12 CFA calculator mistakes, and how to catch them while you work

Every Chartered Financial Analyst (CFA) candidate gets at least one question wrong because of the calculator, not the concept. The mistakes are predictable. Six of them happen on the TI BA II Plus. Six happen on the HP 12C. They look like setup slips. A payment mode left on BGN. A Periods per Year (P/Y) setting that was never reset to 1. Prior Time Value of Money (TVM) values still loaded from the last problem. A sign convention flipped.

Charterly is a free CFA calculator companion that catches all twelve while you work. Pick your device below to see the rule list, then run a problem through the live calculator to watch the warnings fire on your own setup.

Open the BA II Plus calculator

Open the HP 12C calculator

The twelve rules at a glance

Each rule below has the symptom, the keystroke fix, and a short note on how Charterly catches it. None of the warnings block computation. Charterly always shows the result, then flags the risk so you can correct it before you commit the answer.

TI BA II Plus mistakes (M1 through M6)

M1. BGN mode active on a TVM compute

Symptom. Your annuity answer is roughly one period of interest higher than the expected value. The display shows a small BGN indicator in the corner that is easy to miss under exam room light.

Why it happens. 2nd then BGN toggles annuity due mode, and the setting persists through power off. If the last problem was a lease or a beginning of period annuity and you forgot to switch back, every following TVM problem inherits the mode.

Keystroke fix. Press 2nd then BGN. Press 2nd then SET until the screen reads END. Press 2nd then QUIT.

How Charterly catches it. The engine looks at paymentMode on every TVM compute and surfaces a banner that reads, "Annuity due mode is active. If your problem expects end of period payments, switch back to END." The warning fires whether you used keypad mode or form mode.

M2. P/Y is not 1 on an annual problem

Symptom. Your monthly mortgage problem answered correctly. Your next annual problem is off by a factor of 12, or close to it.

Why it happens. 2nd then P/Y is sticky. If you set 12 for a monthly compounding problem and rolled into an annual one without resetting, the BA II Plus quietly divides every rate by 12 before solving.

Keystroke fix. Press 2nd then P/Y. Type 1. Press ENTER. Press 2nd then QUIT.

How Charterly catches it. A warning fires whenever P/Y is not 1, unless the active drill is explicitly tagged as multi-period (for example a monthly mortgage). On the standalone calculator with no drill context, the warning fires whenever P/Y is not 1 so you can confirm it on purpose.

M3. TVM registers still loaded from the prior problem

Symptom. You enter N, I/Y, and PV, press CPT PMT, and the number that comes back ignores a value you typed two problems ago.

Why it happens. The TI BA II Plus does not auto clear TVM. Every register holds its prior value until you press 2nd then CLR TVM. Candidates often only clear the registers they think they touched.

Keystroke fix. Press 2nd then CLR TVM. Do it between every problem during practice so it becomes muscle memory.

How Charterly catches it. The engine records the last TVM touch timestamp for the working register set. In a drill context, any TVM touch before the drill started flags as leftover. In free solve mode, any TVM touch more than 60 seconds before a compute flags as leftover. The timing rule is deliberately conservative so warnings stay below the false-positive target.

M4. PV and FV share the same sign with no PMT

Symptom. The calculator returns Error 5, or a result that does not match any answer choice.

Why it happens. The BA II Plus expects a cash flow story. If PV and FV are both positive and PMT is zero, the engine has no real rate to solve. Money cannot enter your account, stay your money, and end as more of your money without something going the other way.

Keystroke fix. Decide which leg is yours and which is the counterparty's, then flip a sign. Convention: money you give up is negative, money you receive is positive.

How Charterly catches it. The detector checks for PMT of zero plus same-sign PV and FV, then surfaces the warning before the engine reports Error 5. The message explains the sign convention in plain English instead of an opaque error code.

M5. Cash flow worksheet has leftover entries

Symptom. Your Net Present Value (NPV) or Internal Rate of Return (IRR) is off, and the offset matches a cash flow from the previous problem.

Why it happens. The CF worksheet stores up to 24 entries (CF0 through CF23), and CLR WORK only clears them when you are inside the CF worksheet. Pressing CE/C from the home screen does not touch them.

Keystroke fix. Press CF, then 2nd then CLR WORK. Then start entering CF0.

How Charterly catches it. Drill aware. When a question knows its expected CF count, the detector compares actual to expected and warns when actual is higher. Free solve mode does not flag this rule, because the standalone calculator has no expected count to compare against and any warning would be a false positive.

M6. Operating mode is Chn when a problem expects AOS

Symptom. A simple expression like 2 + 3 × 4 gave you 20 instead of 14.

Why it happens. The BA II Plus ships in Chain mode (Chn) out of the box. Chn evaluates strictly left to right. AOS applies algebraic precedence. The default surprises candidates who learned arithmetic the standard way.

Keystroke fix. Press 2nd then FORMAT. Scroll to Chn. Press 2nd then SET to flip to AOS. Press 2nd then QUIT.

How Charterly catches it. Drill aware. When a question's expected keystroke path uses chained arithmetic that requires precedence, the detector flags Chn mode. Free solve respects your preference, because Chn is correct for someone who learned on the device.

HP 12C mistakes (H1 through H6)

H1. BEGIN mode active on a TVM compute

Symptom. Same as M1 on the TI side. Your annuity answer is a period of interest too high.

Why it happens. g BEG is sticky on the HP 12C. The screen shows a small BEGIN annunciator that is easy to overlook.

Keystroke fix. Press g END.

How Charterly catches it. The HP keypad simulator surfaces a parallel warning to M1: "Annuity due mode is active. Press g END to switch back if your problem expects end of period payments."

H2. i entered as annual rate on a multi-period problem

Symptom. A monthly mortgage problem with a 6 percent annual rate gives you a wildly wrong payment.

Why it happens. The HP 12C stores i as the periodic rate. Candidates familiar with the BA II Plus often type the annual rate into i and rely on P/Y to handle the conversion. The HP 12C has no P/Y. You must divide the rate manually before storing it.

Keystroke fix. For a 6 percent annual rate compounding monthly: type 6, press g 12÷ (or compute 6 ENTER 12 ÷), then press i.

How Charterly catches it. Drill aware. When a drill is tagged as multi-period and the stored i is large enough to plausibly be the annual rate, the detector flags it.

H3. TVM registers still loaded from the prior problem

Symptom. Same as M3 on the TI side. Same fix conceptually, different keystrokes.

Keystroke fix. Press f FIN (or f CLX) to clear the financial registers.

How Charterly catches it. Same drill aware logic plus 60 second wall clock fallback as M3, surfaced under code H3.

H4. RPN stack has leftover values

Symptom. A multi-step problem gives you a wrong intermediate value because the X register inherits something from the prior calculation that you forgot to drop.

Why it happens. The HP 12C is RPN. Every typed value pushes onto the stack. If you typed 100 for the prior problem and forgot to clear, your next ENTER 50 + will not give you 50. It will give you 150.

Keystroke fix. Press CLX to clear the X register, or roll the stack down with R↓ until the leftover values fall off.

How Charterly catches it. Drill aware. When a question knows the expected stack depth and the actual depth exceeds it, the detector warns. The free solve HP calculator does not flag this rule, because your stack depth is whatever you intended.

H5. PV and FV share the same sign with no PMT

Same as M4 on the TI side. The HP 12C also uses the cash out negative convention. The detector message is HP specific: "HP TVM expects opposite signs for PV vs FV when PMT is zero. Flip the sign on one of them."

H6. Decimal display set below 4 places

Symptom. A chained calculation drifts a few cents from the expected answer because intermediate values were truncated to two decimals.

Why it happens. The HP 12C ships at f 2, two decimal places. The display setting affects what you see. Internal precision is full. The real exam risk is that you do not notice an intermediate value was rounded in your head because you only saw two digits.

Keystroke fix. Press f 4 (or f 6 for chained tolerance work).

How Charterly catches it. The HP simulator's H6 detector watches the displayed decimal places and surfaces an info level banner when it is below 4. Info level, not warning. It is a hygiene nudge, not an error.

Why a normal calculator does not tell you

A physical BA II Plus or HP 12C is a calculator. It does what you press. It cannot infer that BGN was the right answer for the last problem and the wrong answer for this one. It cannot remember which TVM registers you set on purpose and which are inherited. It cannot tell you that 6 percent looks too big for a monthly periodic rate.

Most CFA prep apps focus on question volume and explanations. They grade your answer but do not look at the keystroke path you used to get there. So a candidate who consistently leaves BGN on, or forgets to clear TVM between problems, sees the right pattern of wrong answers in their mock results but never sees the device state cause underneath. The same mistake recurs on exam day.

Charterly is a free, independent CFA calculator companion that runs all twelve rules while you work. The warnings never block computation. Charterly always shows the result, then flags the risk. The detectors are tuned to keep false positives under 2 percent per device, so the warnings stay credible and you do not learn to ignore them.

Open the BA II Plus calculator

Open the HP 12C calculator

Frequently asked questions

Does Charterly work on the BA II Plus Professional too? The Professional model adds a few keys (breakeven, depreciation methods, conditional cash flows), and every rule on this page applies identically. Charterly's TI engine matches the standard BA II Plus key map. The Professional only features are on the advanced calculators page.

Is Charterly affiliated with Texas Instruments, Hewlett-Packard, or the CFA Institute? No. Charterly is an independent CFA exam companion. TI, BA II Plus, Business Analyst, HP, HP 12C, and CFA are trademarks of their respective owners. Charterly is not endorsed by, sponsored by, or affiliated with any of them.

Will the warnings train me to ignore my real calculator on exam day? No, and that is by design. Charterly's keypad mode mirrors the physical device's key layout and behavior. The mistake detector teaches the checklist. Once you have seen "did I clear TVM" flagged a few hundred times during practice, you start checking it yourself. The warning then has nothing to nudge you about because you already cleared the registers.

Does Charterly auto fix mistakes for me? No. Charterly surfaces the warning and explains the keystroke fix. It never silently changes your inputs. Computing always proceeds. The result is always shown. Mistake detection is a non-blocking layer over the calculator.

How are false positives controlled? The target is under 2 percent per device. Release checks exercise known-good calculator states before detector changes ship, so warnings stay useful without becoming background noise.

Can I turn warnings off? Yes. Each warning is dismissable for the current session. The global setting can be turned off in Settings. The default is on because the cost of a missed mistake is a wrong question.

Do I have to log in to use this? No. The calculator and all twelve mistake rules are free and work without signup. Saving practice history and unlimited drills sit behind Charterly Pro. Everything on this page is free.

Where is the HP 12C version of the practice library? The drill library is calculator agnostic. Every LOS tagged drill works on TI or HP. Where keystroke walkthroughs are involved, the HP keystrokes are shown next to the TI keystrokes. Browse the question library.