How to Reset a Casio fx-991 Scientific Calculator
How to Reset a Casio fx-991 Scientific Calculator
On the fx-991ES PLUS and fx-991EX, press SHIFT 9, choose Initialize All (option 3) and confirm with =. On the fx-991CW, open the Settings key menu, then choose Reset › Initialize All › Yes. A full reset also erases stored data, so read the warning below first.
Data-loss warning. A full reset returns settings to their defaults and clears everything in calculator memory (stored variables, Ans, and similar data). Write down any values you still need before you start. A reset cannot be undone.
Quick reference by model
| Model | Full reset | Clears |
|---|---|---|
| fx-991ES PLUS | SHIFT 9 (CLR) → 3 (All) → = (Yes) | Mode, setup and all memory data |
| fx-991EX | SHIFT 9 (RESET) → 3 (Initialize All) → = (Yes) | Settings and memory data |
| fx-991CW | Open an app, press the Settings key → Reset → Initialize All → Yes | All settings except Contrast and Auto Power Off, plus memory data |
These sequences come from Casio’s own manuals (linked under Sources). Casio sometimes revises firmware and manuals, so if what you see on screen differs from this page, trust the on-screen menu.
Settings reset vs. memory clear vs. full reset
Casio’s reset menu offers more than one option, and they do different jobs:
- Setup/settings reset: puts options such as angle unit and display format back to defaults. Use it when results look wrong, for example an answer in radians when you expected degrees.
- Memory clear: erases stored variables and similar data but leaves your settings alone. Use it when old values interfere with new work.
- Full reset (Initialize All / All): does both. Use it before an exam where a clean calculator is required, when handing the calculator to someone else, or after a battery change.
On the ES PLUS and EX, the menu after SHIFT 9 lists the narrower options as 1 and 2 next to 3. Read the labels on screen before pressing =, because option numbers differ between families. Pressing AC backs out without changing anything.
How to reset a Casio fx-991ES PLUS
- Press SHIFT then 9 to open the CLR menu.
- Press 3 for All. (Per Casio’s guide, this resets the calculation mode and setup and clears all memory data.)
- Press = to answer Yes.
To reset settings only, Casio’s guide gives the sequence SHIFT 9 (CLR) → Setup → Yes, which leaves memory untouched.
Note: this section was checked against the fx-991ES PLUS manual. The older fx-991ES (without PLUS) and regional variants such as the fx-991ES PLUS C use similar menus, but check the manual for your exact model before relying on these keys.
How to reset a Casio fx-991EX
- Press SHIFT then 9 to open the RESET menu.
- Select 3 Initialize All.
- Press = for Yes.
The EX’s manual states this returns the calculator to its initial default settings. Treat it as erasing stored data as well.
How to reset a Casio fx-991CW
The fx-991CW (ClassWiz, newer interface) works differently: there is no SHIFT 9 shortcut in its manual. The procedure is a menu path.
- Press the HOME key and open any calculator app, such as Calculate.
- Press the Settings key (the gear icon).
- Choose Reset, then Initialize All, then Yes.
Casio notes that this initializes all settings except Contrast and Auto Power Off, and also clears data in memory. Casio’s education site also describes lighter reset options in the same Reset menu (for example, keeping or clearing variables only); read the on-screen names if you only need a partial reset.
Battery changes: removing the battery clears the calculator’s memory, and Casio’s manual says to initialize the calculator afterwards. For the fx-991CW sold in the US and Canada, Casio says not to replace the battery yourself; use an authorized service center.
Troubleshooting and common mistakes
- Wrong answers but nothing is broken: check angle unit (Deg/Rad/Gra) and output format before resetting. A settings-only reset is usually enough.
- Pressed the wrong option: if you chose All when you wanted Setup, memory is already gone. Check the label before pressing =.
- Display is dim or blank: this usually points to a weak battery rather than a software fault, and a reset will not fix it.
- Key sequence does nothing: confirm your exact model on the back of the calculator. ES PLUS, EX and CW menus are not interchangeable.
- Still misbehaving after Initialize All: contact Casio support or your seller; the unit may be faulty.
Frequently asked questions
- Does resetting a Casio calculator delete my calculation history?
- A full reset clears memory data, and Casio states the calculator returns to its initial state. Assume anything you want to keep is gone and note it down first.
- Will a reset let me use the calculator in an exam?
- Not necessarily. Exam rules vary and some bodies require specific exam modes or models. Check your exam board’s current rules.
- Is resetting the same as pressing AC?
- No. AC only clears the current entry or screen. It does not touch settings or stored memory.
- Can I reset the CalcNova online calculator the same way?
- No. The CalcNova online calculator is a separate tool; its saved history is stored in your own browser and can be cleared from its History tab.
Next steps
After a reset, you may want to re-set your angle unit and display format. Our guides to Casio scientific calculator functions and fx-991 shortcuts and tips cover those settings and what each key does.