P/Y and C/Y on the BA II Plus

The BA II Plus stores two settings together behind the same key: Periods per Year (P/Y) and Compounding periods per Year (C/Y). On a fresh calculator they are both 1. Once a candidate sets P/Y to 12 for a monthly mortgage problem and forgets to reset, every following annual problem returns an answer off by roughly a factor of 12.

This page covers what P/Y and C/Y do, when to change them, and how to reset them cleanly. Charterly's free BA II Plus calculator catches a non-1 P/Y under rule M2 in its mistake-detection layer.

What P/Y and C/Y mean

• P/Y is the number of payment periods per year. For an annual payment schedule, P/Y is 1. For monthly, 12. For quarterly, 4. For semiannual, 2.
• C/Y is the number of compounding periods per year. For most CFA problems, C/Y equals P/Y. When a question specifies a different compounding period (a stated annual rate compounded continuously, or quarterly compounding on a monthly payment schedule), C/Y and P/Y diverge.

The BA II Plus uses these settings to convert between the rate the user types into I/Y and the rate the engine uses internally. When P/Y is 12 and the user types 6 into I/Y, the engine treats the periodic rate as 6 / 12 = 0.5 percent per month.

The safe CFA default

P/Y is 1. C/Y is 1. The user types the periodic rate directly into I/Y. This is the cleanest setup for CFA problems because most exam questions state the rate per period rather than the stated annual rate, or they make the conversion explicit in the question stem.

A candidate who keeps P/Y at 1 and converts the rate manually has fewer settings to remember. The conversion arithmetic (annual rate divided by frequency) is part of the curriculum anyway, so doing it by hand is not extra work.

When to change P/Y to 12

Two cases:

1. An explicit monthly cash-flow schedule. A 30-year mortgage with monthly payments at a 6 percent annual rate. Set P/Y to 12, set N to 360, type 6 into I/Y, and the engine handles the rate conversion.
2. A nominal annual rate explicitly compounded monthly. A savings account quoted at 6 percent compounded monthly. Same setup as case 1.

The catch is the moment after. The setting persists. The next annual problem requires P/Y back at 1, and many candidates forget.

A worked comparison

Problem A (P/Y = 1). A candidate deposits 1,000 today at 6 percent for 5 years. Compute Future Value (FV).

1. Confirm P/Y = 1.
2. 5 then N. 6 then I/Y. 1000 then +/- then PV. 0 then PMT.
3. CPT then FV. Expected: 1,338.2256.

Problem B (P/Y left at 12 by accident). Same problem, same keystrokes, P/Y forgotten at 12 after a previous monthly problem.

1. P/Y is 12.
2. 5 then N. 6 then I/Y. 1000 then +/- then PV. 0 then PMT.
3. CPT then FV. Returned: roughly 1,025.2513.

The engine treated 5 as 5 months and 6 percent as the annual rate compounded monthly, giving the monthly periodic rate of 0.5 percent for 5 periods. The answer is off by orders of magnitude in terms of the candidate's intended problem.

How to reset P/Y to 1

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

C/Y follows P/Y automatically when you change it through 2nd P/Y. To set C/Y separately (the rare case where they diverge), open the 2nd P/Y worksheet, arrow down to the C/Y field, type the value, press ENTER, then 2nd QUIT.

How Charterly catches this

Rule M2 fires any time P/Y is not 1, unless the active drill is explicitly tagged as multi-period. On the standalone calculator with no drill context, the warning fires whenever P/Y is not 1 so the candidate can confirm the value is intentional. The detector is tuned to stay under Charterly's false-positive target by giving the candidate one tap to dismiss the warning for the current session.

For HP 12C candidates, there is no equivalent setting because the HP 12C stores i as the periodic rate directly. The HP analogue is rule H2, which flags an apparently annual rate stored in i on a multi-period problem. See the HP 12C guide.

Frequently asked questions

Does P/Y affect anything other than TVM? P/Y also influences the AMORT and bond worksheets when they reference the TVM registers. The CF worksheet (NPV and IRR) does not use P/Y. The discount rate stored in NPV is always the periodic rate.

Can I leave P/Y at 12 all the time and convert annual problems manually? Some candidates do. You can multiply N by 12, divide the stated annual rate by 12, and treat every problem as if it were monthly. It works arithmetically. It is fragile in practice because the conversions are easy to mistype under exam time pressure.

Does Charterly auto-reset P/Y? No. Charterly surfaces a warning and explains the keystroke fix. It never silently changes settings (AGENTS section 7).

Where is the full mistake list? The twelve CFA calculator mistakes covers M1 through M6 on the BA II Plus and H1 through H6 on the HP 12C.