# The Black–Scholes Formula for Call Option Price

This example shows how to calculate the call option price using the Black–Scholes formula. This example uses `vpasolve`

to numerically solve the problems of finding the spot price and implied volatility from the Black–Scholes formula.

### Find Call Option Price

The Black–Scholes formula models the price of European call options [1]. For a non-dividend-paying underlying stock, the parameters of the formula are defined as:

$$S$$ is the current stock price or spot price.

$$K$$ is the exercise or strike price.

$$\sigma $$ is the standard deviation of continuously compounded annual returns of the stock, which is called volatility.

$$T$$ is the time for the option to expire in years.

$$r$$ is the annualized risk-free interest rate.

The price of a call option $$C$$ in terms of the Black–Scholes parameters is

$$C=N({d}_{1})\times S-N({d}_{2})\times PV(K)$$,

where:

$${d}_{1}=\frac{1}{\sigma \sqrt{T}}[\mathrm{log}\left(\frac{S}{K}\right)+(r+\frac{{\sigma}^{2}}{2})T]$$

$${d}_{2}={d}_{1}-\sigma \sqrt{T}$$

$$PV(K)=K\mathrm{exp}(-rT)$$

$$N(d)$$ is the standard normal cumulative distribution function, $$N(d)=\frac{1}{\sqrt{2\pi}}{\int}_{-\infty}^{d}\mathrm{exp}(-{t}^{2}/2)\phantom{\rule{0.2777777777777778em}{0ex}}dt$$.

Find the price of a European stock option that expires in three months with an exercise price of $95. Assume that the underlying stock pays no dividend, trades at $100, and has a volatility of 50% per annum. The risk-free rate is 1% per annum.

Use `sym`

to create symbolic numbers that represent the values of the Black–Scholes parameters.

syms t d S = sym(100); % current stock price (spot price) K = sym(95); % exercise price (strike price) sigma = sym(0.50); % volatility of stock T = sym(3/12); % expiry time in years r = sym(0.01); % annualized risk-free interest rate

Calculate the option price without approximation. Create a symbolic function `N(d)`

that represents the standard normal cumulative distribution function.

PV_K = K*exp(-r*T); d1 = (log(S/K) + (r + sigma^2/2)*T)/(sigma*sqrt(T)); d2 = d1 - sigma*sqrt(T); N(d) = int(exp(-((t)^2)/2),t,-Inf,d)*1/sqrt(2*sym(pi))

N(d) =$$\frac{\mathrm{erf}\left(\frac{\sqrt{2}\hspace{0.17em}d}{2}\right)}{2}+\frac{1}{2}$$

Csym = N(d1)*S - N(d2)*PV_K

Csym =$$50\hspace{0.17em}\mathrm{erf}\left(\frac{\sqrt{2}\hspace{0.17em}\left(4\hspace{0.17em}\mathrm{log}\left(\frac{20}{19}\right)+\frac{27}{200}\right)}{2}\right)-95\hspace{0.17em}{\mathrm{e}}^{-\frac{1}{400}}\hspace{0.17em}\left(\frac{\mathrm{erf}\left(\frac{\sqrt{2}\hspace{0.17em}\left(4\hspace{0.17em}\mathrm{log}\left(\frac{20}{19}\right)-\frac{23}{200}\right)}{2}\right)}{2}+\frac{1}{2}\right)+50$$

To obtain the numeric result with variable precision, use `vpa`

. By default, `vpa`

returns a number with 32 significant digits.

Cvpa = vpa(Csym)

`Cvpa = $$12.52792339252145394554497137187$$`

To change the precision, use `digits`

. The price of the option to 6 significant digits is $12.5279.

digits(6) Cvpa = vpa(Csym)

`Cvpa = $$12.5279$$`

### Plot Call Option Price

Next, suppose that for the same stock option the time to expiry changes and the day-to-day stock price is unknown. Find the price of this call option for expiry time $$T$$ that varies from 0 to 0.25 years, and spot price $$S$$ that varies from $50 to $140. Use the values for exercise rate (`K`

), volatility (`sigma`

), and interest rate (`r`

) from the previous example. In this case, use the time to expiry `T`

and day-to-day stock price `S`

as the variable quantities.

Define the symbolic expression `C`

to represent the call option price with `T`

and `S`

as the unknown variables.

syms T S PV_K = K*exp(-r*T); d1 = (log(S/K) + (r + sigma^2/2)*T)/(sigma*sqrt(T)); d2 = d1 - sigma*sqrt(T); Nd1 = int(exp(-((t)^2)/2),-Inf,d1)*1/sqrt(2*pi); Nd2 = int(exp(-((t)^2)/2),-Inf,d2)*1/sqrt(2*pi); C = Nd1*S - Nd2*PV_K;

Plot the call option price as a function of spot price and expiry time.

fsurf(C,[50 140 0 0.25]) xlabel('Spot price') ylabel('Expiry time') zlabel('Call option price')

Calculate the call option price with expiry time 0.1 years and spot price $105. Use `subs`

to substitute the values of `T`

and `S`

to the expression `C`

. Return the price as a numeric result using `vpa`

.

Csym = subs(C,[T S],[0.1 105]); Cvpa = vpa(Csym)

`Cvpa = $$12.5868$$`

### Find Spot Price

Consider the case where the option price is changing, and you want to know how this affects the underlying stock price. This is a problem of finding $$S$$ from the Black–Scholes formula given the known parameters $$K$$, $$\sigma $$, $$T$$, $$r$$, and $$C$$.

For example, after one month, the price of the same call option now trades at $15.04 with expiry time of two months. Find the spot price of the underlying stock. Create a symbolic function `C(S)`

that represents the Black–Scholes formula with the unknown parameter `S`

.

syms C(S) d1(S) d2(S) Nd1(S) Nd2(S) K = 95; % exercise price (strike price) sigma = 0.50; % volatility of stock T = 2/12; % expiry time in years r = 0.01; % annualized risk-free interest rate PV_K = K*exp(-r*T); d1(S) = (log(S/K) + (r + sigma^2/2)*T)/(sigma*sqrt(T)); d2(S) = d1 - sigma*sqrt(T); Nd1(S) = int(exp(-((t)^2)/2),-Inf,d1)*1/sqrt(2*pi); Nd2(S) = int(exp(-((t)^2)/2),-Inf,d2)*1/sqrt(2*pi); C(S) = Nd1*S - Nd2*PV_K;

Use `vpasolve`

to numerically solve for the spot price of the underlying stock. Search for solutions only in the positive numbers. The spot price of the underlying stock is $106.162.

S_Sol = vpasolve(C(S) == 15.04,S,[0 Inf])

`S_Sol = $$106.162$$`

### Find Implied Volatility

Consider the case where the option price is changing and you want to know what is the implied volatility. This is a problem of finding the value of $$\sigma $$ from the Black–Scholes formula given the known parameters $$S$$, $$K$$, $$T$$, $$r$$, and $$C$$.

Consider the same stock option that expires in three months with an exercise price of $95. Assume that the underlying stock trades at $100, and the risk-free rate is 1% per annum. Find the implied volatility as a function of option price that ranges from $6 to $25. Create a vector for the range of the option price. Create a symbolic function `C(sigma)`

that represents the Black–Scholes formula with the unknown parameter `sigma`

. Use `vpasolve`

to numerically solve for the implied volatility.

syms C(sigma) d1(sigma) d2(sigma) Nd1(sigma) Nd2(sigma) S = 100; % current stock price (spot price) K = 95; % exercise price (strike price) T = 3/12; % expiry time in years r = 0.01; % annualized risk-free interest rate C_Range = 6:25; % range of option price sigma_Sol = zeros(size(C_Range)); PV_K = K*exp(-r*T); d1(sigma) = (log(S/K) + (r + sigma^2/2)*T)/(sigma*sqrt(T)); d2(sigma) = d1 - sigma*sqrt(T); Nd1(sigma) = int(exp(-((t)^2)/2),-Inf,d1)*1/sqrt(2*pi); Nd2(sigma) = int(exp(-((t)^2)/2),-Inf,d2)*1/sqrt(2*pi); C(sigma) = Nd1*S - Nd2*PV_K; for i = 1:length(C_Range) sigma_Sol(i) = vpasolve(C(sigma) == C_Range(i),sigma,[0 Inf]); end

Plot the implied volatility as a function of the option price.

plot(C_Range,sigma_Sol) xlabel('Option price') ylabel('Implied volatility')