Trinomial Tree for Pricing European Options

This article presents an Excel spreadsheet and VBA for pricing European options with a trinomial tree.

Binomial trees are often used to price options that have no closed-form analytical solutions. However, they can easily become large and inefficient to implement. Trinomial trees, however, are more efficient and converge more rapidly than their binomial counterparts. Moreover, trinomial trees are only slightly more complex to implement than binomial trees.

The VBA for trinomial pricing lattice is described by this pseudocode

Calculate the jump sizes (u, d)
Calculate the probabilities (p_u, p_m, p_d)
Create a tree of share prices
Calculate the payoff at maturity at the final node
Create the option price lattice through backwards induction. Step back through the lattice to the initial node.

This pseudocode is implemented in the following VBA function.

Function EuropeanOption(OptionType As String, S As Double, X As Double, T As Double, r As Double, div As Double, vol As Double, nSteps As Integer)
 
   Dim C() As Double
   Dim u As Double, d As Double
   Dim Pu As Double, Pd As Double, Pm As Double, cc As Double
   Dim dt As Double, Df As Double, St As Double
   Dim i As Integer, j As Integer, cp As Integer, k As Integer
 
   dt = T / nSteps
   cc = r - div
 
   ReDim C(nSteps * 2 + 1)
 
   'Option Type
   If OptionType = "Call" Then
      cp = 1
   ElseIf OptionType = "Put" Then
      cp = -1
   End If
 
   'Jump Sizes
   u = Exp(vol * Sqr(2 * dt))
   d = Exp(-vol * Sqr(2 * dt))
 
   'Transition Probabilities
   Pu = ((Exp(cc * dt / 2) - Exp(-vol * Sqr(dt / 2))) / (Exp(vol * Sqr(dt / 2)) - Exp(-vol * Sqr(dt / 2)))) ^ 2
   Pd = ((Exp(vol * Sqr(dt / 2)) - Exp(cc * dt / 2)) / (Exp(vol * Sqr(dt / 2)) - Exp(-vol * Sqr(dt / 2)))) ^ 2
   Pm = 1 - Pu - Pd
 
   'Share Price Tree (Forward Tree)
   For i = 0 To (2 * nSteps)
      C(i) = WorksheetFunction.Max(0, cp * (S * u ^ WorksheetFunction.Max(i - nSteps, 0) * d ^ WorksheetFunction.Max(nSteps - i, 0) - X))
   Next
 
   'Calculate Option Price at Maturity (Backwards Induction)
   Df = Exp(-r * dt)
   For k = nSteps - 1 To 0 Step -1
      For i = 0 To (k * 2)
         C(i) = (Pu * C(i + 2) + Pm * C(i + 1) + Pd * C(i)) * Df
      Next
   Next
 
   'Option Price
   EuropeanOption = C(0)
 
End Function