Introduction 
The basics 
Applying the basics to a data set in Excel 
Alternative formulations 
A VBA function 
Two dimensional interpolation 
Bill K on Dec 29, 2011:
Your two dimensional interpolation function works great. You obviously put a lot of work into it. Well done.
DaleW on May 19, 2011:
Nicely done, as usual.
There are a few different types of interpolation techniques. One relies on creating a curve of best fit taking into account the entire data set. This is addressed in the topic Trendline Coefficients and Regression Analysis. Another technique uses piecewise linear interpolation. As the name implies it fits a straight line to each consecutive pair of data points and uses the appropriate straight line for interpolation. It is this technique that is discussed in this section. Other techniques rely on smooth curves that pass through all the data points. These typically are Cubic Spline or Bezier curves and are outside the scope of this section.
An example of piecewise linear interpolation:
Also, suppose that the ranges containing the X and Y values are named XVals and YVals respectively. If necessary, use Insert  Name > Define... to name cell ranges.
Note that the x values must be sorted in ascending order.
Using linear interpolation, given any two points (x_{0}, y_{0}) and (x_{1}, y_{1}) and the value x_{2}, where x_{0} ≤ x_{2} ≤x_{1} the corresponding y_{2} value is given by
y_{2} = y_{0} + (y_{1}y_{0}) / (x_{1}x_{0}) * (x_{2}x_{0})
For example, given two data points (x_{0}, y_{0}) = (10.75, 25.003) and (x_{1}, y_{1}) = (30.8, 33.33) the y value that corresponds to the xvalue given by x_{2} = 11 would be
y_{2} = 25.003 + (33.33  25.003) / (30.8  10.75) * (11  10.75) or y_{2} = 25.1068
Given any data set (such as the one if Figure 1 above), and a target X value, the first step is to find the 2 Xvalues that bracket it. These 2 values will correspond to x_{0} and x_{1}.
Suppose the target X value is given by a cell named TargetVal as in:
Use the MATCH function to find which xvalue corresponds to x_{0} for this targetVal.
Name the cell containing the formula with the MATCH function as MatchIdx.
This is the index into the XVals range that corresponds to the x_{0} value. The index for the x_{1} value, will be one greater than MatchIdx. The actual x_{0} value is given by INDEX(XVals, MatchIdx) and the x_{1} value by INDEX(XVals, MatchIdx+1)
The corresponding y values, i.e., y_{0} and y_{1}, are the same indices applied to the YVals range: INDEX(YVals, MatchIdx) and INDEX(YVals, MatchIdx+1)
Hence, we can now construct the desired result as:
Note this is exactly the same as = y_{0} + (y_{1}y_{0}) / (x_{1}x_{0}) * (x_{2}x_{0})
It is possible to simplify the analysis with other builtin Excel functions. The FORECAST function is normally used for interpolation with an entire data set. However, if its arguments are only two data points, its result will be the same as a piecewise linear interpolation.
One can also use the OFFSET function to further simplify the formula.
There are instances when a VBA function might be easier to use than worksheet formulas. One instance would be error handling. The above solutions work just fine as long as the target X value is well behaved. Enter a value larger than the largest value in the X range and the result will be #REF! or #DIV/0! Yes, one could expand the formula to trap errors but that would make the formula unwieldy.
An alternative formulation would be a VBA function such as the one below
Option Explicit Option Compare Text Function RealEqual(X, Y) As Boolean RealEqual = Abs(X  Y) <= 0.00000001 End Function Function LinearInterp(XVals, YVals, TargetVal) Dim MatchVal On Error GoTo ErrXit With Application.WorksheetFunction MatchVal = .Match(TargetVal, XVals, 1) If MatchVal = XVals.Cells.Count _ And RealEqual(TargetVal, .Index(XVals, MatchVal)) Then LinearInterp = .Index(YVals, MatchVal) Else LinearInterp = .Index(YVals, MatchVal) _ + (.Index(YVals, MatchVal + 1)  .Index(YVals, MatchVal)) _ / (.Index(XVals, MatchVal + 1) _  .Index(XVals, MatchVal)) _ * (TargetVal  .Index(XVals, MatchVal)) End If End With Exit Function ErrXit: With Err LinearInterp = .Description & "(Number= " & .Number & ")" End With End Function
This would be used in a Excel worksheet as:
So far we have looked at interpolation where there is only 1 independent variable (given by x). Now, we look at interpolation with 2 independent variables. In other words, we have some variable z that depends on two independent variables, x and y. This is given by the formula z= f(x,y).
By analogy to the 1D case, interpolation requires that we specify two points (x_{0}, y_{0}, z_{0}) and (x_{1}, y_{1}, z_{1}) and the value (x_{2}, y_{2}) where x_{0} ≤ x_{2} ≤ x_{1} and y_{0} ≤ y_{2} ≤ y_{1}. This will let us calculate the corresponding z_{2} value. The math is not presented here but it is a straightforward extension of the above case. First, calculate two intermediate values based on the known xvalues. Then, we use these intermediate values to calculate the result using the known yvalues.
One example of such a
data set is shown in Figure 21. In this case, one may want to find the value of the function for say x=0.5 and y= 0.5.

Another scenario where interpolation may be required is with missing data. Instead of Figure 21 where each (x,y) pair has a corresponding z value, consider Figure 22 where not every (x,y) pair has a corresponding z value. 
The VBA code below carries out piecewise linear interpolation with two independent variables. There are two functions available for worksheet use.
The first, Interpolate2DArray, applies to the case where one can specify the known x, y, and z values in contiguous ranges. The function itself calculates the required (x_{0}, x_{1}) and (y_{0}, y_{1}) brackets. This corresponds to the data in Figure 21.
Interpolate2DArray (Zvalues, Xvalues, Yvalues, X2, Y2)
Zvalues: Known zvalues. A single 2D
range or a 2D array.
Xvalues: Known xvalues. A single 1D range (or 1D
array) with the same number of rows as in Zvalues.
Yvalues: Known yvalues. A single 1D range (or 1D
array) with the same number of columns as in Zvalues.
X2, Y2: The function returns the z value corresponding to this
(x,y) pair.
An example of this function is shown below.
The second function, Interpolate2D, requires that the brackets be specified and would be more appropriate for Figure 22.
Zvalues: Known zvalues. Four z values corresponding to
the points (x_{0},y_{0}), (x_{0},y_{1}),
(x_{1},y_{0}), and (x_{1},y_{1}).
Does not have to be a single range.
Xvalues: Known xvalues. Two values: x_{0}, and
x_{1}. Does not have to be a single range.
Yvalues: Known yvalues. Two values y_{0}, and
y_{1}. Does not have to be a single range.
X2, Y2: The function returns the z value corresponding to this
(x,y) pair.
The common code required by both functions is at top followed by the two functions themselves.
Option Explicit Option Base 0 Function CellAreaDecode(aRng, ByVal I As Long) As Range Dim AreaI As Long For AreaI = 1 To aRng.Areas.Count If I <= aRng.Areas(AreaI).Cells.Count Then Set CellAreaDecode = aRng.Areas(AreaI).Cells(I) Exit Function Else I = I  aRng.Areas(AreaI).Cells.Count End If Next AreaI End Function Sub MapIn(InVal, ByRef Where) Dim I As Integer, HowMany As Integer If Not (TypeOf InVal Is Range) Then Where = InVal ElseIf InVal.Areas.Count = 1 Then If InVal.Cells.Count = 1 Then Where = InVal.Value ElseIf InVal.Columns.Count = 1 Then Where = Application.WorksheetFunction.Transpose(InVal.Value) Else Where = Application.WorksheetFunction.Transpose( _ Application.WorksheetFunction.Transpose(InVal.Value)) End If Else HowMany = InVal.Cells.Count ReDim Where(HowMany  1) For I = 0 To HowMany  1 Where(I) = CellAreaDecode(InVal, I + 1).Value Next I End If End Sub
Function Interpolate2D(InF, InX, InY, InX2, InY2) 'InX contains two values, x0 and x1 _ InY contains two values, y0 and y1 _ InF contains 4 values, defined at (x0,y0), (x0,y1), _ (x1,y0), (x1,y1) _ InX2 and InY2 define the point at which the value of _ the function is required 'tests to ensure x0<x2<x1 and 'y0<y2<y1 needed Dim F, X, Y, _ X2 As Double, Y2 As Double Dim NoXVals(1) MapIn InF, F MapIn InX, X MapIn InY, Y MapIn InX2, X2 MapIn InY2, Y2 NoXVals(0) = (F(2)  F(0)) / (X(1)  X(0)) * (X2  X(0)) + F(0) NoXVals(1) = (F(3)  F(1)) / (X(1)  X(0)) * (X2  X(0)) + F(1) Interpolate2D = _ (NoXVals(1)  NoXVals(0)) / (Y(1)  Y(0)) * (Y2  Y(0)) _ + NoXVals(0) End Function Function Interpolate2DArray(InF, InX, InY, InX2, InY2) 'Arguments should be in the following format. However, currently _ there is no validation of the arguments. _ Each of the arguments can be either a range or an array. _ InX is a single dimension array of x values sorted ascending. _ InY is a single dimension array of y values sorted ascending. _ InF is a 2D array with 1 entry for each (X, Y) pair of values in _ the InX and InY arrays. _ InX2 is a single value. _ InY2 is a single value. Dim F, X, Y, _ X2 As Double, Y2 As Double, _ XIdx As Long, YIdx As Long Dim NoXVals(1) MapIn InF, F MapIn InX, X MapIn InY, Y MapIn InX2, X2 MapIn InY2, Y2 On Error GoTo ErrXIT XIdx = Application.WorksheetFunction.Match(X2, X, 1) YIdx = Application.WorksheetFunction.Match(Y2, Y, 1) If XIdx = UBound(X) And RealEqual(X2, X(XIdx)) Then If YIdx = UBound(Y) And RealEqual(Y2, Y(YIdx)) Then Interpolate2DArray = F(XIdx, YIdx) Else Interpolate2DArray = F(XIdx, YIdx) _ + (F(XIdx, YIdx + 1)  F(XIdx, YIdx)) _ / (Y(YIdx + 1)  Y(YIdx)) * (Y2  Y(YIdx)) End If ElseIf YIdx = UBound(Y) And RealEqual(Y2, Y(YIdx)) Then Interpolate2DArray = F(XIdx, YIdx) _ + (F(XIdx + 1, YIdx)  F(XIdx, YIdx)) _ / (X(XIdx + 1)  X(XIdx)) * (X2  X(XIdx)) Else NoXVals(0) = F(XIdx, YIdx) _ + (F(XIdx + 1, YIdx)  F(XIdx, YIdx)) _ / (X(XIdx + 1)  X(XIdx)) * (X2  X(XIdx)) NoXVals(1) = F(XIdx, YIdx + 1) _ + (F(XIdx + 1, YIdx + 1)  F(XIdx, YIdx + 1)) _ / (X(XIdx + 1)  X(XIdx)) * (X2  X(XIdx)) Interpolate2DArray = NoXVals(0) _ + (NoXVals(1)  NoXVals(0)) _ / (Y(YIdx + 1)  Y(YIdx)) * (Y2  Y(YIdx)) End If Exit Function ErrXIT: With Err Interpolate2DArray = .Description & "(Number= " & .Number & ")" End With End Function