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Interest Calculation
Introduction to interest calculation formulas that determine how money balances deposited into an interest earning account grow over time.
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The effective annual rate (EAR) is a conversion of interest rates to an equivalent rate with annual compounding.
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Introduction to fixed interest annuity calculation with formulas for present and future value as well as usage examples.

Derivation of Continuous Compounding

The derivation of the formula for continuous compounding starts from interest calculation with discrete compounding. As compounding intervals become shorter the resulting exponential growth converges to an exponential function. Because of the infinitesimally small intervals continuous compounding results in an instantaneous rate of return.

While continuous compounding is next to irrelevant in private banking, it’s of crucial importance in financial calculus when pricing derivatives.

Discrete Compounding

For an account earning fixed interest at regular intervals the development of the balance B(t) over time is given by the following formula:

B(t)&&=&B_0 \cdot (1+r\frac{1}{m})^{t \cdot m}\\
B(t):&& &\mathrm{balance \space at \space time} \space t\\
B_0: && &\mathrm{balance \space at \space time \space zero}\\
t: && & \mathrm{time \space in \space years}\\
r: && & \mathrm{interest \space rate}\\
m: && & \mathrm{compounding}\\
 && & \mathrm{frequency}\\

At fixed intervals of duration Δt=1/m interest earnings are credited to the account B and in turn earn interest in the next interval. For instance, $100 earning a fixed nominal interest rate of 10% over 10 years grow to the following amounts at different compounding intervals Δt:

ΔtBalance B(10)
1 year$259.37
6 months$265.33
3 months$268.51
1 month$270.70
Table 1: Balances of an account credited with $100 earning 10% nominal interest after 10 years for different compounding intervals Δt

A web calculator initialized to match the first table row can be found here.

Note how the differences between ending balances in Table 1 become smaller with decreasing compounding periods Δt. This suggests that account balances should converge towards a finite value as periods Δt become infinitesimally small.

Compound Interest as a Recursion

In order to find a solution for compounding frequency going to infinity, the first step is to rewrite the compound interest formula as a recursion:

\tag{1}B_{t+1} = B_t \cdot (1 + r \frac{1}{m})

Equation (1) has investment B change in discrete time steps Δt=1/m. Now subtract Bt on both sides to get a difference equation:

B_{t+1} - B_t = B_t \cdot r \frac{1}{m}

Finally, with Δt=1/m:

\tag{2}\Delta B_t = B_t \cdot r \Delta t

Differential Calculus

Since time steps Δt tend to zero for continuous compounding, the derivation of a formula for continuous compounding follows from differential calculus. Infinitesimally small Δt turn Equation (2) into a differential equation with instantaneous return r(t).

\tag{3}dB(t) = r(t) B(t) dt

In order to get an easy solution by integration, divide by B(t):

\tag{4}\frac{1}{B(t)} dB(t)=r(t)dt

Subsequently, integrate the interval from t to t+τ:

\tag{5}\int_{B(t)}^{B(t+\tau)}\frac{1}{B(t)} dB(t)\\=\int_{t}^{t+\tau} r(s)ds

The primitive function to the integral on the left hand side of Equation (5) is the natural logarithm, so that the following solution results:

&\int_{B(t)}^{B(t+\tau)}\frac{1}{B(t)} dB(t)\\

Substituting Equation (6) into Equation (5):

\tag{7}ln(\frac{B(t+\tau)}{B(t)})\\=\int_{t}^{t+\tau} r(s)ds

Applying the exponential function to both sides of equation (7) yields the general formula for continuous compounding:

\tag{8}B(t+\tau)=B(t) e^{(\int_{t}^{t+\tau} r(s)ds)}

Since in the general case the instantaneous rate of return r(t) is a function of time, Equation (8) has an integral averaging over the time period τ. But if r is constant, the integral, which is also the right side of Equation (7), simplifies:

\tag{9}\int_{t}^{t+\tau} r ds=r \tau

Inserting Equation (9) into Equation (8) produces the formula for a constant instantaneous rate of return, which may look more familiar.

\tag{10}B(t+\tau)=B(t) \cdot e^{r \tau}

Finally, setting t=0 and naming B(0)=B0 simplifies Equation (10) to the usual formula for continuous compounding with a constant rate r.

\tag{11}B(\tau)=B_0 \cdot e^{r \tau}


Compound Interest, Wikipedia.org

Differential Calculus, Wikipedia.org

Published: November 24, 2022
Updated: November 25, 2022

Financial Algebra
Financial Algebra