals and derivatives. |
| Equation | Quadratics, cubics and systems of two or three linear equations. |
| Matrix / Vector | Determinants, inverses, dot and cross products. |
| Complex | (3 + 2i)(1 − 4i), modulus, argument, polar form. |
| Statistics | Mean, standard deviation, regression from a data table. |
| Base-n | Binary, octal, decimal and hex conversion with the DEC / HEX / BIN / OCT keys. |
| Solver tab | Type a problem in words and get the answer with steps. This goes beyond a physical calculator. |
The One Setting That Causes Most Wrong Answers
Angle mode decides what sin(90) means. In Deg it is 1. In Rad it is 0.894, because 90 is read as 90 radians. The screen shows a small boxed letter for the active unit, and it is the first thing to glance at before any trig question. A common exam slip is to finish a degrees question in radian mode and lose every mark on it.
The same habit applies to output. If a question asks for an exact value and you copy 0.7071 from the screen, you gave a rounded answer. Press S⇔D and write √2/2.
Example Inputs and What the Screen Shows
These inputs show how exact mode, clean rounding and simplified surds behave.
| Input | Screen shows | Why it matters |
|---|---|---|
| sin(180) in Deg | 0 | Many floating-point tools show 1.2e-16 here. Cleaning that noise matters for exact work. |
| 0.1 + 0.2 | 3/10 (0.3) | Plain JavaScript gives 0.30000000000000004. |
| 1/3 + 1/6 | 1/2 | Exact fraction, not 0.5 only. |
| √8 | 2√2 | Simplified surd, as on the handheld. |
Our View: Overrated, Underrated, Wrong
Overrated: memorising every SHIFT function. In practice most students use about a dozen: fractions, powers, roots, trig, log, Ans, STO/RCL, S⇔D and the equation mode. Learn those well before the rest.
Underrated: CALC. Type a formula once, press CALC, and it asks for each variable. For physics or finance formulas with five inputs, it saves minutes per question. Also underrated is the history of previous answers, which makes errors easy to trace.
Wrong: the belief that a calculator answer is automatically correct. It does exactly what you typed. A missing bracket in 1/(2+3) versus 1/2+3 gives two very different results. Estimate first, then compare the screen to your estimate.
Three Worked Examples
1. Quadratic. Solve x² − 5x + 6 = 0. Equation mode gives X = 2 and X = 3. Check: 2² − 10 + 6 = 0.
2. Integral. ∫₀³ x² dx. Use the ∫ key, enter limits 0 and 3, and read 9. By hand: 3³/3 = 9.
3. Compound interest. 5,000 at 6% for 8 years: type 5000×1.06^8. The screen shows 7969.24. Round to cents only at the end.
Online vs the Handheld, and Exams
A browser calculator wins on convenience: it is on every device, shows the full working next to the keys, and explains steps. The handheld wins in the exam hall, because most boards allow only approved models and ban phones. So use this page for practice, then rehearse the same key sequences on your own device. Check your exam board's current approved-calculator list before test day.
Frequently Asked Questions
Is this the same as the Casio fx-991ES PLUS?
It copies the key layout and behaviour so you can practise, but it is an independent tool. It is not a Casio product and is not affiliated with Casio.
Can I use it in an exam?
No. Exams permit approved physical calculators, so use this page to practise and rehearse your key sequences.
How do I switch between degrees and radians?
Press the DRG key, or SHIFT then MODE for SETUP, and watch the angle letter below the screen change.
How do I get a fraction instead of a decimal?
Press S⇔D after the answer, or open the ⋮ menu beside the result and choose View Result Format.
How do I save a result to a variable?
Use the save icon under the result, or the ⋮ menu, and pick a letter. Type that letter in a later calculation to reuse the value.
Does it show steps?
Yes. Use the Solver tab, or the “Solve step by step” button under the result, to see each step.
Is it free?
Yes. No account or sign-up is needed.