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How to Enter Scientific Notation on a Casio fx-991 Calculator: Step-by-Step

Enter scientific notation on a Casio fx-991 using the ×10^x (EXP) key, then type the exponent. Use (-) for negative exponents and SETUP > SCI to control displayed significant digits.

Written by Tommy C. MoranPublished November 4, 2026

Short Answer

Enter scientific notation on a Casio fx-991 using the ×10^x (EXP) key, then type the exponent. Use (-) for negative exponents and SETUP > SCI to control displayed significant digits.

Rule Statement

On a Casio fx-991 calculator, scientific notation is entered as mantissa → ×10x (or EXP) → exponent. The exponent may be negative; use the (-) key, not the subtraction key. The resulting value obeys the general form a × 10n, where 1 ≤ |a| < 10 for normalized scientific notation. This convention is standardized in ISO 80000-1:2009, Clause 7.2, and the BIPM SI Brochure, 9th ed., Section 5.4.3.

Precision and rounding are separate from entry. Typing 6.022×1023 does not force the calculator to store only four significant figures. The Casio fx-991 stores more internal digits than it displays. To control displayed significant digits, use SETUP → Number Format → Scientific (SCI) and choose 1–10 digits. For rounding rules, see Rounding Rules and Significant Figures.

Worked Examples

Example 1: Positive exponent

Target: Enter 6.022×1023.

  1. Key in 6.022.
  2. Press ×10x (or EXP).
  3. Key in 23.
  4. Press =. The display shows 6.022×1023.

Example 2: Negative exponent

Target: Enter 1.602×10−19.

  1. Key in 1.602.
  2. Press ×10x (or EXP).
  3. Press (-) for the negative sign, then 19.
  4. Press =. The display shows 1.602×10−19.

Example 3: Set displayed significant figures

Target: Enter 9.1093837×10−31 and display it with 4 significant figures.

  1. Enter 9.1093837×10x(-)31.
  2. Press SHIFT, then SETUP (or MODE on older models).
  3. Choose Number Format → Scientific → 4.
  4. Press =. The display rounds to 9.109×10−31.

The stored value remains 9.1093837×10−31; only the display is rounded. This distinction is central to Rounding vs Significant Figures.

Counter-Examples

  • Using the minus key in the exponent. Typing 2×10x-3 can be parsed as subtraction, not a negative exponent. Use (-).
  • Forgetting the ×10x/EXP key. Entering 6.02223 instead of 6.022×10x23 gives 6.02223, not 6.022×1023.
  • Assuming the display is exact. A display of 3.333333333×10−1 for 1÷3 is rounded. It is not exact; see Why Sig Figs Are an Approximation.
  • Double rounding. Rounding an intermediate result to 3 sig figs, then calculating, then rounding to 3 sig figs can introduce error. GUM Clause 7.2.6 warns against excessive digits but also supports retaining extra digits until final reporting.
  • Confusing ENG with SCI. Pressing ENG changes the exponent to a multiple of 3, e.g., 0.00045 becomes 450×10−6, not 4.5×10−4.

Convention Comparison Table

Value Casio keystrokes (fx-991EX/CW) SCI 4 display Normalized scientific notation Engineering notation
Speed of light 2.998×10x8 2.998×108 2.998×108 m/s 299.8×106 m/s
Electron charge 1.602×10x(-)19 1.602×10−19 1.602×10−19 C 160.2×10−21 C
Small decimal 0.00045 4.500×10−4 4.500×10−4 450.0×10−6

For more on engineering exponents, see Engineering Notation. For E-style output, see E-Notation.

Standards Citation

“The numerical value of a quantity shall be expressed as a number multiplied by a power of 10.” — ISO 80000-1:2009, Clause 7.2 (paraphrased).

Key references for precision and rounding include:

  • ISO 80000-1:2009, Clause 7.2 — formats for numerical values, including scientific notation.
  • BIPM SI Brochure, 9th ed., Section 5.4.3 — scientific notation and prefix usage.
  • NIST SP 811, Section 7.7 — rounding values and significant figures in SI reporting.
  • ASTM E29-22, Section 6 — using significant digits and rounding for test-data conformance.
  • JCGM 100:2008 (GUM), Clause 7.2.6 — reporting uncertainty with an appropriate number of digits; avoid excessive digits.

These standards reinforce that calculator entry is only one step: you must still choose the displayed and reported precision deliberately. Our GUM Guide and ASTM E29 pages explain the reporting consequences.

Common Mistakes

  1. Negative exponent syntax. Use (-), not −.
  2. Overwriting significant zeros. Enter 2.50×103 to preserve three significant figures; 2.5×103 preserves only two.
  3. Using SCI when FIX is intended. SCI sets significant digits; FIX sets decimal places.
  4. Ignoring the mode indicator. A small SCI or ENG on screen means the display is formatted, not necessarily the stored value.
  5. Rounding before final reporting. Keep intermediate values in memory or variables, then round once at the end.
  6. Assuming all fx-991 models are identical. Key labels and SETUP menus differ across ES Plus, EX, and CW generations.

Software Behavior Note

Casio fx-991 models differ mainly in key labels and menu paths. The fx-991ES Plus uses an EXP key and SHIFT MODE for display settings. The fx-991EX ClassWiz and fx-991CW use a ×10x key and SHIFT SETUP → Number Format. In all cases, the calculator stores more digits than the display shows—typically 15 internal digits and up to 10 displayed mantissa digits in SCI mode. The SCI setting controls significant digits from 1 to 10. The ENG key converts the display to engineering notation with exponents that are multiples of 3. NORM returns to normal display. This behavior mirrors the distinction between Precision and Accuracy vs Precision.

Quick Reference Table

Action Key sequence Example Result
Enter positive exponent mantissa → ×10x → exponent 6.022×10x23 6.022×1023
Enter negative exponent mantissa → ×10x → (-) → exponent 1.602×10x(-)19 1.602×10−19
Set 4 sig figs SHIFT SETUP → Number Format → Scientific → 4 9.1093837×10−31 9.109×10−31
Engineering notation ENG 0.00045 450×10−6
Restore normal SHIFT SETUP → Number Format → Norm 1 (or Norm 2) 4.5×10−4 0.00045

Practice Problems

  1. Enter 3.00×108. Answer: 3.00×108; keystrokes: 3.00 ×10x 8.
  2. Enter 6.62607015×10−34 and set SCI 5. Answer: 6.6261×10−34.
  3. Convert 0.000000123 to engineering notation. Answer: 123×10−9.
  4. A display shows 1.2346×103. How many significant figures are displayed? Answer: 5; the mantissa has five digits.
  5. If the internal value is 1234.56 and SCI 3 is set, what appears? Answer: 1.23×103.

FAQ

Why does my Casio show ×10 instead of E?

Casio’s default scientific-notation display uses the ×10^x form rather than E-notation. E-notation is common in programming and spreadsheets; see E-Notation.

How do I type a negative exponent?

Press ×10^x, then (-), then the exponent digits. Do not use the ordinary subtraction key.

Does entering scientific notation set significant figures?

No. It only enters the value. Use SCI in SETUP to control displayed significant digits, and apply rounding rules for reporting.

What is the difference between SCI and ENG?

SCI uses powers of 10 with a mantissa from 1 to 10. ENG uses powers of 10 that are multiples of 3, with a mantissa from 1 to 1000.

Verified sources

References

  1. ISO 80000-1:2009, Quantities and units — Part 1: General, Clause 7.2.
  2. BIPM, The International System of Units (SI Brochure), 9th ed., Section 5.4.3.
  3. NIST Special Publication 811, Guide for the Use of the International System of Units (SI), Section 7.7.
  4. ASTM E29-22, Standard Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications, Section 6.
  5. JCGM 100:2008, Evaluation of measurement data — Guide to the expression of uncertainty in measurement (GUM), Clause 7.2.6.

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